Voltage Amplification per Stage

Author: J.B. Hoag

In Fig. 13 A, let the input voltage change the grid voltage by an amount e_g. This will change the plate current by the same amount as would μe_g volts change in the plate voltage. The voltage μe_g is shared between the load resistance R₂ and the tube's internal resistance r_p. If these resistances happened to be equal to each other, the output voltage would amount to μe_g,/2. In general, the output voltage is given by μe_gR₂/(r_p + R₂). Hence the ratio of the output to the input voltages, which is the overall gain, will be μ, times [R₂ (r_p + R₂) ]. The term in brackets is always less than unity; hence the overall gain is always less than μ. Fig. 13 D shows the overall gain for different load resistances.

Fig. 13 D. Voltage amplification of a resistance-capacitance-coupled single-stage amplifier

In the transformer-coupled circuit of Fig. 13 B, the voltage gain is nearly equal to the gain of the tube multiplied by the step-up (or step-down) ratio of the transformers.