TRANSISTORSA bipolar transistor consists of a three-layer "sandwich" of doped (extrinsic)

semiconductor materials, either P-N-P or N-P-N. Each layer forming the transistor has a specific name (Emitter, Base and Collector), and each layer is provided with a wire contact for connection to a

circuit

PNP transistor

construction

construction

NPN transistor

FORWARD & REVERSE BIASED

(a) The majority carriers in the emitter p-type material are holes

(b) The base-emitter junction is forward biased to the majority carriers and the holes cross the junction and appear in the base region

(c) The base region is very thin and is only lightly doped with electrons so although some electron-hole pairs are formed, many holes are left in the base region

(d) The base-collector junction is reverse biased to electrons in the base region and holes in the collector region, but forward biased to holes in the base region; these holes are attracted by the negative potential at the collector terminal

(e) A large proportion of the holes in the base region cross the base collector junction into the collector region, creating a collector current; conventional current flow is in the direction of hole movement.

(a) The majority carriers in the n-type emitter material are electrons

(b) The base-emitter junction is forward biased to these majority carriers and electrons cross the junction and appear in the base region

(c) The base region is very thin and only lightly doped with holes, so some recombination with holes occurs but many electrons are left in the base region

(d) The base-collector junction is reverse biased to holes in the base region and electrons in the collector region, but is forward biased to electrons in the base region; these electrons are attracted by the positive potential at the collector terminal

(e) A large proportion of the electrons in the base region cross the base collector junction into the collector region, creating a collector current.

TRANSISTOR AS A SWITCH

Any sufficient source of DC current may be used to turn the transistor on, and that source of current need only be a fraction of the amount of current needed

Construction Stage

Operation • The base of the

NPN transistor must be positive with respect to the emitter,

• And the collector must be more positive than the base.

Operation

Biasing

Biasing

METER CHECK OF A

TRANSISTOR

Testing of Transistor• TESTING A TRANSISTOR to determine if it is good or

bad can be done with an ohmmeter or transistor tester or by the substitution method.

• PRECAUTIONS should be taken when working with transistors since they are susceptible to damage by electrical overloads, heat, humidity, and radiation.

• TRANSISTOR LEAD IDENTIFICATION plays an important part in transistor maintenance because before a transistor can be tested or replaced, its leads must be identified. Since there is NO standard method of identifying transistor leads, check some typical lead identification schemes or a transistor manual before attempting to replace a transistor.

Multimeter without diode check

Meter readings will be exactly opposite, of course, for an NPN

transistor, with both PN junctions facing the

other way

If a multimeter with a “Diode Check" function is used in this test, it will be found that the

emitter-base junction possesses a slightly greater forward voltage drop than the

collector-base junction. This forward voltage difference is due to the disparity in doping

concentration between the emitter and collector regions of the transistor: the emitter is a much

more heavily doped piece of semiconductor material than the collector, causing its junction

with the base to produce a higher forward voltage drop.

Meter touching wire 1 (+) and 2 (-): "OL" Meter touching wire 1 (-) and 2 (+): "OL" Meter touching wire 1 (+) and 3 (-): 0.655 volts* Meter touching wire 1 (-) and 3 (+): "OL" Meter touching wire 2 (+) and 3 (-): 0.621 volts* Meter touching wire 2 (-) and 3 (+): "OL"

Using multimeter with “Diode Check”, the data obtained:

* Indicating forward biasing of the emitter-to-base junction and the collector-to-base junction

In both those sets of meter readings (*), the black (-) meter test lead was touching wire 3, which tells us that the base of this transistor is made of N-type semiconductor material (black = negative). Thus, the transistor is an PNP type with base on wire 3, emitter on wire 1 and collector on wire 2.

Wire 3 is common to both sets of conductive readings. Thus it must be the base connection of the transistor, because the base is the only layer of the three-layer device common to both sets of PN junctions.

Identification

An easy way to identify a specific transistor configuration is to follow three simple steps:

• Identify the element (emitter, base, or collector) to which the input signal is applied.

• Identify the element (emitter, base, or collector) from which the output signal is taken.

• The remaining element is the common element, and gives the configuration its name.

TRANSISTOR RATINGSPower dissipationTransistors are rated in terms of how many watts they can safely dissipate without sustaining damage. High temperature is the mortal enemy of all semiconductor devices, and bipolar transistors tend to be more susceptible to thermal damage than most.

Reverse voltagesAs with diodes, bipolar transistors are rated for maximum allowable reverse-bias voltage across their PN junctions.

Collector currentA maximum value for collector current will be given by the manufacturer in amps.

Saturation voltagesIdeally, a saturated transistor acts as a closed switch contact between collector and emitter, dropping zero voltage at full collector current.

Beta: The ratio of collector current to base current, β is the fundamental parameter characterizing the amplifying ability of a bipolar transistor.

Transistor as Amplifier• The key to understanding how

amplifiers can exist without violating the Law of Conservation of Energy lies in the behavior of active devices.

• The result is a device that appears to magically magnify the power of a small electrical signal (usually an AC voltage waveform) into an identically-shaped waveform of larger magnitude.

Perfect or Imperfect Machine • There does exist, however, a class of machines known as

amplifiers, which are able to take in small-power signals and output signals of much greater power.

• The Law of Conservation of Energy is not violated because :

- The additional power is supplied by an external source, usually a DC battery or equivalent.

- The power output of a machine can approach, but never exceed, the power input for 100% efficiency as an upper limit.

-A realistic machine most often loses some of its input energy as heat in transforming it into the output energy stream.

- Hypothetical “perpetual motion machine” powers itself?

Amplifier

• Amplifier can scale a small input signal to large output, its energy source is an external power supply.

