115A_1_EE115A_W10-Chap5

115A_1_EE115A_W10-Chap5 - Chapter 5 Bipolar Amplifiers h...

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Unformatted text preview: Chapter 5 Bipolar Amplifiers h 5.1 General Considerations 169 h 5.2 Operating Point Analysis and Design h 5.3 Bipolar Amplifier Topologies h 5.4 Summary and Additional Examples Bipolar Amplifiers CH5 Bipolar Amplifiers 170 Voltage Amplifier CH5 Bipolar Amplifiers 171 h In an ideal voltage amplifier, the input impedance is infinite and the output impedance zero. h But in reality, input or output impedances depart from their ideal values. Input/Output Impedances CH5 Bipolar Amplifiers 172 h The figure above shows the techniques of measuring input and output impedances. x x x i V R = Input Impedance Example I CH5 Bipolar Amplifiers 173 h When calculating input/output impedance, small-signal analysis is assumed. π r i v x x = Impedance at a Node CH5 Bipolar Amplifiers 174 h When calculating I/O impedances at a port, we usually ground one terminal while applying the test source to the other terminal of interest. Impedance at Collector CH5 Bipolar Amplifiers 175 h With Early effect, the impedance seen at the collector is equal to the intrinsic output impedance of the transistor (if emitter is grounded). o out r R = Impedance at Emitter 1 1 + = m x x r g i v π CH5 Bipolar Amplifiers 176 h The impedance seen at the emitter of a transistor is approximately equal to one over its transconductance (if the base is grounded). ) ( 1 ∞ = ≈ A m out V g R Three Master Rules of Transistor Impedances CH5 Bipolar Amplifiers 177 h Rule # 1: looking into the base, the impedance is r π if emitter is (ac) grounded. h Rule # 2: looking into the collector, the impedance is r o if emitter is (ac) grounded. h Rule # 3: looking into the emitter, the impedance is 1/g m if base is (ac) grounded and Early effect is neglected. Biasing of BJT CH5 Bipolar Amplifiers 178 h Transistors and circuits must be biased because (1) transistors must operate in the active region, (2) their small- signal parameters depend on the bias conditions. DC Analysis vs. Small-Signal Analysis CH5 Bipolar Amplifiers 179 h First, DC analysis is performed to determine operating point and obtain small-signal parameters. h Second, sources are set to zero and small-signal model is used. Notation Simplification CH5 Bipolar Amplifiers 180 h Hereafter, the battery that supplies power to the circuit is replaced by a horizontal bar labeled Vcc, and input signal is simplified as one node called V in. Example of Bad Biasing CH5 Bipolar Amplifiers 181 h The microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone. h Unfortunately, there’s no DC bias current running thru the transistor to set the transconductance. Another Example of Bad Biasing CH5 Bipolar Amplifiers 182 h The base of the amplifier is connected to V cc , trying to establish a DC bias....
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This note was uploaded on 01/08/2011 for the course EE 115 taught by Professor Kaiser during the Spring '09 term at UCLA.

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115A_1_EE115A_W10-Chap5 - Chapter 5 Bipolar Amplifiers h...

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