Basics of Transistors

8/3/2019 Basics of Transistors

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Link for animated electricals

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That Pair's DARLINGton

The maximum input impedance one can expect from an

emitter follower, is limited by the finite gains of individualtransistors (~ 50 to ~ 350). However, there is a way to

increase the effective gain or transistors by using two

transistors. The total gain of this transistor pair is Gv1 x Gv2= Gv total (Gv ~ 2k - 100k). This is achieved by arranging

the transistors such that the emitter of one is driving the base

of the next and connecting the collectors together. This is

known as a Darlington pair, and can be used as any singletransistor would be: common emitter, emitter follower, etc.

The down side of this arrangement, is reduced speed: because

of the very high gain's effect on the collector to base

capacitance, Co (Ctotal = Co x Hfe).

Darlington Pair

High Input Impedance,Very High Gain Stage

Common Base Stage

Because the base is "grounded", thisconfiguration does not suffer from the MillerEffect, thus yielding the widest bandwidth of allconfigurations. Note that the drive is to theEmitter, and there is no signal inversion.

.

Video Amplifiers

A video amplifier is used to amplify video fromTVs, cameras, computer graphic devices, etc.

Aside from having sufficient bandwidth and the

ability to drive long cables: they cannot invert

the signal's polarity; if they did: unless you were

using an even number of amplifiers in cascade,the image would end up a negative. If you

wanted a gain stage, but didn't want the signalto be inverted, you would drive the emitter

instead of the base. This works, but as you

might imagine, the input impedance is quitelow. So by using what we learned about emitter

followers back in chapter 219, we can

"transform impedances," and now the

Non-Inverting Video Amplifier
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noninverting video amplifier looks better. High Frequency Compensation: Ccp

A Transistor is a Current In/Current Out Device-A Transistor can be thought of as a device that is active in only One Direction: It can draw

more or less current through its load resistor (sometimes referred to as a pull-up resistor).

It can either Source Current or it can Sink Current, it Cannot do Both.-

Sorry, But I Have a Bias

Along comes bias. You have heardabout it, you've read about it, you may

have even dreamed about it: now is

your chance to see-for-yourself--up

close and personal. Before one applies asignal voltage to the base circuit, an

arrangement for a steady voltage to be

applied to the base, such that--with noinput signal--the collector current is the

same as when it is about half way up, or

center of--the linear part of the curve.Now if we apply, say, an AC sinusoid

to the base circuit (through a capacitor),

the collector current--when seen as a

large AC signal voltage at thecollector--will be linear and

undistorted.

A.C. Coupled Amplifier

Common Emitter Amplifier

Hit 'em Again, While He's DownTo further beat a point into the ground:

if one increased the input signal beyondthis level, the output signal would now

start to "Clip" and cause distortion

(sine wave gets flat on top and/orbottom). If the bias point were set either

too low or too high, then the sine wave

would start to clip on the top before the



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bottom, or visa versa (asymmetric

clipping).


Hint #31, Active in Only One Direction

The transistor can be thought of as a device that is active in only one direction: it can draw more

or less current through its load resistor. In the case of a NPN transistor tied as a common emitter

amplifier: the device can only actively sink current through the load resistor (otherwise known as

a pull-up resistor) it cannot source current.

Effects of different Bias Settings

NearCutoff Linear Portion Near Saturation

Let Me Count The WaysBy now you have probably guessed that there are several other ways to "hook-up" the transistor.

In the previous 3 volumes we have discussed using the, so called, common emitter amplifier:

where the only output is at the collector. Now we will introduce you to an interestingarrangement: the common collector, otherwise known as an Emitter Follower, or voltage

follower.


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Now gang, this is where it gets sticky:The definition of an "ideal" voltage source is a source

having zero output impedance, i.e., infinite current can

be drawn, and the voltage stays the same.

Where the common emitter amplifier required a voltage

to current convertor for its current input requirement,this configuration requires voltage input only.

And because there is always a ~ 0.6 volt offset betweenthe base/emitter junction (as did the common emitter),

the emitter sources a voltage that reflects the input

voltage, minus this offset, times the voltage gain:

Vout = [Vin - 0.6 volts] x [Gv = .95].

Emitter Follower

A.K.A., Common Collector

Lets see if I have this right: "Voltage in, voltage out; and it's a current Amplifier?"Bingo! Think about it:

1) The voltage-in is not amplified (Gv ~ .95);

2) There is impedance transformation--high to low; there is power amplification: Therefore there

must be current amplification.

Why a Voltage Gain of Less-Than-One? Good question.

Here goes! In an emitter follower configuration, as voltage equal to--or greater than--0.6 volts is

applied directly to the base, a current is caused to flow through the the emitter resistor resultingin a commensurate voltage drop. This voltage drop is always equal to the input minus ~0.6 volts

multiplied by some value slightly less than one. e.g., .95.

In the previous common emitter amplifier the current into the base was determined by the

relative difference between the base and emitter--above 0.6 volts.

