Looking at http://www.farnell.com/datasheets/8732.pdf and http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf
both from NXP, the BAV21 has a capacitance higher than 1N4148, see Fig. 6 in both datasheets.
You need a zener on the gates to protect them from being over-voltaged when the output stage is driving a low impedance, especially with a tiered high-voltage supply on the front-end.
On Hafler amplifiers driven hard for extended periods of time, the zeners burn up (from lack of limiting in the Vas stage), and then the gates burn up (develop a low resistance from gate to source).
The clamp diodes must be very fast, 1N400x are not fast enough.
APT amplifier.
D5, 6, 8, 9 not needed with IRF type FETs.
On Hafler amplifiers driven hard for extended periods of time, the zeners burn up (from lack of limiting in the Vas stage), and then the gates burn up (develop a low resistance from gate to source).
The clamp diodes must be very fast, 1N400x are not fast enough.
APT amplifier.
D5, 6, 8, 9 not needed with IRF type FETs.
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The person you are asking this to has R.I.P. below his user name!
I think those diodes are used as floating voltage sources to make the clamps go into conduction before the transistors saturate. Without them D7 and D10 would start conducting at about the same voltage as the base-collector junctions of Q13 and Q14, so Q13 and Q14 would still go into deep saturation.
I think those diodes are used as floating voltage sources to make the clamps go into conduction before the transistors saturate. Without them D7 and D10 would start conducting at about the same voltage as the base-collector junctions of Q13 and Q14, so Q13 and Q14 would still go into deep saturation.
That is the Baker Clamp.
Baker clamp - Wikipedia
Here the transistor is Darlington so diode is doubled.
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