A Tutorial on Transistor Based Circuit Design

7/25/2019 A Tutorial on Transistor Based Circuit Design

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A Tutorial on Transistor Based Circuit Design

Compiled by: Sivaranjan Goswami, Assistant Professor

Dept. of ECT, Gauhati University, Guwahati, IndiaContact:[email protected]

Note: This tutorial focuses on the fundamental theory of Bipolar Junction Transistor for its

practical purpose of designing circuits. More complex theories and mathematical derivations

are skipped which you can learn from any standard book on Electronic Devices and Circuits.

However, study of transistors cannot be performed without expressions for its various

voltages and currents, which are included in the tutorial.

It is presumed that the reader has the working level theoretical knowledge of Transistors

including Transistor Configurations, Biasing, AC models and Transistor as Switch (topicscovered in the subject Electronic Devices and Circuits of B. Tech. or B. E. courses). If you

have not studied these topics you are requested to read some book on Electronic Devices and

Circuit Theory. (Suggested books Electronic Devices and Circuit Theory by Robert L.

Boylestadand Louis Nashelsky)

Introduction:

Integrated circuits (ICs) are available for almost all purposes nowadays. But for proper

insight to the working of electronics, there is nothing like transistors. They are the building

blocks of almost all ICs and hence can be used for designing any circuits be it digital or

analog. Software tools like Multisim, PSPICE etc. are available where we can test our circuits

without using any breadboard or PCB. This tutorial covers some basic concepts necessary to

design circuits using BJTs. Moreover, for amplifier application OPAMP based ICs need a

dual power supply which is difficult to make for projects. Using BJT, simple amplifiers can

be made with a single supply. The technique given in this tutorial is highly simplified in this

tutorial with some very simple calculation steps (independent of ).

Outline of the Tutorial:

1. Brushing up the fundamentals of transistor (configuration and biasing)

2. Design of Amplifier using BJT (moderate gain*and high gain)

3. Design of Multivibrator using BJT

4. Transistor as Switch (Normal and Darlington)

5. Design of H-Bridge for Motor Driving using BJT

*Moderate gain is nearly independent of (or hFE) value of transistor
mailto:[email protected]:[email protected]:[email protected]


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Configuration of Transistors:

There are three possible configurations of transistors:

1. Common Base:

This transistor configuration provides a low input impedance while offering a highoutput impedance. Although the voltage is high, the current gain is low and the

overall power gain is also low when compared to the other transistor configurationsavailable. The other salient feature of this configuration is that the input and output

are in phase.

Fig 1: Common base transistor configuration

As can be seen from the diagram, in this transistor configuration, the base electrode is

common to both input and output circuits.

2. Common Collector

This transistor configuration is also known as the emitter follower because the emitter

voltage follows that of the base. Offering a high input impedance and a low output

impedance it is widely used as a buffer. The voltage gain is unity, although currentgain is high. The input and output signals are in phase.

Fig 2: Common collector transistor configuration

As can be seen from the diagram, in this transistor configuration, the collector

electrode is common to both input and output circuits.

3. Common Collector:

This transistor configuration is probably the most widely used. The circuit provides a

medium input and output impedance levels. Both current and voltage gain can bedescribed as medium, but the output is the inverse of the input, i.e. 180 phase

change. This provides a good overall performance and as such it is often thought of asthe most widely used configuration.


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Fig 3: Common emitter transistor configuration

As can be seen from the diagram, in this transistor configuration, the emitter electrode

is common to both input and output circuits.

The choice of the transistor configuration which is most applicable will depend upon manycharacteristics. Input impedance, output impedance, gain and also the phase relationships all

have a bearing.

Transistor Biasing:

A transistor is a non-linear device. But in order to use it as amplifiers we have to make it

linear. Here comes the concept of biasing.

For designing bias, we have to look at the transfer characteristics of the transistor and find a

Quotient Point (Q-point) for which the transistor behaves linearly.

Fig 4: Static characteristic of BC547 transistor

Let us consider the case of a common emitter NPN transistor (which is most widely used in

transistors and switches). Its transfer characteristics can be found at the datasheet. Let us

consider the case of BC547.