In the case of the emitter follower, as the base voltage is increased, there is a corresponding

tracking of the base/emitter differential: the emitter rises to--or follows--the base's change. If the

output follows the input, there can never be enough current drawn by the base to cause a voltage

drop across the emitter which exceeds the input voltage--hence no voltage gain. This is anelegant case of (internal) negative feedback.

The amount of base current required to cause some larger current to flow through the emitter

resistor (and corresponding voltage drop) is dependent on the gain--Hfe--of the transistor and theemitter load (emitter resistor and load).

Another way of thinking about this relationship, is as input impedance: if the transistor had

infinite gain, there would be no base current, resulting in infinite input impedance.

If the transistor had zero gain, the input impedance would be directly dependent on the emitter

resistor, i.e., base current = emitter current.


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If the transistor had some finite gain, the input impedance would be finite, i.e., base current

would be dependent on the emitter resistor modified by the transistor's finite gain (Hfe), i.e., basecurrent ~= emitter current/Hfe.

In all of this, one can think of it as a sort of internal feedback, or bootstrapping of the input

impedance.

Why is an emitter follower so stable? Another good question. Easy to answer: As long as thegain is 1 or less than 1, it can never oscillate. Oscillation requires a positive feedbackand a gain

of greater than 1 to sustain oscillation (of which instability is a precursor).

`

A.C. Coupled Common Emitter Amplifier

No Feedback

A Common Emitter Amplifier

Without FeedbackA simple common emitter transistor

amplifier--having no negative

feedback--is not an ideal amplifier. This

is because of the variability of gainfrom one transistor to another making

uniform gain from amplifier to

amplifier impossible. Also, withoutfeedback some amplifiers--having

transistors with excessive gain--might

be unstable and prone to be oscillate, aswell as, poor signal to noise and

distortion ratios (S/N+D); low input

impedance (poor impedance matching

between stages, etc.), and susceptibilityto temperature extremes. Without

negative feedback, high ambient

temperatures can raise the operatingpoint, thus heating the device further;

ending with this positive (thermal)

feedback, bringing on the transistor'spermanent failure.


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Phase Invertor

So That's Feedback, ah...

When (negative) feedback is introduced, most of

these problems diminish or disappear, resultingin improved performance and reliability. Thereare several ways to introduce feedback to this

simple amplifier, the easiest and most reliable of

which is accomplished by introducing a smallvalue resistor in the emitter circuit. The amount

of feedback is dependent on the relative signal

level dropped across this resistor, e.g., if theresistor value approached that of the collector

load resistor, the gain would approach unity (Gv

~ 1).

Emitter and Collector Feedback

And to beat a simple point into

Terra firma: with no emitterfeedback (no Re), the gain would

be essentially that of the

transistor.

Another feedback technique isthe introduction of some fraction

of the collector signal back to the

base circuit. This is most easilydone via the positive biasing

resistor (Rb1) --as in the figure.

A third --but by no means last--approach is to use a

combination of feedback

techniques


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The Miller EffectIt's Miller Time

In a gain stage (common emitter) there is a limit

to the achievable bandwidth at some set gain:

i.e., the higher the gain, the lower thebandwidth; conversely, the lower the gain, the

wider the bandwidth. This is the now famous,Gain Bandwidth Product. The dominant

mechanism for this is found in the intrinsicfeedback capacitance, Ccb, between the collector

and the base. The effect--as frequency

increases--is to increase feedback via Ccb'scapacitive reactance, XCcb, thus reducing the

overall gain. To compound this problem: XCcb is

dependent on the intrinsic capacitance, Ccb,multiplied by the gain, i.e., as the gain is

reduced, the bandwidth is increased. There are

ways of reducing this effect, such as peakingcoils in the collector (Xl cancels Xc); pre-

emphasis of the signal's higher frequencies at

the input; frequency selective feedback, etc...

The Miller Effect

Gain Bandwidth Product

Using several lower gain stages in cascade is a

strategy that also works. And, a very direct and

effective solution is a common base

configuration, in which the input signal drives theemitter, and the base is grounded, which has the

effect of breaking the collector/base feedbackpath. Frequency dependent feedback In the

figure, the capacitor, Ce, across the emitter

resistor, Re, causes the gain of this device to begreater at higher frequencies. As capacitive

reactance, Xc, approaches the value of Re, a

rapid increase in gain occurs. The effect, of

course, is to reduce the negative feedback athigher frequencies. This is often done to

compensate for the limited bandwidth of thetransistor stage.


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Common Base Stage

Because the base is "grounded", thisconfiguration does not suffer from the MillerEffect, thus yielding the widest bandwidth of allconfigurations. Note that the drive is to the

Emitter, and there is no signal inversion.

The Differential AmplifierDifferential amplifiers are everywhere: input stages of Op Amps; comparator inputs; some videoamps; balanced line receivers for digital data transmission ; etc... It is not one of the more easily

understood combinations of transistors, however, I shall attempt to explain this "not-so-little-

bugger."

