Help with HV common emitter and push pull stage amplifier

Hello all,

I'm designing a high voltage amplifier and running into problems that
I'm sure that one of you might be able to guide me through.

Presently I have an operational amplifier (OPA37) driving a
common-emitter amplifier stage (NTE287) feeding into a push-pull stage
(NTE287 and NTE288) and then feeding back to the opamp.

Link to circuit diagram:
http://www1.pacific.edu/~nnjegova/Circuit.TIF

(disregard the split collector resistor it was there to allow for a
terminal that I could bootstrap to the input which by the way did not
help with gain)

I'll need the following specifications:


input: +/- 15V (from another opamp)
freq = (100Hz to 10kHz) possible > most signals will be near 1kHz

The output compliance must be at least +/- 180V (ideally +/- 200V take
a diode and current limiting resistor drop), and will be driving no
more than 1mA (typically 20uA) through 1M (that's right, MegaOhm) of
impedance.

My supply rails can reach +/-(200V to 500V) and source 3mA of current
out of each rail at maximum load

Right now I can input +/- 10 V (the output clips at anything above 10V)
at 1kHz and get an approx +/- 100V output. However I have thermal
instability with that gain (15 min and the output is distorted) and a
problem with attenuation at the other frequencies. I've tried a bypass
capacitor at the emitter in order to avoid thermal instability but when
I connect it I find that my output gets extremely distorted.

I've been trying different resistor values, especially with a higher
collector and lower emitter but I can't seem to get anymore gain out of
this circuit, just DC offset. I would like to just use bjts but I have
a couple of high voltage mosfets if needed. Also, I've been searching
this group and found the great diagrams that Dr. Hill posted for the
"basic high voltage mosfet dc amplifier" (by the way thanks for
authoring that great book) and I might go down that road if I cant get
my design working, but I was hoping that I could get an idea of why my
design won't give me the gain I was hoping for, as well as why it is
thermally unstable. I have the second edition of AoE if anyone wants to
reference it in their reply.

Thanks in advance and thanks for the discussions that you have all had
in the past about this topic.

Nik

.

Relevant Pages

  • Re: common emitter configuration- voltage divider biasing.
    ... then the emitter is essentially grounded above that frequency point. ... What it means is that the gain is then ... > In other words, you design the DC operating point by ignoring Rac and Cac, ... > gain of the amplifier at those still lower frequencies declines until the ...
    (sci.electronics.basics)
  • Re: Help with HV common emitter and push pull stage amplifier
    ... I'm designing a high voltage amplifier and running into problems that ... a diode and current limiting resistor drop), ... However I have thermal instability with that gain and a problem with attenuation at the other frequencies. ... a couple of high voltage mosfets if needed. ...
    (sci.electronics.design)
  • Re: common emitter configuration- voltage divider biasing.
    ... effectively bypass the emitter R at some low frequency and for those frequencies ... What it means is that the gain is then based ... In other words, you design the DC operating point by ignoring Rac and Cac, at ... gain of the amplifier at those still lower frequencies declines until the Z is ...
    (sci.electronics.basics)
  • Re: Help with HV common emitter and push pull stage amplifier
    ... I'm designing a high voltage amplifier and running into problems that ... However I have thermal instability with that gain and a problem with attenuation at the other frequencies. ... a couple of high voltage mosfets if needed. ... my design working, but I was hoping that I could get an idea of why my ...
    (sci.electronics.design)
  • Re: Help with HV common emitter and push pull stage amplifier
    ... >I'm designing a high voltage amplifier and running into problems that ... >help with gain) ... >a couple of high voltage mosfets if needed. ... >my design working, but I was hoping that I could get an idea of why my ...
    (sci.electronics.design)
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