The Impedance-Coupled Amplifier

Author: E.E. Kimberly

In the circuit of Fig. 27-13, E_bb must be great enough to cause the direct plate current to flow through the combined resistance of r_p and R_L - If R_L be replaced by an inductance L_p, less voltage E_bb is needed for the same direct plate current; and at the same time the plate circuit impedance ωL_p 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

At lower frequencies K_a 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 L_p is proportional to frequency. At very high frequencies also K_a will decrease worse than for a resistance-capacitance-coupled amplifier because the distributed capacity inherent in a coil used for L_p acts like a condenser in parallel with L_p and lowers the available voltage across L_p.

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