A differential amplifier is an amplifier that has two inputs, each of which is sensitive to theopposite polarity of the other, i.e., if the inverting input has a positive going signal, and the non-

inverting input has the negative version, then there is an output equal their difference (multiplied

by some gain, Gv). Conversely, if both inputs happen to be at the same value, then there is nooutput signal: they cancel one another, i.e., both signals (being the same polarity and amplitude)

make no change is the shared emitter resistor's current, therefore, neither signal affects the other:

there is "cancellation," otherwise known as Common Mode Rejection, CMR. Another way of

saying the same thing is: if both inputs have the opposite polarity (or phase) signal, the sharedemitter resistor draws current equal to the algebraic summation of both transistors.

Deja vu All Over Again

You may have noticed that the configuration of the transistors in a differential amplifier are acombination of common emitter and emitter follower. OK? OK.

OK, Point #1:

A signal into either input's base, causes an inverted

signal at its collector, and simultaneously, a smaller,non-inverted output at the (shared) emitter resistor.

OK, Point #2:

Any signal at the emitter will appear at the collector

as a non-inverted version of this signal--but amplified(remember the video amp?).

OK, Point #3:

Therefore, any signal at one transistor's input is not

only seen at its collector, but is also seen at the othertransistor's collector, enabled by the action of the

shared emitter resistor (Points #1 & #2).


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What, Not a Restatement of the Same Old Thing!

This amplifier consists of two or three transistors (two in the simple version, three or more in themore precision version). These two input transistors are coupled to each other, via each's emitter,

and share the same emitter resistor . At this common connection each input transistor affects the

output of itself, as well as, the other transistor's output.

"I Lied." Or Did He?Now that you think you understand howa "Differential Pair" works, there is just a little more to the story.

Previously I said that the two input transistors share the same

emitter resistor, leaving the impression that a signal voltage wasat the junction of the emitters and Re. If you think about it, when

one transistor is increasing in current, e.g., positive alternation

of a sine wave; the other transistor is decreasing in current, byan equal amount, for the negative alternation. Since the pair is

sharing the one resistor, one can deduce that, ideally, there is

always a constant current in that resistor. Ideally, it is desiredthat the emitters transfer all of their signal to the other

transistor's emitter.

Click Me!

see a constantcurrent source


Using several lower gain stages in cascade is a

strategy that also works. And, a very direct andeffective solution is a common baseconfiguration, in which the input signal drives the

emitter, and the base is grounded, which has the

effect of breaking the collector/base feedbackpath. Frequency dependent feedback In the

figure, the capacitor, Ce, across the emitter

resistor, Re, causes the gain of this device to begreater at higher frequencies. As capacitive

reactance, Xc, approaches the value of Re, a

rapid increase in gain occurs. The effect, ofcourse, is to reduce the negative feedback at

higher frequencies. This is often done to

compensate for the limited bandwidth of thetransistor stage.

Common Base Stage

Because the base is "grounded", thisconfiguration does not suffer from the MillerEffect, thus yielding the widest bandwidth of allconfigurations. Note that the drive is to theEmitter, and there is no signal inversion.
http://williamson-labs.com/images/aa-diff2amp.gif


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The Differential AmplifierDifferential amplifiers are everywhere: input stages of Op Amps; comparator inputs; some video

amps; balanced line receivers for digital data transmission ; etc... It is not one of the more easily

understood combinations of transistors, however, I shall attempt to explain this "not-so-little-

bugger."

A differential amplifier is an amplifier that has two inputs, each of which is sensitive to theopposite polarity of the other, i.e., if the inverting input has a positive going signal, and the non-

inverting input has the negative version, then there is an output equal their difference (multipliedby some gain, Gv). Conversely, if both inputs happen to be at the same value, then there is no

output signal: they cancel one another, i.e., both signals (being the same polarity and amplitude)


there is "cancellation," otherwise known as Common Mode Rejection, CMR. Another way ofsaying the same thing is: if both inputs have the opposite polarity (or phase) signal, the shared

emitter resistor draws current equal to the algebraic summation of both transistors.


You may have noticed that the configuration of the transistors in a differential amplifier are acombination of common emitter and emitter follower. OK? OK.

OK, Point #1:A signal into either input's base, causes an inverted

signal at its collector, and simultaneously, a smaller,

non-inverted output at the (shared) emitter resistor.

OK, Point #2:

Any signal at the emitter will appear at the collectoras a non-inverted version of this signal--but amplified

(remember the video amp?).

OK, Point #3:

Therefore, any signal at one transistor's input is not

only seen at its collector, but is also seen at the other

transistor's collector, enabled by the action of theshared emitter resistor (Points #1 & #2).

What, Not a Restatement of the Same Old Thing!

This amplifier consists of two or three transistors (two in the simple version, three or more in themore precision version). These two input transistors are coupled to each other, via each's emitter,and share the same emitter resistor . At this common connection each input transistor affects the

output of itself, as well as, the other transistor's output.

