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Voltage Amplification per Stage

Author: J.B. Hoag

In Fig. 13 A, let the input voltage change the grid voltage by an amount eg. This will change the plate current by the same amount as would μeg volts change in the plate voltage. The voltage μeg is shared between the load resistance R2 and the tube's internal resistance rp. If these resistances happened to be equal to each other, the output voltage would amount to μeg,/2. In general, the output voltage is given by μegR2/(rp + R2). Hence the ratio of the output to the input voltages, which is the overall gain, will be μ, times [R2 (rp + 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.

Last Update: 2009-11-01