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From the transfer characteristic three regions are clearly visible:

1. Saturation Region:If VCE is less than saturation voltage and the IB is large enough

(IB>50mV in the figure), the collector current (IC) varies linearly as VCE. This mode isused to implement transistor as switch (ON state). When the switch is ON, that is I Bis

large, then the input at collector will get transmitted to the emitter.

2. Cut off Region: If IBis very small, whatever input voltage is applied across collector

and emitter (VCV) there will be no flow of current. This mode is used to implement

OFF state of a transistor switch.

3. Active Region:This is the region which is of concern for designing amplifiers. The

Q-point must lie in this region. For a common emitter amplifier, the input voltage is

applied at the Base and the output is obtained across Emitter and ground. Thus I B is

varied to get some variation in IC.

We know that

IC= hFEIB. (hFEor value can be obtained from datasheet).

If IBis set such that IC>Imax, then the output voltage will be clipped.

Again, if IBbecomes very small, then the transistor will get cut off. That is why it is

wise to take Q-point at Imax/2.

Similar is the case with voltage (VCE) also. As IC changes, VCE will also change.Therefore, it is wise to take Q-point at VCE_max/2.

However, if we are dealing with very small voltages and currents (in millivolts and micro-

ampere range), it is possible to set the Q-point much bellow. Because in this case the risk of

the signal being clipped is very less.

This is especially important because it gives us the freedom to take very small power supply

to run our devices. It is seen that many electronic devices we use run on batteries ranging

from 1.5 V to 6 V. It can never give us a high Q-value. But they work because the signal they

deal with is very small voltage and current.


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Practical Steps for Design of an Amplifier using BJT

Part -1 : Moderate gain amplifier (Common Emitter Configuration with Potential

Divider Biasing)nearly independent of or hFE

Fig 5: Basic NPN common-emitter amplifier stage. Component selection to establish the

design stage gain and properly bias the transistor is discussed in the text.

A simple and effective way to construct a transistor gain stage is to supply the transistors

base bias using a voltage divider and to AC couple the input and output signals as shown in

Figure 5. The big advantage of this circuit is that it can be designed to work successfully

almost completely independently of the transistors gain , so that it will work with nearly

any available transistor and is very tolerant of circuit temperature and power supply voltage

variations. In this section we will go through a design procedure for this circuit so that you

can successfully assign the proper values to the resistors and capacitors in the circuit.

The limitation of this design is that here the gain, G is moderate (5|G|20), so as the stage

gain goes up, the circuits input impedance will have to drop and the output impedance will

rise, although these problems may be mitigated somewhat by a judicious choice of power

supply voltage. If you need a large gain, it will probably require you to cascade several

amplifier stages to achieve this result.


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The gain of the amplifier is given by:

=

Here goes the design steps of a Potential Divider, Common Emitter amplifier with moderategain.

1.

To minimize the distortion of the transistor,

= 12. Thus we can calculate

= 1 + 1

= + 1

+ 1

3.

Using these values calculate maximum output voltage swing

0( )= 0.95( 1)1 +

1||

4.

Now calculate the quiescent values of Emitter and Collector voltages VE0 and VC0respectively:

0 = 0.525(+ 1) + 1

= || 0 5. Now find the quiescent values of Collector Current IC and Base Current IB. Before

that select an arbitrary value ofRC(collector resistance as shown in Fig 5)

0 = 0

0 = 0

It is seen that the value of (also known as hFEin device datasheets) is necessary to

compute a term in the design procedure. We want the circuit design to accommodate a fairly

large variation in transistor without significantly affecting the amplifiers performance.

Calculate the maximum value of IB0you may expect by considering a reasonably lower value

of .


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6. Now find the quiescent value of Base Voltage. It must establish a 1 diode drop (0.7V

for silicon) across the base emitter junction. Thus

0= 0+ 0.7

7.

In Fig 5, we can see that to establish the critical voltage VB0the resistors RB1and RB2are used. The quiescent Base Current IB0 is supplied from VCC through the voltage

divider resistors RB1 and RB2. We want the circuit design to accommodate a fairly

large variation in transistor without significantly affecting the amplifiers

performance. While calculatingIB0we have considered a reasonably lower bound on

so that we can get the maximum possible value of IB0. Now to minimize the impact of

in the amplifier, select the resistors RB1and RB2such that the current through RB2is

10IB0. Thus

2=

0100

1= 2 0 1

8. Now we have all values of resistors except RE. RC is assigned, RB1 and RB2 are

calculated. Now, to find the REwe use the relation G = RC/RE.

