Bipolar Junction Transistors Topics Covered in Chapter : 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 Chapter 28 © 2007 The McGraw-Hill Companies, Inc. All rights reserved.

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. McGraw-Hill© 2007 The McGraw-Hill Companies, Inc. All rights reserved.

28-1: Transistor Construction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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 pass on to the collector.  The collector region is moderately doped and is the largest of all three regions.

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 V EE and the collector-base supply voltage is designated V CC.

28-2: Proper Transistor Biasing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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, V EE.  V CC reverse-biases the CB junction.  Fig (b) illustrates currents in a transistor.

28-3: Operating Regions  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  Saturation  Breakdown  Cutoff  Active Fig. 28-6: Common-emitter connection (a) circuit. (b) Graph of I C versus V CE for different base current values.

28-3: Operating Regions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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, I B.  Usually, the second approximation of a diode is used.  If the transistor is silicon, assume that V BE equals 0.7 V.

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

28-5: Checking a Transistor with an Ohmmeter Fig  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.

28-5: Checking a Transistor with an Ohmmeter Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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 R R /R F should be equal to or greater than 1000:1.

28-5: Checking a Transistor with an Ohmmeter Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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 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.

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  Voltage-divider  Emitter

28-6: Transistor Biasing Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Fig (a) shows the simplest way to bias a transistor, called base bias.  V BB is the base supply voltage, which is used to forward-bias the base-emitter junction.  R B is used to provide the desired value of base current.  V CC 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.

28-6: Transistor Biasing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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

28-6: Transistor Biasing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig Fig illustrates a dc load line showing the end points I C (sat) and V CE (off), as well as the Q point values I CQ and V CEQ.

28-6: Transistor Biasing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  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

28-6: Transistor Biasing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  Fig shows the dc load line for voltage-divider biased transistor circuit in Fig  End points and Q points are  I C (sat) = mA  V CE (off) = 15 V  I CQ = 7 mA  V CEQ = 6.32 V

28-6: Transistor Biasing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  If 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.  An example of emitter bias is shown in Fig