Bipolar Junction Transistors

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Chapter 2828 BipolarBipolar JunctionJunction TransistorsTransistors

Topics Covered in Chapter 28 28-1: Transistor Construction 28-2: Proper Transistor Biasing 28-3: Operating Regions 28-4: Transistor Ratings 28-5: Checking a Transistor with an Ohmmeter 28-6: Transistor Biasing

A transistor has three doped regions, as shown in Fig. 28-1 (next slide). Fig. 28-1 (a) shows an npn transistor, and a pnp is shown in (b). For both types, the base is a narrow region sandwiched between the larger collector and emitter regions.

The emitter region is heavily doped and its job is to emit carriers into the base. The base region is very thin and lightly doped. Most of the current carriers injected into the base from emitter pass on to the collector. The collector region is moderately doped and is the largest of all three regions. Fig. 28-1

Collector Collector

N P P Base N Base N P Emitter Emitter

C B E 2828 --2:2: ProperProper TransistorTransistor BiasingBiasing

For a transistor to function properly as an amplifier, the emitter-base junction must be forward -biased and the collector-base junction must be reverse -biased. The common connection for the voltage sources are at the base lead of the transistor.

The emitter-base supply voltage is designated V EE and the collector-base supply voltage is designated V CC . For silicon, the barrier potential for both EB and CB junctions equals 0.7 V SchematicSchematic SymbolSymbol Transistor Biasing

Collector

N IC Reverse bias P Base N

IE = I B + I C I Emitter B

I Forward E bias 2828 --2:2: ProperProper TransistorTransistor BiasingBiasing

Fig. 28-4 shows transistor biasing for the common-base connection. Proper biasing for an npn transistor is shown in (a). The EB junction is forward-biased by the emitter supply voltage, V EE . VCC reverse-biases the CB junction. Fig. 28-4 (b) illustrates currents in a transistor. CE voltage of an npn transistor must be positive

Ratio of I C to I E is called DC alpha αdc

Fig. 28-4

Since emitter lead is common, this connection is called common-emitter connection

Collector current I C is controlled solely by the base current, I B. By varying I B, a transistor can be made to operate in any one of the following regions Active Saturation Breakdown Cutoff

Ratio of I C to I B is called DC beta βdc Fig. 28-6: Common-emitter connection (a) circuit. (b) Graph of I C versus V CE for different base current values. 2828 --3:3: OperatingOperating RegionsRegions

Active Region Collector curves are nearly horizontal

IC is greater than I B (I C = βdc X I B) Saturation

IC is not controlled by I B Vertical portion of the curve near the origin Breakdown Collector-base voltage is too large and collector-base diode breaks down Undesired collector current Cutoff

IB = 0

Small collector current flows I C ≈ 0 TransistorTransistor CurrentsCurrents

IE = I B + I C

IC = I E –IB

IB = I E –IC

IC βdc = IB IC αdc = IE

βdc αdc = 1 + βdc ExampleExample 2828 --44

A transistor has the following currents:

IE = 15 mA IB = 60 µA

Calculate αdc, and βdc

IC = I E –IB = 14.94 mA

αdc = 0.996

βdc = 249 2828 --3:3: OperatingOperating RegionsRegions

Fig. 28-7 shows the dc equivalent circuit of a transistor operating in the active region.

The base-emitter junction acts like a forward-biased diode with current, I B. Usually, the second approximation of a diode is used.

If the transistor is silicon, assume that V BE equals 0.7 V.

Fig. 28-7

A transistor, like any other device, has limitations on its operations. These limitations are specified in the manufacturer’s data sheet. Maximum ratings are given for Collector-base voltage Collector-emitter voltage Emitter-base voltage Collector current Power dissipation 2828 --5:5: CheckingChecking aa TransistorTransistor withwith anan OhmmeterOhmmeter

An analog ohmmeter can be used to check a transistor because the emitter-base and collector-base junctions are p-n junctions. This is illustrated in Fig. 28-8 where the npn transistor is replaced by its diode equivalent circuit.

