Electronics > Beginners
Depletion mode mosfets in push-pull circuit?
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TheBaconWizard:
Is there any reason why, with appropriate biasing, one could not use an enhancement-mode mosfet paired with a depletion-mode mosfet for a push-pull circuit, working with a DC sine-wave (rather than true AC) on the gates?
T3sl4co1l:
"DC sine" is self contradictory.

"DC" is the part that isn't moving. AC is the part that is moving.  These two components are independent.  It's not meaningful to make a distinction between "DC", "pulsed DC", "AC" and so on, without supplying further meaning (i.e., "pulsed DC" is usually meant to mean a DC source turning on and off, i.e. the source resistance varying widely, which has different effects on different kinds of loads, hence it's not to be used as a general term).

If you mean simply biasing the gates wherever as needed, and you mean using the same polarity (e.g. both N channel), then you must:
- Supply complementary polarity AC, because one is a source follower (noninverting) and the other is common source (inverting)
- Either supply equal amplitudes to the gates, with isolated drive to the high side gate (effectively turning it into a common source configuration), or setting the drive amplitudes (with respect to ground) inversely proportional to the average gains of each transistor (because gain(source follower) ~= 1, while gain(common source) might be ~20.

Solutions to these problems are ancient, because vacuum tubes are only depletion mode N channel.  Consider these:
- White CF
- SEPP (single ended push-pull)
- Mu follower
- Futterman OTL
(and others I've probably forgotten)

Tim
TheBaconWizard:
Having briefly researched the things you suggest, it seems at first glance like a Single-ended push-pull kind of idea is going to be ideal, thanks :)
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