"I Lied." Or Did He?Now that you think you understand how

a "Differential Pair" works, there is just a little more to the story.Previously I said that the two input transistors share the same

Click Me!
http://williamson-labs.com/480_xtor.htm#selection


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emitter resistor, leaving the impression that a signal voltage was

at the junction of the emitters and Re. If you think about it, when

one transistor is increasing in current, e.g., positive alternation

of a sine wave; the other transistor is decreasing in current, byan equal amount, for the negative alternation. Since the pair is

sharing the one resistor, one can deduce that, ideally, there isalways a constant current in that resistor. Ideally, it is desiredthat the emitters transfer all of their signal to the other

transistor's emitter.see a constantcurrent source

..

Enter the Oft-Maligned Constant Current Source

Because of a non-ideal world and the non-ideal transistors that

cohabit it, a constant current source (generator) is substituted for

Re. A constant current generator is a circuit in which a fixedvoltage source (Zener diode) is applied to the base, along with

some current determining resistor in the emitter circuit. The

result is a collector that will furnish a constant current over a

wide range of voltages.

On the Level -----------------------------------------

Differential stages are also useful for level translation. Either input can be driven (biased) toaffect the operating point of both transistors in a complementary fashion, and therefore theoutput (collector) offset voltage. This is what allows the Op Amp's offset voltages to be trimmed

to zero. (See figures)

Don't lose your TemperatureAs mentioned, one of the features of a differential amplifier is its ability to reject common mode

signals (CMRR), i.e., if the same signal is on both inputs in equal amounts the output does not

change. This works because of "common" signal cancellation that occurs within that first

differentail stage, between the inverting & non-inverting inputs. The degree of precision of this
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effect is dependent directly on how closely the two transistors are matched (gain, etc.). Typically

both transistors share the same substrate and/or package; these appear as one transistor but are, infact, a pair--sometimes refereed to as a "differential pair."

As you might guess, when packaged like this, they also share the same temperature gradients.

However, if the two transistors are separated, the slightest change in temp that is not shared can

cause a large shift in offset voltages as seen at the output (e.g., between both collectors). This

might appear as a change in gain, but it is really more a "shift" in its quiescent voltages.However, if there is any cancellation going on, this shift might reduce the cancellation which

would appear as a change in gain...

Spring as Load Analogy Common Emitter Common Base

Differential Amplifier Differential input Common Mode input

Transistor Models-

The Rheostat as a Transistor

The transistor can be thought of as a device that is like a rheostat

(potentiometer). If you think of a pot tied to a fixed resistor as atransistor amplifier: the pot is working against the fixed resistor--the

collector load resistor. This means the transistor cannot generate a

positive and a negative signal, it can only draw more or less current,e.g., the pot decreases its resistance, causing more current through

the "load" resistor, thus causing the voltage dropped across that

resistor to increase; the pot increases its resistance, causing lesscurrent through the load resistor, and this causes less voltage to be

dropped across the load resistor. If we think of the extremes of

current as being the equivalent of the positive and negative

alternations of a sine wave, then it follows that the equivalent ofzero is some current equidistant between the two.
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There's an Echo in Here

A NPN transistor connected as a common emitter amplifier: the base needs current to do its

thing.The collector cannot output voltage, it can only cause more or less current to be drawn through

its load resistor. If a voltage is applied to the base resistor a current now flows into the base

(base emitter junction). If a resistor is connected between the collector and a positive supply

voltage: the collector current flowing through the collector or load resistor causes a voltage to bedropped across said load resistor.

Diodes as TransistorWe can simulate a NPN transistor using two diodes and connecting both anodes together. One

cathode is tied to common (the emitter); the other cathode (the collector) goes to a load resistor

tied to the positive supply. Now connect a 1k resistor to the junction of the two anodes (the

base), and using a signal generator, apply a 0 to 2 volt P-P sine wave to the other end. Using adual beam oscilloscope, observe the signal at both ends of the resistor, i.e., the generator and the

"base."

The results should resemble the figure: the diode signal

starts up unimpeded until it reaches ~ 0. 6 volts peak (1.2

volts P - P), at which point the voltage at the "base"

appears to stop increasing, even though the signalgenerator is still increasing in amplitude. No matter how

much the voltage applied from the generator increases

(within reason), the "base" voltage appears to notincrease. However, the current into that junction (two

anodes) increases linearly: I = [E - 0.6]/R.

Now at this point, the analogy falls apart: these two diodes have no gain, as the transistor we are

trying to simulate would have. However, let us pretend that it does: the "collector" is a highimpedance current source and if a resistor (the load resistor) is connected between the "collector"

and the positive supply, a voltage is seen at the collector. This changing voltage drop across the

resistor--caused by the changing collector current--will change correspondingly to the "base"

current.

Now follow me, just a few more words, and You've got it! As the voltage at the generator goes

more positive; the base current increases; the collector current increases; the voltage drop acrossthe collector resistor increases; and the voltage at the collector goes less positive or lower.