• Amplifiers, like all machines, are limited in efficiency to a maximum of 100 percent.

• Usually, electronic amplifiers are far less efficient than that, dissipating considerable amounts of energy in the form of waste heat.

• Because the efficiency of an amplifier is always 100 percent or less, one can never be made to function as a “perpetual motion” device.

Amplifier • The requirement of an external source of power is common to

all types of amplifiers, electrical and non-electrical. • A common analogy of a non-electrical amplification system

would be power steering in an automobile, amplifying the power of the driver's arms in turning the steering wheel to move the front wheels of the car. The source of power necessary for the amplification comes from the engine. The active device controlling the driver's “input signal” is a hydraulic valve shuttling fluid power from a pump attached to the engine to a hydraulic piston assisting wheel motion. If the engine stops running, the amplification system fails to amplify the driver's arm power and the car becomes very difficult to turn.

Connection

COMMON-BASE AMPLIFIER

Both the signal source and the load share the base lead as a common connection point

Gain• The term hfe used in place of b. The terms hfe and b are

equivalent and may be used interchangeably. This is because "hfe" means:

h = hybrid (meaning mixture) f = forward current transfer ratio e = common emitter configuration The resistance gain of the common emitter can be found in a

method similar to the one used for finding beta: • Once the resistance gain is known, the voltage gain is easy to

calculate since it is equal to the current gain multiplied by the resistance gain (E = bR).

• And, the power gain is equal to the voltage gain multiplied by the current gain b (P = bE).

Example: Measurements at several points of interest using oscilloscope

COMMON-EMITTER AMPLIFIER

Both the signal source and the load share the emitter lead as a common connection point

Example: Measurements at several points of interest using oscilloscope

COMMON-COLLECTOR AMPLIFIER

Both the signal source and the load share the collector lead as a common connection point

Example: Measurements at several points of interest using oscilloscope

Transistor's Characteristic Curves

ACTIVE MODE OPERATIONWhen a transistor is in the fully-off state (like an open switch), it is said to be cutoff.

Conversely, when it is fully conductive between emitter and collector (passing as much current through the collector as the collector power supply and load will allow), it is said to be saturated.

CLASS A AMPLIFIER Class A operation is where the entire input waveform is faithfully reproduced.

Class A operation can only be obtained when the transistor spends its entire time in the active mode, never reaching either cutoff or saturation

CLASS B AMPLIFIER

Class B operation is the transistor spent half its time in active mode and the other half in cutoff

with the input voltage too low (or even of the wrong polarity!) to forward-bias its base-emitter junction.

Class Input and Output• FIDELITY and EFFICIENCY

are two terms used in conjunction with amplifiers.

• Fidelity is the faithful reproduction of a signal, while

• Efficiency is the ratio of output signal power compared to the total input power.

• The class A amplifier has the highest degree of fidelity, but the class C amplifier has the highest efficiency.

Typical Configuration• This illustration is a class A amplifier

configured as a common emitter using fixed bias. From this, you should be able to conclude the following:

• Because of its fixed bias, the amplifier is thermally unstable.

• Because of its class A operation, the amplifier has low efficiency but good fidelity.

• Because it is configured as a common emitter, the amplifier has good voltage, current, and power gain.

• In conclusion, the type of bias, class of operation, and circuit configuration are all clues to the function and possible application of the amplifier.

Example• If the input current (IB) in a common emitter changes

from 75 mA to 100 mA and the output current (IC) changes from 1.5 mA to 2.6 mA, the current gain (b) will be 44.

• This simply means that a change in base current produces a change in collector current which is 44 times as large.

Amplifier Rating • Because amplifiers have the ability to increase the

magnitude of an input signal, it is useful to be able to rate an amplifier's amplifying ability in terms of an output/input ratio.

• The technical term for an amplifier's output/input magnitude ratio is gain.

• As a ratio of equal units (power out / power in, voltage out / voltage in, or current out / current in), gain is naturally a unitless measurement.

• Mathematically, gain is symbolized by the capital letter “A”.

Example• If an amplifier takes in

an AC voltage signal measuring 2 volts RMS and outputs an AC voltage of 30 volts RMS, it has an AC voltage gain of 30 divided by 2, or 15:

• If an amplifier with an AC current gain of 3.5 is given an AC input signal of 28 mA RMS, the output will be 3.5 times 28 mA, or 98 mA:

Rating…

• Amplifiers often amplify changes or variations in input signal magnitude (AC) at a different ratio than steady input signal magnitudes (DC).

Rating

Multistage Amplifier • If multiple amplifiers are staged, their respective

gains form an overall gain equal to the product (multiplication) of the individual gains. (Figure below)

• A 1 V signal were applied to the input of the gain of 3 amplifier in Figure above, a 3 V signal out of the first amplifier would be further amplified by a gain of 5 at the second stage yielding 15 V at the final output.

Heat Sink

Application of Electronics• From electric to electronic • Active versus passive devices• Diode• Transistor

• MICROELECTRONICS is a broad term used to describe the use of integrated circuits to miniaturize electronic equipment.

• A PRINTED CIRCUIT BOARD (PCB) is a flat, insulating surface upon which printed wiring and miniaturized components are connected in a predetermined design and attached to a common base.

• MODULAR CIRCUITRY is an assembly technique in which printed circuit boards are stacked and connected together to form a module. This technique increases the packaging density of circuit components and results in a considerable reduction in the size of electronic equipment.

• An INTEGRATED CIRCUIT is a device that integrates (combines) both active components (transistors, diodes, etc.) and passive components (resistors, capacitors, etc.) of a complete electronic circuit in a single chip.