Stabilized D.C. Amplifiers

Author: J.B. Hoag

Fig. 24 B. A balanced d.c. amplifier

One of the greatest difficulties in connection with the amplifier of Fig. 24 A is that caused by small changes in the batteries, or by the heating of the elements inside the tube. Figures 24 B and 24 C show two attempts to stabilize the circuit against these disturbing variations.

Fig. 24 C. A simple, stabilized d.c. amplifier

In Fig. 24 B, the triodes are selected to have as nearly identical characteristic curves as possible. The circuit is balanced so that there is no current through G. A change in voltage in the A, B, or C batteries will (theoretically) cause no flow of current through G. It is only when a voltage is applied between A and B that a deflection is observed. If A is made positive, and B negative, the plate current of one tube is increased, that of the other decreased, the voltage drops across the two plate resistors are no longer the same, and the balance of the "bridge" is destroyed.

In the circuit of Fig. 24 C, an increase in the "B" battery voltage does three things. It increases the B and the C voltages and also heats the filament hotter. If the various resistors are properly chosen, these three changes cancel each other and the d.c. balance remains constant.

Fig. 24 D. An improved form of a balanced d.c. amplifier

In Fig. 24 D is shown a much more satisfactory stabilizing system,wherein the extra grid of the tube serves to compensate for changes in the plate, filament, and grid voltages when the battery voltage changes.