Impedance-Coupled Amplifiers

Author: J.B. Hoag

The principle of the impedance-coupled amplifier was discussed previously. The addition of the power supplies, with their attendant filtering and decoupling circuits, follows the same lines of reasoning as for the R-C coupled amplifiers.

The inductance of the coupling impedance is made as large as possible ; by winding its turns on an iron core when amplifying audio frequencies. The iron core is of the closed-shell type in order that the magnetic field will be confined as much as possible to that particular coil; so that it will not spread out to cut the wires and coils in other parts of the circuit, with the attendant induction of e.m.f.'s in undesired parts of the circuit. The higher the inductance L of the coupling impedance, the greater will be the reactance (2πfL) of the coil, and the greater will be the a.c. voltages across it, i.e., the higher the amplification. The need for a large L is particularly great at the low frequencies, where 2πfL is small because f is small. Values of L range from 10 to 800 henries in audio amplifiers. The inductance of an iron core coil decreases as the d.c. current through its windings is increased above a certain small value. This is due to a decrease in the magnetic property (called the permeability) of the iron with increase in magnetizing force of the larger currents. In order to keep L large and independent of the d.c. plate current, the plate circuit is sometimes split into two parallel branches, one for the d.c, the other (containing the coil) for the a.c, as in Fig. 25 F.

Fig. 25 F. Coupling unit for an impedance-coupled a.f. amplifier using parallel plate feed

The blocking condenser C can be chosen so that its reactance is numerically equal to that of the coil L for a given frequency. Parallel resonance then occurs in the circuit RCL. The voltages which develop across L at the resonant frequency are comparatively large. If L = 125 h. and C = 0.05 μfd., then f_r = 60 cycles. Then the gain of the amplifier at the low frequency end will be greatly augmented. In fact, the frequency whereat the resonant character of the RCL circuit is developed can be chosen at will, thus increasing the gain at low, at intermediate, or at high frequencies. If R is kept to a low value, the resonant peak will be sharp, and vice versa. Thus the frequency-response curve of this type of amplifier can be made to have a wide variety of shapes. In conclusion, we may say that impedance-coupled amplifiers give somewhat greater gain per stage than do R-C coupled amplifiers, they do not require as high a voltage for the B supply, and they do not (in general) have as constant an amplification at different frequencies.