BJT vs MOSFET - taking the best of both

[Circlotron]

Originally posted by x-pro
Hi all, for 10 years I was making amplifiers with N-channel only MOSFETs in the output and really like these devices.

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.

[x-pro]

Check the link in my post in the "Quasi-complementary Hexfet monstrosity" thread

Al

[Circlotron]

Originally posted by x-pro
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

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.

[x-pro]

Originally posted by Circlotron

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.

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

[gwolf]

Al,

I never found a Spice model for the BUK (555, ...) devices. Do you have them?

Cheers,

Gerhard

[x-pro]

BUK555 are obsolete anyway. Try BUK9540-100A as a replacement

x-pro

[gwolf]

Originally posted by x-pro
BUK555 are obsolete anyway. Try BUK9540-100A as a replacement

x-pro

Yes, I know. But I still have a bunch of BUK555-100B in my drawer. But even from the BUK9540 I could find no usable spice model. Any idea?

Gerhard