Thus,

=

Note that the negative sign is there because a common emitter amplifier always gives

a phase shift of 180 degree.

9.

We have designed the amplifier block. The purpose of the coupling capacitors C in and

Coutare to act as highpass filter and block the DC values. The cut-off frequencies of

the two highpass filters are given by:

= 12 =1

2 1

1 +1

2 +1

Similarly,

= 12 ( + )=1

2 +

Thus we have to select the capacitors Cin and Cout such that the desired operating

frequency of the antenna is higher than bothfc(in)andfc(out).

Note that

= 1 || 2|| = 11 + 1

2 + 1

1

-is the input impedance of the amplifier.

Zout= RC- is the output impedance of the amplifier.


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Practical Steps for Design of an Amplifier using BJT

Part -2 : High gain amplifier (Common Emitter Configuration with Fixed Bias)

Fig. 6: High-gain NPN common-emitter amplifier stage.

Although the high-gain amplifier circuit shown in Figure 6 is much simpler than the previous

amplifier design, it will be quite dependent on the actual transistor for its DC bias

conditions and its resulting gain and output voltage range. The absence of an emitter resistor

means that the transistors dynamic emitter resistance rewill determine the circuit gain along

with the collector resistorRC.

Select a value of RB1. For silicon transistor, the value of RCcan be found using the following

relation:

We can see that the transistors actual gain is proportional to the actual value of . Thus

practical values ofRB1andRCare to be trimmed to match the desired gain at desired circuit

conditions.

The input impedance is (RB1 || .re). Accordingly we have to select the coupling capacitor

keeping in mind the band of operation.


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Practical Steps for Design of a Multivibrator using BJT

A MULTIVIBRATOR is an electronic circuit that generates square, rectangular, pulse

waveforms, also called nonlinear oscillators or function generators.

Those who are familiar with ICs, can ask why go for such complex BJT based circuits tomake multivibrators, when they can be easily implemented using ICs such as 555 Timer. The

answer is pretty simple. If you look at the data-sheet of a LM555, you will see that the IC has

more transistors than any of the schematic you will see in this section. Thus the power

consumption will be very high. If you want your design to run on battery, you can increase

the battery life tremendously with this design.

Multivibrator is basically a two amplifier circuits arranged with regenerative feedback.

There are three types of Multivibrator:

1. Astable Multivibrator:Circuit is not stable in either stateit continuously oscillates

from one state to the other. (Application in Oscillators)

2. Monostable Multivibrator: One of the state is stable but the other is not.

(Application in Timer)

3. Bistable Multivibrator: Circuit is stable in both the state and will remain in

eitherstate indefinitely. The circuit can be flipped from one state to the other by

anexternal event or trigger. (Application in Flip flop)

Astable Multivibrator:

Fig 7: Astable Multivibrator circuit


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There are four stages involved in this:

1. When we first turn ON the circuit, both transistors will be in OFF state.

2. Both VB1 and VB2 rise via base resistor R3 and R2 respectively. Any one of the

transistor will conduct faster than other due to some circuit imbalance. We cannot say

which transistor will turn on first so for analysis purpose we assume Q 1conducts first

and Q2off (C1is fully charged).

Fig 8(a): When power to the multivibrator is turned ON

3.

Since Q1conducts and Q2off hence Vc1= 0V and Vc2= VCC. - state1 (for time T2)

Due to higher voltage at Vc2, capacitor C2will be charged via R4(low resistance path

because R4 R1).

Time taken to discharge C1(T1= R2C1) > time taken to charge C2 (T2 =R4C2)

(Correction: T2= 0.693 R4C2, T1= 0.693 R2C1)

Fig 8 (b): Current through charging and discharging

4.

When C2is fully charged then left plate of C2will be atVCCwhich switch off the Q1.

When C1is fully discharged then left plate of C1will be at +VCCwhich switch on theQ2.State 2


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When VB2reaches Von, the circuit enters in state 1 again, and the process repeats.

[THIS CIRCUIT HAS BEEN TESTED USING MULTISIM]

Monostable Multivibrator Circuit:

Fig 9: Monostable Multivibrator Circuit

One of the states is stable but the other is not. For that capacitive path between VC2

andVB1 removed.

