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CEAmpSu10 - c Copyright 2010 W Marshall Leach Jr Professor...

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c ° Copyright 2010. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of Electrical and Computer Engineering. The Common-Emitter Ampli fi er Basic Circuit Fig. 1 shows the circuit diagram of a single stage common-emitter ampli fi er. The object is to solve for the small-signal voltage gain, input resistance, and output resistance. Figure 1: Single-stage common-emitter ampli fi er. DC Solution (a) Replace the capacitors with open circuits. Look out of the 3 BJT terminals and make Thévenin equivalent circuits as shown in Fig. 2. V BB = V + R 2 + V R 1 R 1 + R 2 R BB = R 1 k R 2 V EE = V R EE = R E (b) Make an “educated guess” for V BE . Write the loop equation between the V BB and the V EE nodes. V BB V EE = I B R BB + V BE + I E R EE = I C β R BB + V BE + I C α R EE (c) Solve the loop equation for the currents. I C = αI E = βI B = V BB V EE V BE R BB + R EE (d) Verify that V CB > 0 for the active mode. V CB = V C V B = ( V CC I C R CC ) ( V BB I B R BB ) = V CC V BB I C ( R CC R BB ) 1
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Figure 2: Bias circuit. Small-Signal Solution It will be assumed that the base spreading resistance r x is non zero. This is a resistance in series with the base lead in the small signal models.
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