I would guess it should be high enough to not load driver but low enough to turn on mosfet quickly enough.
I would also guess the gate capacitance has something to do with it but mosfet gate capacitance has different values for Cgs and Cgd.
I have a mosfet power amp that uses 390R gate resistors with IRFP240/9240 and it seems to be OK up to about 20Khz then it tails off quickly.
Is there any danger in lowering gate resistors to get better bandwidth ?
Clearly the Vas capacitor has an effect on the bandwidth too but I have got this down to about 100pf and now the effect of changing it is minimal.
I would also guess the gate capacitance has something to do with it but mosfet gate capacitance has different values for Cgs and Cgd.
I have a mosfet power amp that uses 390R gate resistors with IRFP240/9240 and it seems to be OK up to about 20Khz then it tails off quickly.
Is there any danger in lowering gate resistors to get better bandwidth ?
Clearly the Vas capacitor has an effect on the bandwidth too but I have got this down to about 100pf and now the effect of changing it is minimal.
Hi nigel
I don't quite understand. Are you saying that when you do a frequency sweep with a 8 ohm load, the amplitude drops rapidly after 20kHz?
If you can post a schematic, it'll be easier to identify the problem.
Regards
Mike
Yes a sweep tails off quickly after 20KHz.
An externally hosted image should be here but it was not working when we last tested it.
It's hard to see but it seems your R13 is 47K.
I suggest you change R13 to 1K. The capacitor after that can be anywhere from 100pF to 470pF.
Is this a simulation or actual full power bench testing?
I suggest you change R13 to 1K. The capacitor after that can be anywhere from 100pF to 470pF.
Is this a simulation or actual full power bench testing?
The 390R gate resistor doesn't cut at 20khz. If we approximate the Miller capacitance to 100pf( high) then the bw will be at least 4mhz.
Actually any value between 220 and 470 will do.
If your amp cuts at 20khz, then it has other causes. I can't see the value of the VAS miller cap.
Actually any value between 220 and 470 will do.
If your amp cuts at 20khz, then it has other causes. I can't see the value of the VAS miller cap.
I forgot about the RF filter on the front end.
That 1k makes a big difference.
It has much better top end now.
Glad to hear it worked out.
You may want to add a small cap across the feedback resistor.
Also a zobel on the output.
Input RFI filters are always good to have for a quiet amp. I typically use a 2k2 between input and gate/base of input stage and 220pF to GND. Like the ALPHA 20:
The amp was designed as a minimal circuit.
Its called "cheapest amp".
It sounds pretty good now and between tracks is dead silent.
It has brought out the full range sound of my pair of Fane 12-250TC's.
I have hammered it with square waves to see if it oscillates under duress but it seems to be fine.
That's great.
Q4 looks like a MPSA 42. It must be connected to the heatsink to prevent thermal runaway.
You can change that to a MJE340 if you want. It'll be easier to mount it on the heatsink.
Q4 looks like a MPSA 42. It must be connected to the heatsink to prevent thermal runaway.
You can change that to a MJE340 if you want. It'll be easier to mount it on the heatsink.
I usually put the mpsa42 Vbe multiplier next to the heat sink so it can be glued to it. Not sure why I didn't with this one.
Will change it on next revision.
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