1. Bipolar Junction Transistors
Chapter 28
Bipolar Junction Transistors
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
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2. 28-1: Transistor Construction
A transistor has three doped regions, as shown in Fig (next slide).
Fig (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.
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3. 28-1: Transistor Construction
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
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4. Bipolar Transistors Collector Collector Base Base Emitter Emitter C B

5. 28-2: Proper Transistor Biasing
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 VEE and the collector-base supply voltage is designated VCC.
For silicon, the barrier potential for both EB and CB junctions equals 0.7 V

6. Schematic Symbol

7. Transistor Biasing Collector Base Emitter N IC Reverse bias P N
IE = IB + IC
Emitter IB IE
Forward
bias

8. 28-2: Proper Transistor Biasing
Fig 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, VEE.
VCC reverse-biases the CB junction.
Fig (b) illustrates currents in a transistor.
CE voltage of an npn transistor must be positive
Ratio of IC to IE is called DC alpha αdc
Fig. 28-4
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9. 28-3: Operating Regions
Since emitter lead is common, this connection is called common-emitter connection
Collector current IC is controlled solely by the base current, IB.
By varying IB, a transistor can be made to operate in any one of the following regions
Active
Saturation
Breakdown
Cutoff
Ratio of IC to IB is called DC beta βdc
Fig. 28-6: Common-emitter connection (a) circuit. (b) Graph of IC versus VCE for different base current values.

10. 28-3: Operating Regions Active Region Saturation Breakdown Cutoff
Collector curves are nearly horizontal
IC is greater than IB (IC = dc X IB)
Saturation
IC is not controlled by IB
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 IC  0

11. Transistor Currents IE = IB + IC IC = IE – IB IB = IE – IC dc = αdc =

12. Example 28-4 A transistor has the following currents:
IE = 15 mA
IB = 60 µA
Calculate αdc, and dc
IC = IE – IB = mA
αdc = 0.996
dc = 249

13. 28-3: Operating Regions
Fig 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, IB.
Usually, the second approximation of a diode is used.
If the transistor is silicon, assume that VBE equals 0.7 V.
Fig. 28-7
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14. 28-4: Transistor Ratings
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

15. 28-5: Checking a Transistor with an Ohmmeter
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 where the npn transistor is replaced by its diode equivalent circuit.
Fig. 28-8

16. Using a DMM to check a Diode
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, RR, to forward resistance, RF, should be very large such as 1000:1 or more

17. 28-5: Checking a Transistor with an Ohmmeter
To check the base-emitter junction of an npn transistor, first connect the ohmmeter as shown in Fig (a) and then reverse the ohmmeter leads as shown in (b).
For a good p-n junction made of silicon, the ratio RR/RF should be equal to or greater than 1000:1.
Fig. 28-9
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18. 28-5: Checking a Transistor with an Ohmmeter
To check the collector-base junction, first connect the ohmmeter as shown in Fig (a) and then reverse the ohmmeter leads as shown in (b).
For a good p-n junction made of silicon, the ratio RR/RF 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
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19. 28-6: Transistor Biasing
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

20. 28-6: Transistor Biasing
Fig (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, RC, provides the desired voltage in the collector circuit
Fig
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21. Transistor Biasing: Base Biasing
A more practical way to provide base bias is to use one power supply.
VCC - VBE
IB = RB
IC  dc x IB
VCE  VCC - ICRC

22. 28-6: Transistor Biasing
The dc load line is a graph that allows us to determine all possible combinations of IC and VCE for a given amplifier.
For every value of collector current, IC, the corresponding value of VCE can be found by examining the dc load line.
A sample dc load line is shown in Fig
Fig
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23. 28-6: Transistor Biasing Midpoint Bias
Without an ac signal applied to a transistor, specific values of IC and VCE 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 IC and VCE are greatly affected by any change in the transistor’s beta value

24. 28-6: Transistor Biasing
Fig illustrates a dc load line showing the end points IC (sat) and VCE (off), as well as the Q point values ICQ and VCEQ.
Fig
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25. Base Bias – Example 1 Solve for IB, IC and VCE
Construct a dc load line showing the values of IC(sat), VCE(off), ICQ and VCEQ

26. Base Bias – Example 2 Solve for IB, IC and VCE
Construct a dc load line showing the values of IC(sat), VCE(off), ICQ and VCEQ

27. 28-6: Transistor Biasing
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 R2
VB =
X VCC
R1 + R2
VE = VB - VBE
Fig
IE  IC
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28. Voltage Divider Bias – Example
Solve for VB, VE, IE, IC, VC and VCE
Construct a dc load line showing the values of IC(sat), VCE(off), ICQ and VCEQ

29. 28-6: Transistor Biasing End points and Q points are
Fig shows the dc load line for voltage-divider biased transistor circuit in Fig
End points and Q points are
IC (sat) = mA
VCE (off) = 15 V
ICQ = 7 mA
VCEQ = 6.32 V
Fig
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30. 28-6: Transistor Biasing
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
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31. Emitter Bias – Example
Solve for IE, and VC