Head2right for ac signals the capacitor is short and

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head2right For ac signals, the capacitor is short and the amplifier is degenerated by R E .
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Example: Design CE Stage with Degeneration as a Black Box CH5 Bipolar Amplifiers 210 head2right If g m R E is much greater than unity, G m is more linear. E m m in out m E m in m out A R g g v i G R g r v g i V + = + + = = - 1 ) ( 1 1 π
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Degenerated CE Stage with Base Resistance = V CH5 Bipolar Amplifiers 211 1 1 ) 1 ( . + + + - + + + - = = β β β π B E m C v B E C in out A out in A in out A R R g R A R R r R v v v v v v v v
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Input/Output Impedances E B in E in A R r R R R r R V = + + + = + + = = ) 1 ( ) 1 ( 2 2 1 β β π π CH5 Bipolar Amplifiers 212 head2right R in1 is more important in practice as R B is often the output impedance of the previous stage. C out R R
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Emitter Degeneration Example III CH5 Bipolar Amplifiers 213 1 2 2 1 || ) 1 ( 1 1 ) || ( R R R R r R R R g R R A C out in B m C v = + + = + + + - = β β π
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Output Impedance of Degenerated Stage with V A < CH5 Bipolar Amplifiers 214 head2right Emitter degeneration boosts the output impedance by a factor of 1+g m (R E ||r π ). head2right This improves the gain of the amplifier and makes the circuit a better current source. [ ] [ ] ) || ( 1 ) || )( 1 ( || ) || ( 1 π π π π r R g r R r R r g r R r R r r R g R E m O out E O m O out E O E m out + + + = + + =
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Two Special Cases O m O out E r r g r R r R ) 1 ( + >> π π β ) 1 CH5 Bipolar Amplifiers 215 O E m out E r R g R r R ) 1 ( + << π ) 2
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Analysis by Inspection CH5 Bipolar Amplifiers 216 head2right This seemingly complicated circuit can be greatly simplified by first recognizing that the capacitor creates an AC short to ground, and gradually transforming the circuit to a known topology. [ ] 1 2 || ) || ( 1 R r r R g R O m out π + = [ ] O m out r r R g R ) || ( 1 2 1 π + = 1 1 || out out R R R =
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Example: Degeneration by Another Transistor CH5 Bipolar Amplifiers 217 head2right Called a “cascode”, the circuit offers many advantages that are described later in the book. [ ] 1 1 2 1 ) || ( 1 O O m out r r r g R π + =
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Study of Common-Emitter Topology head2right Analysis of CE Core Inclusion of Early Effect head2right Emitter Degeneration 218 Inclusion of Early Effect head2right CE Stage with Biasing
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Bad Input Connection CH5 Bipolar Amplifiers 219 head2right Since the microphone has a very low resistance that connects from the base of Q 1 to ground, it attenuates the base voltage and renders Q 1 without a bias current.
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Use of Coupling Capacitor CH5 Bipolar Amplifiers 220 head2right Capacitor isolates the bias network from the microphone at DC but shorts the microphone to the amplifier at higher frequencies.
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DC and AC Analysis CH5 Bipolar Amplifiers 221 head2right Coupling capacitor is open for DC calculations and shorted for AC calculations. O C out B in O C m v r R R R r R r R g A || || ) || ( = = - = π
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Bad Output Connection CH5 Bipolar Amplifiers 222 head2right Since the speaker has an inductor, connecting it directly to the amplifier would short the collector at DC and therefore push the transistor into deep saturation.
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Still No Gain!!!
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