Amplification of Alternating Voltage

Author: E.E. Kimberly

In the circuit of Fig. 27-13 let it be assumed that an a-c generator impresses a voltage eg between the grid and the cathode. This voltage is superposed on the bias voltage Ecc and therefore adds to and subtracts from Ecc on the dynamic characteristic. This action is shown in Fig. 27-12. A positive increase in grid voltage (reducing the grid bias) causes an increase in ib. An increase in ib causes an increase in ibRL across the load resistor, thus leaving a smaller voltage eb across the triode. The voltage ep is therefore reversed in phase from ec. The instantaneous plate current is

The instantaneous component of voltage across the triode is

This is the output voltage available between terminals A and B of Fig. 27-13.

If the dynamic characteristic were linear, the output voltage would have the same wave form as the grid input signal. Only a portion of the dynamic characteristic is acceptably straight, however, and it is important that the bias voltage Ecc be so chosen that the swings of eg will not overreach the straight portion. Regardless of the bias point chosen, excessive grid signal voltage will cause more than the straight part of the dynamic characteristic to be used and distortion of the output voltage across RL will occur.