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Cascade DesignAuthor: Leonard Krugman
The input resistance of the last stage (equation 313) is expressed as: and the current gain (equation 38) is: Since r_{m} is close to the value of r_{c}, the intermediate stage is restricted to the grounded emitter connection. For this stage:
Since the input resistance of the last stage is the output resistance of the intermediate stage, R_{L} = 5,000 ohms. The input resistance of the intermediate stage is and the current gain is: Since a low value of R_{g} is specified, the first stage must use either the grounded emitter or the grounded base connection. The load of the first stage equals the input resistance of the intermediate stage and is a low value. Therefore, the best choice for the first stage is the grounded emitter connection. Since R_{g} was specified as being adjustable, its value will be made equal to the input resistance, The current gain is: The overall currentgain of the cascaded system α = α_{1}α_{2}α_{3} = (18.75) (18.1) (19.99) = 6,780 The operating gain (equation 51) is The resulting cascade circuit is shown in Fig. 517. This circuit does not include biasing arrangements, coupling networks and feedback loops. The values of the elements necessary for introducing these requirements may be computed by the methods in preceding paragraphs. The cascade system may be changed considerably by the addition of external resistance arms to the circuits. These have the effect of increasing the effective values of the transistor parameters. For example, consider the effect of adding a stabilizing resistor R_{E} = 50 ohms in series with the emitter arm of the input stage. The effective resistance of the emitter is now r_{e} f R_{E} = 50 f 50 = 100 ohms, and the general fourterminal parameters are now:
The input resistance and the current gain
The overall current gain
and the operating gain Thus, a simple change reduces the overall system gain by a factor of onehalf. It is evident that even after the basic stage connections are fixed, a considerable variation in the cascade performance and resistance terminal characteristics can be attained by changes in the effective value of the transistor parameters.


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