In stable state any one transistor conducts and other is off.

Application of external trigger (negative) changes the state.

When the external signal goes high,

VB2charges up to VCCthrough R2

After a certain time T, VB2=VON, Q2turns on

VC2pulled to 0V, Q1turns off.

When the external signal goes high

VB2charges up to VCCthrough R2

After a certain time T, VB2=VON, Q2turns on

VC2 pulled to 0V, Q1turns off.

Enters state 1 and remains there

When VB2is momentarily pulled to ground by an external signal

VC2rises to VCC Q1turns on


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VC1pulled to 0V

Bistable Multivibrator

Fig 10: Bistable Multivibrator

Try to analyze yourself.


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Transistor as Switch

Nowadays, digital electronic devices are widely used to control the operation of various

motors in industries. But the digital circuits cannot provide sufficient power to run those

devices. In this case a transistor switch can be used.

When the transistor is in ON state, it is in saturation mode and when it is in OFF state, it is in

cut-off mode.

In the example given in Fig 11, a microcontroller is used to run a relay using a transistor as

switch. The relay is connected to an even higher load that is not of our importance.

Fig 11: Example of transistor being used as a switch to run a relay

Here, 4 mAis required to run the relay. Thus,IC=4mA. The value of is 200.

Therefore,

= =4

200 = 20We have to find the value of the Base resistor (Rb) required to switch the load fully ON

when the input terminal voltage exceeds 2.5v.

Thus

= ( )

=2.5 0.7

20 106 = 90

The Flywheel diode is used to bypass the current produced by the back EMF at the coil of the

relay.


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Use of Darlington Pair for Switch

Fig 12: NPN Darlington Pair

The above NPN Darlington transistor switch configuration shows the Collectors of the two

transistors connected together with the Emitter of the first transistor connected to the Base

terminal of the second transistor therefore, the Emitter current of the first transistor becomes

the Base current of the second transistor switching it ON.

The first or input transistor receives the input signal to its Base. This transistor amplifies itin the usual way and uses it to drive the second larger output transistors. The second

transistor amplifies the signal again resulting in a very high current gain ( gets multiplied).

One of the main characteristics of Darlington Transistors is their high current gains

compared to single bipolar transistors.

As well as its high increased current and voltage switching capabilities, another advantage of

a Darlington Transistor Switchis in its high switching speeds making them ideal for use in

inverter circuits, lighting circuits and DC motor or stepper motor control applications.

One difference to consider when using Darlington transistors over the conventional singlebipolar types when using the transistor as a switch is that the Base-Emitter input voltage (Vbe)

needs to be higher at approx 1.4V (20.7V) for silicon devices, due to the series connection

of the two PN junctions.


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H-Bridge for Motor Driving Circuit

Fig: H-Bridge for motor driver

(Disclaimer: This schematic is for explanation purpose only. The actual circuit may need

additional components such as free wheel diodes and current limiting resistors in series with

transistors. The values of the resistors will need calculation as per the ratings of your motor

and transistors. Darlington pairs may also be required if current ratings of the motor is veryhigh. Please go through some dedicated tutorial on this topic before proceeding with

hardware implementation.)

Case 1: When switch S1 is shorted and S2 is open, the transistors Q1and Q4are ON whereas

the transistors Q2 andQ3are OFF. Thus current will flow from left to right.

Case 2: When switch S2 is shorted and S1 is open, the transistors Q2and Q3are ON whereas

the transistors Q1 andQ4are OFF. Thus current will flow from right to left.

As the rotation of a DC motor depends upon the direction of flow of current, this can be used

to drive a motor. Practically, the switches are replaced by some digital control circuit or

microcontroller.

Case 3: When both S1 and S2 are open, all transistors will be OFF and there will be no

rotation of the motor.

Case 4: When both S1 and S2 are short, all the four transistors will be ON. Ideally there

should be no rotation of the motor in this case too. However, there is an wastage of power

and as we are not using any resistors to limit the currents across the transistors so this case

may harm the transistors (over current and heating that may burn the transistors), so this case

is suggested to be avoided.


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You may try L293D motor driver IC that can run 4 motors together. The principle in the IC is

same as H-bridge, but they have used diodes instead of transistors. Refer to datasheet for

more details about the IC and its application in circuit.

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A Tutorial on Transistor Based Circuit Design