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lecture_06_single-stage_amplifiers

# lecture_06_single-stage_amplifiers - Handout#6 EE 214...

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B. Murmann, B. Wooley EE214 Winter 2008-09 1 Single-Stage Amplifiers B. Murmann and B. A. Wooley Stanford University Handout #6 EE 214 Winter 2009 B. Murmann, B. Wooley EE214 Winter 2008-09 2 Common-Emitter (C-E) Stage Provides both voltage and current gain ~ v i V i V CC Q 1 R L R S V o +v o + DC input bias voltage, V i , biases Q 1 in the forward active region. Typically, want V o ! V CC /2. V o V CC I B V i V i

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B. Murmann, B. Wooley EE214 Winter 2008-09 3 For dc analysis with Q 1 in the active region, can often model the B-E diode as a constant voltage, V BE(on) . Then I B = V i ! V BE(on) R S V o = V CC ! I C R L = V CC ! " F I B R L = V CC ! " F R L R S V i ! V BE(on) ( ) The dependence on ! F makes this “direct voltage biasing” impractical. How to generate V i so as to control V o ? Practical approaches include using feedback, ac coupling and/or differential circuits. I B I s e qV BE kT R L R S + V BE V i V CC I C = ! F I B + V o B. Murmann, B. Wooley EE214 Winter 2008-09 4 Small-Signal Equivalent Circuit for C-E Stage For hand analysis, typically neglect r c and r e , and neglect r b or include it with R S . Then, at low frequencies A V (0) ! v o v i ! = 0 = " g m r # R S * + r # \$ % & ( ) (r o ||R L ) where R S * = R S + r b Note that for R S ! 0 and R L ! " A V (0) ! " g m r o = " 1 # = " V A kT q r " C " C μ g m v 1 r o r c R L R S r b i i v i ~ + v 1 + v o i o i 2
B. Murmann, B. Wooley EE214 Winter 2008-09 5 Miller Approximation Referring to the small-signal circuit for # > 0 i 2 = (v 1 ! v o )(sC μ ) where R L * ! R L ||r o (neglect r c ) If i 2 << g m v 1 , then i 2 v 1 = s(1 + g m R L * )C μ = sC M C M is referred to as the “Miller Capacitance” v o = ( ! g m v 1 + i 2 )R L * C M = (1 + g m R L * )C μ = (1 + A V )C μ where A V = v o v 1 = voltage gain from the "internal" base B. Murmann, B. Wooley EE214 Winter 2008-09 6 Miller Approximation, cont’d With the Miller approximation the equivalent circuit for the C-E stage can be reduced to: C t = C ! + (1 + g m R L * )C μ R S * = R S + r b R L * = r o ||R L r " C t g m v 1 v i ~ + v 1 + v o R S * R L * A V (s) = v o (s) v i (s) = A V (0) 1 1 ! s p 1 " # \$ % & where A V (0) = ! g m R L * r " R S * + r " # \$ % & ( p 1 = !

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lecture_06_single-stage_amplifiers - Handout#6 EE 214...

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