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Figure 1036 Approximate low-end frequency response of a common-emitter ampli er, assuming nite coupling capacitors
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Transistor Ampli ers and Switches
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Figure 1037 High-frequency BJT model and equivalent circuit
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It can be shown that at high frequency both capacitances contribute to a low-pass response with cutoff frequency given by: T = 1 RT CT (1062)
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where RT = RS R1 R2 r and CT is an equivalent capacitance related to CCB and CBE The calculation of CT requires making use of Miller s theorem, which is well beyond the scope of this book The effect of these capacitances on the frequency response of the ampli er is depicted in Figure 1038 Figure 1039 summarizes the discussion by illustrating that a practical BJT common-emitter ampli er stage will be characterized by a band-pass frequency response with low cutoff frequency low and high cutoff frequency high
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low
high
Figure 1038 Low-pass lter effect of parasitic capacitance in a common-emitter ampli er
Figure 1039 Frequency response of a common-emitter ampli er
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A completely analogous discussion could be made for FET ampli ers, where the effect of the input and output coupling capacitors and of the source bypass capacitor is to reduce the ampli er frequency response at the low end, creating a high-pass effect Similarly, a high-frequency small-signal model valid for all FETs will include the effect of parasitic capacitances between gate, drain, and source terminals This model is shown in Figure 1040 Multistage Ampli ers The design of a practical ampli er involves a variety of issues, as mentioned earlier in this chapter To resolve the various design trade-offs and to obtain acceptable performance characteristics, it is usually necessary (except in the simplest applications) to design an ampli er in various stages In general, three stages are needed to address three important issues:
Figure 1040 High-frequency FET model
Part II
Electronics
1 Choosing an appropriate input impedance for the ampli er, so as not to load the small-signal source The input impedance should, in general, be large 2 Providing suitable gain 3 Matching the output impedance of the ampli er to the load This usually requires choosing a low output impedance Each of these tasks can be accomplished in a different manner and with more than one ampli er stage, depending on the intended application Although the task of designing a multistage ampli er is very advanced, and beyond the scope of this book, the minimum necessary tools to understand such a design process have already been introduced Each ampli er stage can, through the use of small-signal models, be represented in the form of a two-port circuit and characterized by a gain and an input and an output impedance; the overall response of the ampli er can then be obtained by cascading the individual two-port blocks, as shown in Figure 1041
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Figure 1041 Block diagram of a multistage ampli er
A three-stage ampli er is shown in Figure 1042 The input stage consists of a MOSFET ampli er; the choice of a MOSFET for the input stage is quite
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Figure 1042 Three-stage ampli er
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Transistor Ampli ers and Switches
natural, because of the high input impedance of this device Note that the rst stage is AC-coupled; thus, the ampli er will have a band-pass characteristic, as discussed in the preceding section The choice of a MOSFET as the input stage of a linear ampli er is acceptable in spite of the nonlinearity of the MOSFET transfer characteristic, because the role of the rst stage is to amplify a very low-amplitude signal, and therefore the MOSFET is required to amplify in only a relatively small operating region Thus, the linear transconductance approximation will be valid, and relatively little distortion should be expected The second stage consists of a BJT common-emitter stage, which is also AC-coupled to the rst stage and provides most of the gain The output stage is a BJT emitter follower, which produces no additional gain but has a relatively low output impedance, needed to match the load Note that this last stage is DC-coupled to the preceding stage but is AC-coupled to the load The ampli er of Figure 1042 will be further explored in the homework problems
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