Re: Help with HV common emitter and push pull stage amplifier

On 29 Jun 2005 11:34:39 -0700, nikNjegovan@xxxxxxxxx wrote:

>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


I posted a circuit for a +-200 volt amp, made of one cheap opamp and
two high-voltage optocouplers. It's around here somewhere...

John

.

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