Hang on! Stay with me!

Conversely, when the voltage at the generator goes less positive; the base current decreases; the

collector current decreases; the voltage drop across the collector resistor decreases; and the

voltage at the collector goes more positive or higher. Feel better now OK, So I Lied: There is just

a little more to the story. Remember when the base reached ~0.6 volts? well the collector output

is only that part of the signal that caused the base to conduct current. In other words: until the


14/28

base rises to ~ 0.6 volts and there is base current, there is no change at the collector--no collector

output.

Make a ListThe following list of attributes may, at first glance, seem confusing and contradictory, however

they are all true and are offered as clues to the puzzle of: 'how does a transistor really work?'

Abstractly, here are some Characteristics:

1. An equivalent circuit of a NPN transistor is two diodes tied anode to anode; one cathode

being the emitter, the other the collector, and the junction of the anodes is the base.

2. When a NPN transistor is doing-its-thing, there is always a constant 0.6 volt drop betweenthe base and emitter, i.e., the base is always ~ 0.6 volts more positive than the emitter--always!

3. There is no output at the collector, until the base has reached ~ 0.6 volts and the base is

drawing current, i.e., any signal that appears at the base that is not up to ~ 0.6 volts (and notdrawing base current), is never seen at the collector.

4. The base requires a current, not a voltage to control the collector current.

5. The collector is a current source: it does not source a voltage.

6. The collector appears to output a voltage when a resistor is connected between it and power.

7. The collector is a high impedance when compared to the emitter.

8. The transistor can output an amplified signal either from the collector or the emitter (orboth).

9. When operating with a collector resistor (RL): the output voltage from the collector is an

amplified voltage.

10. When operating with only an emitter resistor (Re): the output voltage from the emitter is not

an amplified voltage, because it is always ~ 0.6 volts, below the input (base) voltage--hence thename voltage follower. But because the emitter can source large amounts of current to the

"LOAD," it can be said, there was CURRENT amplification.

11. The collector--being high impedance--cannot drive a low impedance load.

12. The emitter--being a low impedance--can drive a low impedance load.

13. The voltage gain from the collector is greater than one (Gv > 1).

14. The voltage gain from the emitter is less than one (Gv < 1).


15/28

15. Both the collector and the emitter: output ~ the same power: E x I = P.

Because a transistor is a current device: if you cause some current to flow in the base, a largeramount of current is caused to flow in the collector. There's that pesky echo again.

Looking at the common emitter circuit in the figure: while measuring the voltage and the current,one starts to apply a voltage to the base of the transistor through the base resistor.

As the voltage increases from, zero there is no current

flowing. At 0.1 volt, no current; 0.2 volt, no current; 0.5 volt,still no current; as the voltage at the base approaches 0.6

volts--where there was no current--all of a sudden a small

current starts to be drawn by the base, and the voltage at thebase slows its rate of increase--and remains at ~ 0.6 volts. As

the voltage from the source increases, the voltage at the base

remains ~ 0.6 volts, and the current increases--as well as thecorresponding collector current.

At some point, as the currents increase, the increase in the collector current starts to slow, until it

stops increasing altogether, at this point it is said to be in Saturation (if this transistor was being

used as a switch or as part of a logic element, then it would be considered to be switched on).

What have we learned?

First, as the input voltage is

increased from 0 volts towards

0.6 volts, there is an abrupt

change in current, i.e., from

zero current to some small

current flow. Just below this

point where there is no current

flow, the device is said to be in

Cutoff. This low end region is

considered a nonlinear part of

the operating curve (see the

curves). Next, consider the

other extreme: as the currentsin the base and collector are

increasing (base and collector

are tracking), and the collector

current is starting to no longer

track the input base current:

this too is considered a

nonlinear part of the operating

curve, and is in saturation

Direct Coupled Amplifier (A.K.A., D.C.

Coupled)

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(again refer to the curves).

Now, to the heart of the

matter!We have an operating curve

consisting of a fairly linear

segment bounded by twononlinear ends: cutoff and

saturation.

Operating in the Middle

The transistor will operatevery nicely if one could insure

that no input voltage, i.e.,

signal voltage--would causethe collector current to ever

operate beyond either end of

the linear portion of theoperating curve.

Base Current verses Collector

Current

Now, to the heart of the

matter!

We have an operating curveconsisting of a fairly linear

segment bounded by two

nonlinear ends: cutoff and

saturation.

Operating in the Middle

The transistor will operate

very nicely if one could

insure that no input voltage,i.e., signal voltage--would

cause the collector current to

ever operate beyond eitherend of the linear portion of

the operating curve.

Base Current verses Collector Current

Sorry, But I Have a BiasAlong comes bias. You have heard

about it, you've read about it, you mayhave even dreamed about it: now is

your chance to see-for-yourself--up

close and personal. Before one appliesa signal voltage to the base circuit, an

arrangement for a steady voltage to be

applied to the base, such that--with no



17/28

input signal--the collector current is the

same as when it is about half way up, or

center of--the linear part of the curve.

