BJT buffer?

Discussion in 'General Electronics Chat' started by Gibson486, Apr 1, 2014.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    I was trying to drive a transformer with an op amp. The issue is that the frequency I am running at (350kHz) is at the -3 db frequency. As a result, over time, my output drops when I try to reach my max amplitude (400V) since I am already losing half the power.

    Looking for an opamp that could do this alone seems to be a hassle since high power and "high frequency" do not play together. I am guessing I need to use a BJT or mosfet at the output. Is this a step in the right direction? I attached a pic of what I was thinking about doing....
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    What is the amplitude and DC offset of the input sinewave? If it is centered about GND, then you have isgnificant DC flowing through the primary. Unless V- is something like GND, then you have no output at all.

    What is the amp, what is the transistor, what is V-... A litt help...?

    ak
     
  3. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    It's just a generic drawing. the answer to all those questions is that i do not know yet. All I know is that I need 17V Pk to pk at the transformer input to get my desired output. All i am looking for is seeing if I am headed in the right direction. I am not looking for anyone to design this for me. the op amp was the OPA564.
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    784
    NS did some fast and ultra-fast buffers, generally they're configured like op-amps, but some versions came in cases like TO3 but with more pins. Various other manufacturers have done high power video buffers in similar case styles.

    These days you might find something in a 5 or 7 pin TO220 style casing.
     
  5. #12

    Expert

    Nov 30, 2010
    16,298
    6,811
    This chip (OPA564) can supply +/- 1.5 amps and 19V p-p if you keep it cooled correctly.
    Adding a transistor to disable its ability to supply negative current puzzles me.
    The picture below is what comes to mind.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The original circuit looks like a voltage to current converter. Is it a requirement to drive the transformer with a current source rather than a voltage source?

    And again with the DC - There may be siginficant DC flowing in the transformer primary, and this could cause the low output signal you describe.

    The datasheet shows about 30 dB of open loop gain left at 400 KHz. That should be enough for the unity gain circuit in post #1. What is the 17V, 350 KHz signal? What is the load on the transformer secondary? And is there a reason you are not using a standard voltage follower circuit?

    ak
     
  7. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    I just used a standard voltage follower in the end (using opa 564). The signal is a either a sin or triangle wave. I wanted to use a BJT originally because I am not a huge fan of the OPA564 because it is sort a pain to deal with, especially during testing. Just PM me and I can personally tell you my pain with this IC.
     
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