Power Amplifiers & Gain

[Kaidanovsky]

Hullo all,

After many an hour reading M.J. and researching on the Infobahn I'm still confused about total gain. If for example an output valve requires 50Vrms on the grid and the input sensitivity is 1Vrms, surely the entire amplifier only needs an amplification factor of 50 (ignoring for the moment headroom for NFB etc.)

If this is the case, why do many amplifiers have stages where overall the gain factor is in the hundreds (two halves of a 6SN7 for example having ~19x19 = 361?) M.J. refers to amplifiers with 2,500 times amplification, if my memory serves.

Are there rules of thumb relating to this? Surely I must have something wrong otherwise there'd be no reason for amplifiers to have more than one stage, unless other driving reasons came into play!

Many thanks for any clarification.

[Defiant]

You answered your own question in the parenthesis in your second sentence. For every 6dB of negative feedback you apply, you need to double your overall gain. So if you apply 12dB negative feedback, your 50x gain requirement just jumped to 200x. Many SS amps will run 60+dB of negative feedback, which require gains in the thousands. Many op-amps have overall gains in the hundreds of thousands to several million (100+dB gain), and use large amounts of negative feedback to achieve their performance levels.

[DF96]

Also, many circuits do not achieve the full amplification from the valves but perhaps 60-80% for each stage.

Total gain does not stop at the output grid, but must include the output stage and the OPT.

[Kaidanovsky]

Thanks a lot chaps. I haven't moved (yet) into calculating NFB so only had a vague idea of how it was applied - in the books I'm reading gain's talked of in multiples and NFB's in dB, so thanks for the conversion!