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Unformatted text preview: IV. Transistor Amplifiers 4.1 Introduction Amplifiers are two-port networks in which the output voltage or current is directly propor- tional to either input voltage or current (see discussions of Sec. 1.4). Four different kinds of amplifiers exit: Voltage amplifier: A v = v o /v i = constant Current amplifier: A i = i o /i i = constant Transconductance amplifier: G m = i o /v i = constant Transresistance amplifier: R m = v o /i i = constant The above definitions represent “ideal” amplifiers, i.e., the above relationship should hold for any “source” and “load” conditions. However, we noted in Sec. 1.4 that in real circuits the “input” and “output” resistances have a dramatic impact on the response of the two-port network. This observation leads to models for “real” linear amplifiers. Let’s take the case of a voltage amplifier. We note that the for a given input voltage, the maximum output voltage occurs for R L → ∞ (open-loop transfer function) and the output voltage drops when a load attached to the circuit via v o = v o ( open − loop ) × R L / ( R L + R o ). Therefore, one should assume that in a real “linear” voltage amplifier the open-loop gain, A vo , is a constant as opposed to A v = v o /v i . As such, the output part of a voltage amplifier can be modeled by two circuit elements as shown below (note for amplifiers we usually use A v instead of H v to underscore that this is an amplifier). We also assume that the load ( R L ) does not affect the input resistances of the circuit. Therefore, we model the input stage with a simple resistor (or an impedance). i V- + o I V o o i-- + i A V vo + Voltage Amplifier Model R R As discussed in Sec. 1.4, for “maximum” voltage transfer between stages, we need to have a large in- put resistance and a small output resistance. Thus, a good voltage amplifier has a large input resistance, R i , and a small output resistance, R o . An “ideal” voltage amplifier has, R i → ∞ and R o → 0. In this case, A v = v o /v i = constant for all source and load conditions. For a current amplifier, the situation is analogous. We note that the for a given input current, the maximum output current occurs for R L → 0 (short-circuit) and the output current drops when a load attached to the circuit via i o = i o ( short − circuit ) × R L / ( R L bardbl R o ) (see Sec. 1.4.4). Therefore, for a real linear current amplifier, the short-circuit gain, A is , is a constant ECE65 Lecture Notes (F. Najmabadi), Fall 2009 116 (as opposed to A i = i o /i i ). As such, the output part of a current amplifier can be modeled by two circuit elements as shown below. We also assume that the load ( R L ) does not affect the input resistances of the circuit. Therefore, we model the input stage with a simple resistor (or an impedance)....
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