Class "A" Operation

Author: J.B. Hoag

Let the A, B, and C voltages of the amplifiers shown in Figs. 13 A and 13 B be so adjusted at the start that the tubes are operating at point P in the middle of the straight portion of the dynamic characteristic curve of Fig. 13 C. The filament is operating at its proper temperature, the C-bias is, say, —10 volts, and the plate potential is, say, 200 volts positive above the filament. These so-called d.c. operating conditions are given by the manufacturer of the tube.

Next, suppose an alternating signal voltage is applied to the input terminals. It will develop a voltage drop across the grid resistor R₁ of Fig. 13 A, or across the secondary of the transformer T₁ of Fig. 13 B. These voltages are obviously in series with the C-bias voltage and cause the grid voltage to alternate back and forth about the fixed C-bias value, as indicated by the (e_g — t) curve in Fig. 13 C. Whenever the grid becomes less negative, the plate current increases and vice versa, as shown by the (i_p — t) curve of this figure.

Fig. 13 C. Principle of Class A operation. The output wave-form duplicates the input wave-form

Provided the signal voltages are not so great as to exceed the straight portion of the characteristic curve, the shape of the fluctuations in the plate current will exactly reproduce the voltage changes of the input circuit. This faithful reproduction of the wave form of the incoming signal is the outstanding characteristic of Class A amplifiers. In addition, these amplifiers are characterized by (1) low efficiency and consequent high cost of adequate tubes, (2) a steady d.c. plate drain, which simplifies the plate supply problems.

More will be said concerning Class A amplifiers in Chapter 23. In addition Class AB₁, AB₂, B and C amplifiers are taken up at that point.