Fig. 28-8 UsingUsing aa DMMDMM toto checkcheck aa DiodeDiode

Ohmmeter ranges in DMMs do not provide the proper forward bias to turn on the diode Set DMM to the special diode range In forward-bias, digital display indicates the forward voltage dropped across the diode In reverse-bias, digital display indicates an over range condition For silicon diode, using an analog meter, the ratio of

reverse resistance, R R, to forward resistance, R F, should be very large such as 1000:1 or more 2828 --5:5: CheckingChecking aa TransistorTransistor withwith anan OhmmeterOhmmeter

To check the base-emitter junction of an npn transistor, first connect the ohmmeter as shown in Fig. 28-9 (a) and then reverse the ohmmeter leads as shown in (b).

For a good p-n junction made of silicon, the ratio R R/R F should be equal to or greater than 1000:1.

Fig. 28-9

To check the collector-base junction, first connect the ohmmeter as shown in Fig. 28-10 (a) and then reverse the ohmmeter leads as shown in (b).

For a good p-n junction made of silicon, the ratio R R/R F should be equal to or greater than 1000:1. Although not shown, the resistance measured between the collector and emitter should read high or infinite for both connections of the meter leads.

Fig. 28-10

For a transistor to function properly as an amplifier, an external dc supply voltage must be applied to produce the desired collector current. Bias is defined as a control voltage or current. Transistors must be biased correctly to produce the desired circuit voltages and currents. The most common techniques used in biasing are Base bias Voltage-divider bias Emitter bias 2828 --6:6: TransistorTransistor BiasingBiasing

Fig. 28-12 (a) shows the simplest way to bias a transistor, called base bias .

VBB is the base supply voltage, which is used to forward-bias the base-emitter junction.

RB is used to provide the desired value of base current.

VCC is the collector supply voltage, which provides the reverse-bias voltage required for the collector-base junction.

The collector resistor, R C, provides the desired voltage in the collector circuit

Fig. 28-12

A more practical way to provide base bias is to use one power supply.

VCC -VBE IB = RB

IC ≈ βdc x I B

VCE ≈ VCC -ICRC 2828 --6:6: TransistorTransistor BiasingBiasing

The dc load line is a graph that allows us to determine all possible combinations

of I C and V CE for a given amplifier.

For every value of collector current, I C, the corresponding value of V CE can be found by examining the dc load line.

A sample dc load line is shown in Fig. 28-14.

Without an ac signal applied to a transistor, specific values of

IC and V CE exist at a specific point on a dc load line This specific point is called the Q point (quiescent currents and voltages with no ac input signal) An amplifier is biased such that the Q point is near the center of dc load line

ICQ = ½ IC(sat)

VCEQ = ½ VCC

Base bias provides a very unstable Q point, because I C and VCE are greatly affected by any change in the transistor’s beta value 2828 --6:6: TransistorTransistor BiasingBiasing

Fig. 28-15 illustrates a dc load line showing the end points I C (sat) and VCE (off) , as well as the Q point values I CQ and V CEQ .

Fig. 28-15

Solve for I B, I C and V CE

Construct a dc load line showing the values of I C(sat) , VCE(off) , I CQ and V CEQ BaseBase BiasBias –– ExampleExample 22

Solve for I B, I C and V CE Construct a dc load line showing the values of

IC(sat) , V CE(off) , I CQ and V CEQ 2828 --6:6: TransistorTransistor BiasingBiasing

The most popular way to bias a transistor is with voltage-divider bias .

The advantage of voltage-divider bias lies in its stability .

An example of voltage-divider bias is shown in Fig. 28-18.

R2 VB = X V CC R1 + R 2

VE = V B -VBE Fig. 28-18

IE ≈ IC

Solve for V B, V E, I E, I C, V C and V CE

Construct a dc load line showing the values of I C(sat) , VCE(off) , I CQ and V CEQ 2828 --6:6: TransistorTransistor BiasingBiasing

Fig. 28-19 shows the dc load line for voltage-divider biased transistor circuit in Fig. 28-18. End points and Q points are

IC (sat) = 12.09 mA VCE (off) = 15 V ICQ = 7 mA VCEQ = 6.32 V

Fig. 28-19

Both positive and negative power supplies are available

Emitter bias provides a solid Q point that fluctuates very little with temperature variation and transistor replacement.

Fig. 28-23

Solve for I E, and V C