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Old 28th Jun 2017, 12:13 am   #215
Radio Wrangler
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Default Re: 807 (maybe) amplifier build. Now EL34

Don't worry about a loop being closed. You can get all the info needed for calculating its stability margins from open loop analysis.

When the loop is closed the classic feedback equation comes into play.

Let's say the gain of the amplifier without feedback is G where G is an equation with poles and zeroes as required. And the feedback arrangement has a transfer function of B (traditionally the letter Beta)

Closed loop gain = G/(1+BG)

If G becomes very large, then the 1 in the denominator becomes trivial compared to G, and so it approximates to:

Closed loop gain = G/(BG)

Which simplifies to Closed loop gain =1/B Which is the approximation result used for opamps.

But here we're interested in the case there G still has some effect, so we must use the true gotm

CLG = G/(1+B)

G and B can be simple numbers or full blooded equations but whatever they are, they combine this way.

You can consider bringing in the feedback gradually, moving smoothly up to its full intended level. The poles and zeroes on its s-plane move in accordance with some simple rules (look up root-locus techniques) and lets you estiate behaviousr closed-loop.

David
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