isolated high voltage amp

[unclejed613]

as far as output impedance is concerned, that is mostly determined by the feedback ratio. the actual output resistance is only a very small factor. basically the output impedance is the output resistance divided by the feedback ratio. an amplifier with feedback will correct for discrepancies introduced by output resistance, and that effectively lowers the output impedance. the power supply and output devices do place limitations on how well the amp can maintain that low output impedance (there's no such thing as a free lunch).

[wg_ski]

Originally posted by wg_ski
Both sides behave as common-drain and have equal voltage gain.

Correction - both behave as common *source* with equal voltage gain/output impedance.

It is commonly mistaken to think of the upper unit as common drain and the lower one as common source, which would have unequal gains. But the input signals to both are in the same place - straight across Vgs. Neither signal is ground-referred, which is why you can drive staggeringly high voltages with this.

Back in The Day, we used to do this with a pair of PNP Ge's with a dual-secondary transformer. The transformer works well for driving FETs as well, and removes the speed/bandwidth limitations of an unassisted optoisolator. Speaking of which, does anyone know of a decent source for this type trafo these days (other than an old vintage amp)?

[Dr.EM]

I suppose you could use an amp like this to directly drive electrostatic panels without the step-up transformer?

[unclejed613]

there ya go.... i was thinking of modifying the circuit for lower voltage/more current, but somebody spotted a use for it as-is...... cool