Now if we apply, say, an AC sinusoid

to the base circuit (through a capacitor),the collector current--when seen as a

large AC signal voltage at thecollector--will be linear and

undistorted.


Again, While He's DownTo further beat a point into the ground:

if one increased the input signal beyond

this level, the output signal would nowstart to "Clip" and cause distortion

(sine wave gets flat on top and/or

bottom). If the bias point were set either

too low or too high, then the sine wavewould start to clip on the top before the

bottom, or visa versa (asymmetric

clipping).



Hint #31, Active in Only One Direction

The transistor can be thought of as a device that is active in only one direction: it can draw more

or less current through its load resistor. In the case of a NPN transistor tied as a common emitter

amplifier: the device can only actively sink current through the load resistor (otherwise known as

a pull-up resistor) it cannot source current.

Effects of different Bias Settings


18/28

Near Cutoff Linear Portion Near Saturation

.

Let Me Count The WaysBy now you have probably guessed that there are several other ways to "hook-up"

the transistor. In the previous 3 volumes we have discussed using the, so called,

common emitter amplifier: where the only output is at the collector. Now we will

introduce you to an interesting arrangement: the common collector, otherwise

known as an Emitter Follower, or voltage follower.

Now gang, this is where it gets sticky:

The definition of an "ideal" voltage source is a

source having zero output impedance, i.e.,

infinite current can be drawn, and the voltage

stays the same.

Where the common emitter amplifier required avoltage to current convertor for its current input

requirement, this configuration requires voltage input

only.

And because there is always a ~ 0.6 volt offsetbetween the base/emitter junction (as did the common

emitter), the emitter sources a voltage that reflects the

input voltage, minus this offset, times the voltage

gain:Vout = [Vin - 0.6 volts] x [Gv = .95].

Emitter Follower

A.K.A., Common Collector

Lets see if I have this right: "Voltage in, voltage out; and it's a current

Amplifier?"

Bingo! Think about it:

1) The voltage-in is not amplified (Gv ~ .95);

2) There is impedance transformation--high to low; there is power amplification: Therefore there


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must be current amplification.

.

Some other attributes are:

It has a voltage gain of less than one (Gv ~ .95); it is not easy to cutoff, or saturated

the transistor. Unlike the common emitter, it does not invert the polarity of theinput signal; it is among the most stable of amplifiers--yes it is an amplifier, even if

it has a voltage gain below one. Because it has high input impedance, and low

output impedance, it is often used for transforming a high impedance output, to a

low impedance output: it is often used to drive transmission lines, e.g., video cable

from camera to monitor. Also, it is often used as the output stage (pass transistor)

of linear voltage regulators. If a 5.6 volt voltage source (low impedance) is

connected to the base, the emitter output will try to maintain that voltage minus 0.6

volts: 5.60 - 0.6 = 5.0 volts (how well it maintains this voltage is dependent on the

transistor's gain: Hfe = large number).

Another attribute is its excellent high frequency response. Because there is no voltage gain, or

because it has a gain of ~ 1, the bandwidth is equal to the cutoff frequency of the transistor, Ft(where Ft = [Hfe = 1]: BW = Ft).

.

Note: because there is no voltage gain, there is no multiplication of thebase/collector capacitance (Co) which reduces the high frequency response of

common emitter amplifiers; see, also: Miller effect.

Why a Voltage Gain of Less-Than-One? Good question.Here goes! In an emitter follower configuration, as voltage equal to--or greater

than--0.6 volts is applied directly to the base, a current is caused to flow through

the the emitter resistor resulting in a commensurate voltage drop. This voltage drop

is always equal to the input minus ~0.6 volts multiplied by some value slightly less

than one. e.g., .95.

In the previous common emitter amplifier the current into the base was determined by the

relative difference between the base and emitter--above 0.6 volts.

In the case of the emitter follower, as the base voltage is increased, there is a corresponding

tracking of the base/emitter differential: the emitter rises to--or follows--the base's change. If theoutput follows the input, there can never be enough current drawn by the base to cause a voltage

drop across the emitter which exceeds the input voltage--hence no voltage gain. This is an

elegant case of (internal) negative feedback.

The amount of base current required to cause some larger current to flow through the emitter

resistor (and corresponding voltage drop) is dependent on the gain--Hfe--of the transistor and the

emitter load (emitter resistor and load).
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Another way of thinking about this relationship, is as input impedance: if the transistor had

infinite gain, there would be no base current, resulting in infinite input impedance.

If the transistor had zero gain, the input impedance would be directly dependent on the emitterresistor, i.e., base current = emitter current.

If the transistor had some finite gain, the input impedance would be finite, i.e., base current

would be dependent on the emitter resistor modified by the transistor's finite gain (Hfe), i.e., base

current ~= emitter current/Hfe.

