Basic Electronic Devices and Circuits EE 111 Electrical Engineering Majmaah University 2 nd Semester 1432/1433 H Chapter 6 BJT Amplifiers 1
Introduction The things you learned about biasing a transistor in Chapter 5 are now applied in this chapter where BJT circuits are used as small-signal amplifiers. The term small-signal refers to the use of signals that take up a relatively small percentage of an amplifier s operational range. Additionally, you will learn how to reduce an amplifier to an equivalent dc and ac circuit for easier analysis. 2
AC Quantities AC quantities are indicated with a lowercase italic subscript; V rms: 1/ 2 0.707 avg: 2/π 0.6366 rms values are assumed unless otherwise stated. The figure shows an example of a specific waveform for the collector-emitter voltage. Notice the DC component is V CE and the ac component is V ce. rms avg V CE 0 0 v ce V ce V ce V ce V ce t Resistance is also identified with a lowercase subscript when analyzed from an ac standpoint. 3
Linear Amplifier A linear amplifier produces a replica of the input signal at the output. +V CC I c V b I CQ V BQ R 1 R C V ce C 2 R s C 1 I b V CEQ I BQ V s R 2 R E R L For the amplifier shown, notice that the voltage waveform is inverted between the input and output but has the same shape. 4
AC Load Line V CC /(R C R L +R E ) I C I b I BQ I c Operation of the linear amplifier can be illustrated using an ac load line. The ac load line is different than the dc load line because a capacitor looks open to dc but effectively acts as a short to ac. Thus the collector resistor R C appears to be in parallel with the load resistor R L. I CQ Q 0 V CE V ce V CEQ ac ground ac short; Z C = 1/( j ω C ) 5
Solution: Projections on the graph of Figure 6 4 show the collector current I c varying from 6 ma to 4 ma for a peak-to-peak value of 6 4 = 2 ma and the collector-to-emitter voltage V ce varying from 1 V to 2 V for a peak-to-peak value of 2 1 = 1 V. 6
Transistor AC Model very large very small 7
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Transistor AC Model The five resistance parameters (r-parameters) can be used for detailed analysis of a BJT circuit. For most analysis work, the simplified r-parameters give good results. The simplified r-parameters are shown in relation to the transistor model. An important r-parameter is r e '. It appears as a small ac resistance between the base and emitter. ' r e 25 mv = I assuming an abrupt junction between the n and p regions, and an ambient temperature of 20 C E 9 B C E r e β ac I b B I b C r e E β ac I b
Quiz Q. The equation for finding the ac emitter resistance of a BJT is a. b. c. d. ' r e r ' e r ' e r ' e 25 mv = I B E B 25 mv = I E 0.7 V = I 0.7 V = I 10
I E = 25 mv / 8 Ω = 3.125 ma 11
Comparison of β ac to β DC Curve of I C versus I B is nonlinear! slope slope 12
h (hybrid) parameters 13
Relationships of h Parameters and r Parameters forward current gain, common-base forward current gain, common-emitter voltage feedback ratio, common-emitter output admittance (conductance), common-emitter input impedance (resistance), common-emitter 14