# Lectw10 - npn BJT Amplifier Stages: Common-Emitter (CE) 1....

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EE 105 Fall 2000 Page 1 Week 10 npn BJT Amplifier Stages: Common-Emitter (CE) 1. Bias amplifier in high-gain region Note that the source resistor R S and the load resistor R L are removed for determining the bias point; the small-signal source is ignored, as well. Use the load-line technique to find V BIAS = V BE and I C = I SUP . 2. Determine two-port model parameters EE 105 Fall 2000 Page 2 Week 10 Small-Signal Model of CE Amplifier n The small-signal model is evaluated at the bias point; we assume that the current gain is β o = 100 and the Early voltage is V An = 25 V: g m = I C / V th (at room temperature) r π = β o / g m = 10 k r o = V An / I C = 100 k n Substitute small-signal model for BJT; V CC and V BIAS are short-circuited for small-signals

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EE 105 Fall 2000 Page 3 Week 10 Two-Port Model: CE Amplifier n Use transconductance amplifier form for model ( not mandatory) n R in = r π , R out = r o || r oc , G m = g m by inspection n Compare with CS amplifier inferior input resistance superior transconductance about the same output resistance (assuming r o dominates) EE 105 Fall 2000 Page 4 Week 10 Common-Base Amplifier Input current is applied to the emitter (with a bias current source) and the output current is taken from the collector
EE 105 Fall 2000 Page 5 Week 10 Common Base Two-Port Model n See text for details of nodal analysis , , n CB stage is an excellent current buffer Comparison with the CG stage: note the effect of the source resistance on the output resistance if R S is much greater than r π , then the output resistance is approximately: R i n 1 g m 2245 R ou t r o c r o 1 g m r π R S ( 29 + ( 29 [ ] 2245 A i β o 1 β o + ( 29 1 2245 = R ou t r o c β r o [ ] EE 105 Fall 2000 Page 6 Week 10 Common-Collector Amplifier n Circuit configuration n Biasing: if transistor is “on” (i.e., not cutoff), then V BIAS -

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EE 105 Fall 2000 Page 7 Week 10 Common Collector Two-Port Model n Two-port model: presence of r π makes the analysis more involved than for a common drain Note 1: both the input and the output resistances depend on the load and source resistances, respectively (note typo in Fig. 8.47 in text) Note 2: this model is approximate and can give erroneous results for extremely low values of R L . However, it is very convenient for hand analysis. Comparison with CD stage:
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## This note was uploaded on 10/23/2011 for the course SDASD 102 taught by Professor Dsfas during the Spring '11 term at Baptist Bible PA.

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Lectw10 - npn BJT Amplifier Stages: Common-Emitter (CE) 1....

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