In all of this, one can think of it as a sort of internal feedback, or bootstrapping of the inputimpedance.

Why is an emitter follower so stable? Another good question. Easy to answer: As long as the

gain is 1 or less than 1, it can never oscillate. Oscillation requires a positive feedbackand a gain

of greater than 1 to sustain oscillation (of which instability is a precursor).

.

That Pair's DARLINGton

The maximum input impedance one can expect from

an emitter follower, is limited by the finite gains of

individual transistors (~ 50 to ~ 350). However,

there is a way to increase the effective gain or

transistors by using two transistors. The total gain of

this transistor pair is Gv1 x Gv2 = Gv total (Gv ~ 2k

- 100k). This is achieved by arranging the transistors

such that the emitter of one is driving the base of

the next and connecting the collectors together. This

is known as a Darlington pair, and can be used as

any single transistor would be: common emitter,

emitter follower, etc.

The down side of this arrangement, is reduced speed:

because of the very high gain's effect on the collector to basecapacitance, Co (Ctotal = Co x Hfe).

Darlington Pair

High Input Impedance,

Very High Gain Stage

An Ideal Amplifier

An ideal amplifier is one that is made up of some gain device (transistors) that has very muchmore gain than the finished amplifier. If this gain device had infinite gain, then the amplifier's

gain would be completely dependent on the gain setting resistors: which set the gain bydetermining the amount offeedbackused to overcome the amplifier's open loop gain (e.g., Op

Amps). In the case of simple single transistor gain stages, the control exerted by the gain setting

resistors is limited and has less effect on the stage's overall performance, i.e., the transistor'sinherent gain is dominant. However, realize that the greater the ratio of final amplifier gain to the

maximum possible gain (no feedback) of the transistor, the less vulnerable the gain of the


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amplifier is to variations of the individual transistor's gain (within limits).

. .

A.C. Coupled Common Emitter Amplifier

No Feedback

A Common Emitter

Amplifier Without Feedback

A simple common emitter

transistor amplifier--having no

negative feedback--is not an

ideal amplifier. This is because

of the variability of gain from

one transistor to another

making uniform gain from

amplifier to amplifier

impossible. Also, without

feedback some amplifiers--having transistors with

excessive gain--might be

unstable and prone to be

oscillate, as well as, poor signal

to noise and distortion ratios

(S/N+D); low input impedance

(poor impedance matching

between stages, etc.), and

susceptibility to temperature

extremes. Without negativefeedback, high ambient

temperatures can raise the

operating point, thus heating

the device further; ending with

this positive (thermal)

feedback, bringing on the

transistor's permanent failure.

.

Phase Invertor So That's Feedback, ah...

When (negative) feedback is introduced,

most of these problems diminish or

disappear, resulting in improved

performance and reliability. There are

several ways to introduce feedback to

this simple amplifier, the easiest and

most reliable of which is accomplished by
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introducing a small value resistor in the

emitter circuit. The amount of feedback is

dependent on the relative signal leveldropped across this resistor, e.g., if the

resistor value approached that of the

collector load resistor, the gain would

approach unity (Gv ~ 1).

.

Emitter and Collector Feedback

And to beat a simple point

into Terra firma: with no

emitter feedback (no Re),

the gain would be

essentially that of the

transistor.

Another feedback technique

is the introduction of somefraction of the collector signal

back to the base circuit. This ismost easily done via the

positive biasing resistor (Rb1)

--as in the figure. A third --but

by no means last --approach isto use a combination of

feedback techniques.

.


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The Miller Effect

It's Miller Time

In a gain stage (common emitter) there

is a limit to the achievable bandwidth at

some set gain: i.e., the higher the gain,

the lower the bandwidth; conversely, the

lower the gain, the wider the bandwidth.This is the now famous, Gain Bandwidth

Product. The dominant mechanism for

this is found in the intrinsic feedback

capacitance, Ccb, between the collector

and the base. The effect--as frequency

increases--is to increase feedback via

Ccb's capacitive reactance, XCcb, thus

reducing the overall gain. To compound

this problem: XCcb is dependent on the

intrinsic capacitance, Ccb, multiplied bythe gain, i.e., as the gain is reduced, the

bandwidth is increased. There are ways

of reducing this effect, such as peaking

coils in the collector (Xl cancels Xc); pre-

emphasis of the signal's higher

frequencies at the input; frequency

selective feedback, etc...

The Miller Effect

.


Using several lower gain stages in

cascade is a strategy that also works.

And, a very direct and effective solution

is a common base configuration, in

which the input signal drives the emitter,

and the base is grounded, which has the

effect of breaking the collector/base

feedback path. Frequency dependent

feedback In the figure, the capacitor, Ce,

across the emitter resistor, Re, causes

the gain of this device to be greater athigher frequencies. As capacitive

reactance, Xc, approaches the value of

Re, a rapid increase in gain occurs. The

effect, of course, is to reduce the

negative feedback at higher frequencies.

This is often done to compensate for the

limited bandwidth of the transistor


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stage.

