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Registered Member #2411
Joined: Sat Oct 03 2009, 05:50PM
Location: South Texas, USA
Posts: 11
Recently, I have been researching high voltage amplifiers, and i ran across this circuit Here (4th page down - "High-voltage amplifier uses simplified circuit") I cant seem to get my head around it. They "stacked" the nmos and the pmos to create "huge" mosfets? I would really like to know how this works.
Also, wouldnt q5 be always off? pmos with a + voltage at the gate (or is it depletion type?). The nmos are enhancement type. The data sheet for the pmos shows positive breakdown and threshold voltages.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The circuit may well have some mistakes. It would make a lot more sense if the supply to Q5 gate were -12V. I can also see some missing connection dots and the like. Nobody makes depletion mode MOSFETs any more.
Another thing that makes me WTF is that they say that they chose the OP-07 for its low offset, but then operated it with a gain of only 2. So the offset performance will actually be limited by the LF356, and the money spent on the OP-07 is wasted. This shows a lack of brainpower on the part of the designer that would make me doubt the rest of the circuit even more.
Stacking the MOSFETs in series is an old trick that's explained in The Art Of Electronics. One of them has its gate driven and acts as an amplifier, and the rest of them are source followers that share the voltage drop, hopefully more or less equally, as controlled by the divider chain of resistors.
Registered Member #2411
Joined: Sat Oct 03 2009, 05:50PM
Location: South Texas, USA
Posts: 11
Thanks for your reply,
I thought something was up with this circuit. Isnt a connection dot added near the -1000v supply at the 4M resistor at the gate of Q10? (any more?) Say the connection dots were there and Q5's source was -12 instead of +12, is this circuit still feasible? I say this because im trying to build the same thing (a high voltage amplifier) only i just need ~450-500v pk-pk from a small controllable sine wave (<200hz).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you're not so confident with electronics and want to build a high voltage amp for some hobby experiment, I'd seriously recommend that you use vacuum tubes. They're still widely available, and very easy and forgiving to work with.
The MOSFET HV amp circuits, on the other hand, can be a bit fiddly. I used to work for a company that had one in its product line for a piezo driver. They had to discontinue it because more often than not they would explode on the test bench, and nobody ever figured out why.
There is another MOSFET HV amp circuit that I'd trust more in The Art Of Electronics.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I second that suggestion on the tube amps!
I finally have a viable ~100V amp from 30K to 1MHz, but had I put my money into the tube side of things I would have been better off. Solid state takes a hell of a lot of effort! and its only 25W.
To get big power I have to sum a bunch of amps together with ferrite transformers, balanced and matched as power combiners.
Just go for tubes, you'll be happer and the results will come much faster, that is unless you like spending a year researching amps.
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High Voltage Amplifier Circuit
