BJT vs MOSFET - taking the best of both

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#41
x-pro said:
Hi all, for 10 years I was making amplifiers with N-channel only MOSFETs in the output and really like these devices.
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Hi x-pro. I like them very much too. would you like to post some info on your knowledge of them, particularly the gate drive / phase spltting circuitry needed to drive the lower mosfet. I think this kind of cct is becoming a dying art.

Right - back to the thread.
 
#43
x-pro said:
The main advantage of the MOSFET ouput stage, in my view, is that it has low output impedance even without NFB, and does not change the load of the voltage amplifier as much as BJT output does with the load change. Al
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Could this be a property peculiar to logic level fets? My experience with normal n-channel type switching hexfets in linear mode is that they have a fairly "soft" and squishy output impedance with voltage changing under load from zero to full maybe 5 to 10% depending on output level. This improves greatly though when you crank up the bias to something like Class A levels because the transconductance increases with current so the output impedance drops a fair bit.
 
#44
Circlotron said:

Could this be a property peculiar to logic level fets? My experience with normal n-channel type switching hexfets in linear mode is that they have a fairly "soft" and squishy output impedance with voltage changing under load from zero to full maybe 5 to 10% depending on output level. This improves greatly though when you crank up the bias to something like Class A levels because the transconductance increases with current so the output impedance drops a fair bit.
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In my circuit I did measure less than 0.5 Om output impedance with about 100 mA idle. Logic level devices certanly have higher transconductance - it helps. However they also much easier destroyed by an overvoltage on the gate. Thankfully, in my design it never goes over 2xVth.

Al
 
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