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Fig. 5-4 Circuit for cathode peaking.

The transistor because of its inherently lower impedance levels requires large capacitors. Fortunately the d-c voltage levels which these capacitors must withstand are also small, so that physically small, high-capacitance electrolytic capacitors may be used. In many cases even the coupling capacitor must be of the electrolytic type. If only small amounts of sag can be tolerated, better performance may be obtained by using no resistance in the emitter lead and resorting to feedback biasing, as shown in Fig. 3-26. [A resistor from the base to a positive supply (assuming a PNP transistor) may be necessary so that the resistor shown as R/ may be sufficiently reduced to give the necessary bias stability.]

In the vacuum-tube case the cathode bypass capacitor may be removed 1 to eliminate the greatest cause of sag, but the gain of the stage is also reduced by the factor 1/(1 + gmRk)- This reduction of gain is not so great as that caused by leaving the transistor emitter unbypassed for typical circuit conditions.

5-4 Cathode Peaking. Where strict requirements on slope make it desirable to leave the cathode bias resistor unbypassed, it becomes necessary to reevaluate the short-time transient response. In so doing it has been found beneficial to add a small capacitor across the bias resistor, instead of having none at all. This capacitance is so small, however, that its effect on the long-time response is negligible. In the short-time response, though, the rise time can be improved, with beneficial effects similar to those of shunt peaking, and hence the name "cathode peaking" is usually ascribed to this technique. The analysis proceeds from the circuit of Fig. 5-4.

Removing Ck may have one undesirable side effect: the heater-to-cathode leakage current may cause a heater-frequency noise to appear across Rk and thus in the output.

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