Common Base Stage

Because the base is "grounded", thisconfiguration does not suffer from the Miller

Effect, thus yielding the widest bandwidth of allconfigurations. Note that the drive is to theEmitter, and there is no signal inversion.

.

Video Amplifiers

A video amplifier is used to amplify

video from TVs, cameras, computer

graphic devices, etc. Aside from havingsufficient bandwidth and the ability to

drive long cables: they cannot invert the

signal's polarity; if they did: unless you

were using an even number of amplifiers

in cascade, the image would end up a

negative. If you wanted a gain stage, but

didn't want the signal to be inverted,

you would drive the emitter instead of

the base. This works, but as you might

imagine, the input impedance is quite

low. So by using what we learned about

emitter followers back in chapter 219,

we can "transform impedances," and

now the noninverting video amplifier

looks better.

Non-Inverting Video Amplifier

High Frequency Compensation: Ccp

.

The Differential AmplifierDifferential amplifiers are everywhere: input stages of Op Amps; comparator inputs;

some video amps; balanced line receivers for digital data transmission ; etc... It isnot one of the more easily understood combinations of transistors, however, I shall

attempt to explain this "not-so-little-bugger."

A differential amplifier is an amplifier that has two inputs, each of which is sensitive to the

opposite polarity of the other, i.e., if the inverting input has a positive going signal, and the non-

inverting input has the negative version, then there is an output equal their difference (multipliedby some gain, Gv). Conversely, if both inputs happen to be at the same value, then there is no

output signal: they cancel one another, i.e., both signals (being the same polarity and amplitude)


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there is "cancellation," otherwise known as Common Mode Rejection, CMR. Another way of

saying the same thing is: if both inputs have the opposite polarity (or phase) signal, the sharedemitter resistor draws current equal to the algebraic summation of both transistors.


You may have noticed that the configuration of the transistors in a differential amplifier are a

combination of common emitter and emitter follower. OK? OK.

OK, Point #1:

A signal into either input's base, causes an

inverted signal at its collector, and

simultaneously, a smaller, non-inverted output

at the (shared) emitter resistor.

OK, Point #2:

Any signal at the emitter will appear at the collectoras a non-inverted version of this signal--but amplified

(remember the video amp?).

OK, Point #3:

Therefore, any signal at one transistor's input is notonly seen at its collector, but is also seen at the other

transistor's collector, enabled by the action of the

shared emitter resistor (Points #1 & #2).

What, Not a Restatement of the Same Old Thing!This amplifier consists of two or three transistors (two in the simple version, three or more in the

more precision version). These two input transistors are coupled to each other, via each's emitter,

and share the same emitter resistor . At this common connection each input transistor affects theoutput of itself, as well as, the other transistor's output.

"I Lied." Or Did He? Now that you think you

understand how a "Differential Pair" works, there is

just a little more to the story. Previously I said that the

two input transistors share the same emitter resistor,leaving the impression that a signal voltage was at the

junction of the emitters and Re. If you think about it,

when one transistor is increasing in current, e.g.,

positive alternation of a sine wave; the other transistor

is decreasing in current, by an equal amount, for the

negative alternation. Since the pair is sharing the one

resistor, one can deduce that, ideally, there is always a

constant current in that resistor. Ideally, it is desired

Click Me!


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that the emitters transfer all of their signal to the other

transistor's emitter.

see a constant current

source

..

Enter the Oft-Maligned Constant Current Source

Because of a non-ideal world and the non-ideal

transistors that cohabit it, a constant current source

(generator) is substituted for Re. A constant current

generator is a circuit in which a fixed voltage source

(Zener diode) is applied to the base, along with some

current determining resistor in the emitter circuit. The

result is a collector that will furnish a constant current

over a wide range of voltages.

On the Level -----------------------------------------

Differential stages are also useful for level translation. Either input can be driven

(biased) to affect the operating point of both transistors in a complementary

fashion, and therefore the output (collector) offset voltage. This is what allows the

Op Amp's offset voltages to be trimmed to zero. (See figures)

Don't lose your Temperature

As mentioned, one of the features of a differential amplifier is its ability to reject common modesignals (CMRR), i.e., if the same signal is on both inputs in equal amounts the output does not

change. This works because of "common" signal cancellation that occurs within that first
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differentail stage, between the inverting & non-inverting inputs. The degree of precision of this

effect is dependent directly on how closely the two transistors are matched (gain, etc.). Typically

both transistors share the same substrate and/or package; these appear as one transistor but are, infact, a pair--sometimes refereed to as a "differential pair."

As you might guess, when packaged like this, they also share the same temperature gradients.

However, if the two transistors are separated, the slightest change in temp that is not shared can

cause a large shift in offset voltages as seen at the output (e.g., between both collectors). Thismight appear as a change in gain, but it is really more a "shift" in its quiescent voltages.

However, if there is any cancellation going on, this shift might reduce the cancellation which

would appear as a change in gain...


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