Post on 11-Jan-2023
Bipolar Junction TransistorsBipolar 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
ChapterChapter
2828
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
2828--1: Transistor Construction1: Transistor Construction
� 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.
McGraw-Hill © 2007 The McGraw-Hill Companies, Inc. All rights reserved.
2828--1: Transistor Construction1: Transistor Construction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 28-1
� 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.
2828--2: Proper Transistor Biasing2: 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
2828--2: Proper Transistor Biasing2: Proper Transistor Biasing
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Fig. 28-4
� 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, VEE.� 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 IC to IE is called DC alpha αdc
2828--3: Operating Regions3: 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.
2828--3: Operating Regions3: Operating Regions
� Active Region
� Collector curves are nearly horizontal
� IC is greater than IB (IC = βdc 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
Transistor CurrentsTransistor Currents
� IE = IB + IC� IC = IE – IB� IB = IE – IC
� βdc =
� αdc =
� αdc =
IC
IBIC
IE
βdc
1 + βdc
Example 28Example 28--44
� A transistor has the following currents:
IE = 15 mA
IB = 60 µA
Calculate αdc, and βdc
� IC = IE – IB = 14.94 mA
� αdc = 0.996
� βdc = 249
2828--3: Operating Regions3: Operating Regions
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Fig. 28-7
� 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, IB.
� Usually, the second approximation of a diode is used.
� If the transistor is silicon, assume that VBE equals 0.7 V.
2828--4: Transistor Ratings4: 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
2828--5: Checking a Transistor 5: Checking a Transistor
with an Ohmmeterwith an Ohmmeter
Fig. 28-8
� 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.
Using a DMM to check a DiodeUsing 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
2828--5: Checking a Transistor 5: Checking a Transistor
with an Ohmmeterwith an Ohmmeter
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Fig. 28-9
� 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 RR/RF should be equal to or greater than 1000:1.
2828--5: Checking a Transistor 5: Checking a Transistor
with an Ohmmeterwith an Ohmmeter
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 28-10
� 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 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.
2828--6: Transistor Biasing6: 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
2828--6: Transistor Biasing6: Transistor Biasing
Fig. 28-12
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� 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, RC, provides
the desired voltage in the collector circuit
Transistor BiasingTransistor Biasing: Base Biasing
� A more practical way to provide base bias is to use
one power supply.
IB = VCC - VBE
RB
IC ≈ βdc x IB
VCE ≈ VCC - ICRC
2828--6: Transistor Biasing6: Transistor Biasing
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Fig. 28-14
� 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. 28-14.
2828--6: Transistor Biasing6: Transistor BiasingMidpoint BiasMidpoint Bias
� Without an ac signal applied to a transistor, specific values ofIC 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
2828--6: Transistor Biasing6: Transistor Biasing
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Fig. 28-15
Fig. 28-15 illustrates a dc load lineshowing the end points IC (sat) and VCE (off), as well as the Q point values ICQ and VCEQ.
Base Bias Base Bias –– Example 1Example 1
� Solve for IB, IC and VCE
� Construct a dc load line showing the values of IC(sat), VCE(off), ICQ and VCEQ
Base Bias Base Bias –– Example 2Example 2
� Solve for IB, IC and VCE
� Construct a dc load line showing the values of IC(sat), VCE(off), ICQ and VCEQ
2828--6: Transistor Biasing6: Transistor Biasing
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Fig. 28-18
� 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.
VB = X VCC
R2
R1 + R2
VE = VB - VBE
IE ≈ IC
Voltage Divider Bias Voltage Divider Bias –– ExampleExample
� 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
2828--6: Transistor Biasing6: Transistor Biasing
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Fig. 28-19
� 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
2828--6: Transistor Biasing6: Transistor Biasing
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Fig. 28-23
� 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.