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I believe that is true.whatever the gain is, do you agree that the small signal transfer function of the error amplifier that includes the BJT will be just the same as the transfer function without the BJT, but multiplied by the gain of the BJT?
-the problem I the topswitch-jx expects a feedback current, not a voltage. We have to amplify the opto current and feed that amplified current into the control pin of the topswitch-jx.So to take the BJT current gain out of the equation, convert the current from the opto transistor directly to a voltage using an amp with a gain determined largely by feedback
How about amplifying the optocoupler output using a Howland current output circuit.-the problem I the topswitch-jx expects a feedback current, not a voltage. We have to amplify the opto current and feed that amplified current into the control pin of the topswitch-jx.
Okay.thanks, though its actually a current amplifier that we need, we need to amplify a current of 100uA to 1 milliamp by seven times.
Thanks, but we don't know to what value it gets limited to.Sigh, by fitting degenerations R's to the npn opto and the other part of the darlington, the max gain can be limited....
Why? The original circuit is a plain current amplifier (with loosely defined gain), the new circuit is a precise current amplifier.Actually sorry, Crutschow’s circuit (post #32) is great but it would need a different small signal feedback loop transfer function to be derived for it.
Double sigh.Sigh, by fitting degenerations R's to the npn opto and the other part of the darlington, the max gain can be limited....
My simulations show the circuit reaches 90° of phase shift at 160kHz where the gain has dropped by over 13dB (below).yes , but I am referring to the topswitch schematic of fig23, page 14 of AN-57 (linked in post #36).
This has different dynamics than what it would be with Crutschow's circuit in there. The small signal transfer function would be different. That is, different than equation 25, page 14 of AN-57.
The new transfer function would have to be derived first.
Yes, it's amazing how the high-open loop gain of an op amp used in a feedback circuit can greatly minimize the effects of circuit component variations as well as diode and base-emitter offset voltages, and thus improve the stability and accuracy of a circuit..................
.......your circuit is exactly what we want...its so good that I am wondering if it can be for real.
The only problem I can see is the sensed voltages being close to the rail, but that can be solved by just putting in an extra resistor, or indeed supplying the opamp from a bit higher voltage.
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