Cefreqrev - c Copyright 2010 W Marshall Leach Jr Professor Georgia Institute of Technology School of Electrical and Computer Engineering Frequency

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c c Copyright 2010. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of Electrical and Computer Engineering. Frequency Response of the CE Ampli±er Midband Voltage Gain The Fgure shows the signal circuit of the common-emitter ampliFer. There are three capacitors in the circuit. At the mid frequency band, these are considered to be short circuits. When r 0 is neglected except in calculating the collector output impedance r ic , the midband voltage gain from v s to v o can be written A v = R 1 b R 2 R s + R 1 b R 2 × 1 r ie + R te × α × ( - r ic b R C b R L ) R tb = R S b R 1 b R 2 R te = R E b R 3 r ie = R tb + r x + r π 1 + β r ic = r 0 p 1 + βR te R tb + r x + r π + R te P + R te This solution corresponds to the third solution found in the class notes on the common-emitter ampliFer. E²ect of C 1 At low frequencies, C 1 is an open circuit and the gain is zero. Thus C 1 has a high pass e±ect on the gain, i.e. it a±ects the lower cuto± frequency of the ampliFer. To account for C 1 , A v is multiplied by the high-pass transfer function T 1 ( s ) = τ 1 s 1 + τ 1 s where τ 1 is the time constant for C 1 . The worst case time constant for the calculation of the lower cuto± frequency is the smallest value, i.e. the value which predicts the highest pole frequency. ²or this to be the case, the base input resistance r ib must be calculated with C 3 a short circuit. This makes r ib its smallest possible value. Imagine C 1 being replaced with an ohmmeter with the source zeroed. The time constant is given by the resistance measured by the ohmmeter multiplied by C 1 . τ 1 = ( R S + R 1 b R 2 b r ib ) C 1 r ib = r x + r π + (1 + β ) R te The pole frequency is given by f 1 = 1 2 πτ 1 1
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Efect oF C 2 Capacitor C 2 also has a high pass efect on the gain. To account For C 2 , A v is multiplied by the high-pass transFer Function T 2 ( s
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This note was uploaded on 05/26/2011 for the course ECE 3050 taught by Professor Hollis during the Summer '08 term at Georgia Institute of Technology.

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Cefreqrev - c Copyright 2010 W Marshall Leach Jr Professor Georgia Institute of Technology School of Electrical and Computer Engineering Frequency

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