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Design Considerations  Overall Power GainAuthor: Leonard Krugman In any given problem requiring more than one stage of amplification, several cascade arrangements are possible. This flexibility is a desirable design feature; however, it complicates the problem of selecting the best combination of the three general forms of transistor connections with respect to the input and output resistances, and to the required gain of the system Every design is fixed to some extent by the function of the circuit, but the requirement for maximum gain is invariably included. Fig. 516. Block schematic of cascade operation Figure 516 is the block schematic of a three stage circuit. It is evident from inspection that the overall current gain of the system is the product of the individual stage gains, thus α = α_{1}α^{2}α_{3}. The operating gain as defined in equation 345 is which can be modified to Since the current gain as defined in equation 38 is:, and the input resistance as defined by equation 313 is r_{i} = r_{11} , these values may be substituted in the operating gain equation, which then becomes
This is a useful form of the equation. For the cascade stages, illustrated in Fig. 516, the overall power gain based on equation 51 can now be written as On this basis, a cascade system has maximum gain when each of the stages is separately designed for a maximum value of its associated gain factors. Selection of Stage Connection. The first stage requires that its gain factor be as large as possible. The following general rules for this stage are based on an analysis of the gain factor vs R_{g }characteristic:
It must be noted that the intermediate stage represented by the current gain α_{2} in this discussion may actually consist of several intermediate stages having a total current gain equal to α_{2}. This analysis of the threestage circuit of Fig. 514, therefore, is applicable to any number of cascaded stages. In the final stage, the gain factor R_{L}α_{3}^{2} is made as large as possible. The following general rules for this stage are based on the analysis of the gain factor vs R_{L} characteristic for the three basic transistor connections.
(The numerical values listed above apply to those junction transistors with characteristics similar to the Western Electric Type 1752 transistor; however, the general values can be extended on a relative basis to cover all types.) Based on the foregoing rules, it might appear that the choice of the grounded emitter connection is the best under all conditions. However, specific design problems often dictate the use of grounded base and grounded collector circuits when the coupling network, biases, feedback, and other factors are taken into consideration.


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