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The Impedance-Coupled Amplifier

Author: E.E. Kimberly

In the circuit of Fig. 27-13, Ebb must be great enough to cause the direct plate current to flow through the combined resistance of rp and RL - If RL be replaced by an inductance Lp, less voltage Ebb is needed for the same direct plate current; and at the same time the plate circuit impedance ωLp can be kept large. Such an amplifier is really an inductance-capacitance amplifier, but the term impedance-coupled is generally accepted. The amplification constant is

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At lower frequencies Ka for an impedance-coupled amplifier will decrease worse than for a resistance-capacitance-coupled amplifier because for a particular value of alternating-current plate current the reactance of Lp is proportional to frequency. At very high frequencies also Ka will decrease worse than for a resistance-capacitance-coupled amplifier because the distributed capacity inherent in a coil used for Lp acts like a condenser in parallel with Lp and lowers the available voltage across Lp.

A comparison of characteristics of impedance-coupled amplifiers and resistance-capacitance-coupled amplifiers is shown in Fig. 27-17.

Fig. 27-17. Comparative Characteristics of Impedance-Coupled and Resistance-Capacitance-Coupled Amplifiers



Last Update: 2010-10-06