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noise problem in voltage booster circuit

Discussion in 'Electronic Design' started by jsmith, Apr 8, 2007.

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  1. jsmith

    jsmith Guest

    hi all

    my voltage booster circuit {http://john111smith.googlepages.com/
    vbstr.png} show the following noise levels:
    A: <+-50mv
    B:~ +-200mv at 67v output
    D: <+-10mv

    How can I reduce noise level at point B to about +-50mv?
     
  2. jsmith

    jsmith Guest

    excuse me noise level at point D is about Zero;
     
  3. Jamie

    Jamie Guest

    Well, you're going to get noise there because transistors in
    general make noise at certain regions. Thermoshot noise comes
    to mind also!, you're using a 741 which isn't the cleanest Op-amp
    there is.
    use a small capacitor to ground on the Base at C and Collect at
    B,. also, a small choke in series with the collector then cap to
    common helps.
    some where in the hood of .1uf should do it.
     
  4. Guest

    jsmith,

    First of all, what is "vin"? DC? AC? Signal? What frequencies?

    Also, what type is Q2? And what value/type is L3? And C54 and C55?

    Some possible suggestions:

    This appears to be an oscillator. I didn't have a 741 model to
    simulate it with. But, with 2V DC for vin, and an OP275 (and 500K for
    the 1M pot), it oscillated at about 180 kHz, after about 70 ms. And
    with an LT1007, it oscillated at about 55 kHz, after about 49 ms.

    And if the bottom of the 1M pot is connected to the opamp's negative
    input, instead of to the positive input, it appears to just add 75 to
    90V DC to the input signal (depending on the 1M pot's value), while
    attenuating an AC vin input signal by about 80%.

    But, _assuming_ that you just drew the circuit's schematic
    incorrectly, for this post only:

    Can you use a better opamp?

    Can you lower the resistances' values?

    Depending on what vin is and what it's for, you could try a capacitor
    from point B to GND.

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
  5. jsmith

    jsmith Guest

    Thanks Jamie
    I before replaced 741 with OP07 but the result is the same.
    Also before there was a RC(1K,2n2) between collector and base that
    force circuit to oscilate, other values for C (20p to 100n) didnt
    produce good results.
    I will try with choke.
     
  6. jsmith

    jsmith Guest

    Thanks Tom

    Vin is DC
    Q2 is KSP42
    L3 680 uH plastic axial
    C54,55 are 1uF multilyer
    U10 is OP07

    because of low output impedance of +100V supply source, I have to use
    this resistances values, but if you think change of resistors, produce
    considerable change in noise, I will replace +100V supply.
    I will try with change configuration of inputs of opamp and polarity
    of DC Vin.
    The cap between B and GND make circuit oscillate with this
    configuration.
     
  7. You might try adding a 1 nF cap (1000pF) between the opamp -
    input and its output, to roll the response off at about 1
    kHz. This eliminates most of the frequency response that
    might contribute to oscillation.
     
  8. jsmith

    jsmith Guest

    Because of high voltage at B point I cant control PNP transistor with
    opamps with +-15V power supply, so the bottom of the 1M pot is
    connected to the opamp's posetive input istead of negative input with
    PNP transistor control
     
  9. Jamie

    Jamie Guest

    I ment between the Collector to Grd and another from base to grd.
     
  10. jsmith

    jsmith Guest

    Thanks John
    I will try it.
     
  11. jsmith

    jsmith Guest

    Dear Jamie
    if you ment, use both of them together, i will try it
     
  12. I took a better look at your circuit, and it would be a lot
    better if you could use a lower impedance drive to the
    transistor than 330k and lower the voltage gain of the
    transistor stage with emitter degeneration.

    For instance, if you added a 15k emitter resistor to the -15
    volt supply, eliminate the base diode and 330k base resistor
    and replace them with a 12k resistor opamp output to base
    and 10k from base to -15 V supply, the stability and noise
    of the high voltage stage improves dramatically. This
    biasing allows the output to follow an input all the way
    down to zero volts. And you don't need any of that output
    filter stuff.

    I would still use a high frequency roll off capacitor opamp
    output to - input, but it could be reduced to something like
    22 to 47 pF to extend the closed loop response out to
    almost 10 kHz.
     
  13. jsmith

    jsmith Guest

    Thanks John
    I will try it.
     
  14. What is the ripple voltage, now at the 100V supply, and what
    filter capacitance does it now have across it?
     
  15. jsmith

    jsmith Guest

    Thanks
    only 1nf feedback cap was good and decreased noise to 50mv.
    FFT of noise shows 100Hz peak noise harmonies. how can i calculate
    L(or LC) filter for decrease this noise (from +100v power supply)?
     
  16. jsmith

    jsmith Guest

    It is only the above +-50mv noise.
    parallel 220uF, 100nF, 10nF after bridge diode + LRC [680uH,
    20K,parallel 100nF,10nF,1nF] before load
     
  17. In that case, you need more loop gain to reduce the ripple
    with feedback. But your original circuit is not stable
    unless you reduce the loop gain with the capacitor I
    mentioned. However, the later version I recommended does
    have about 10 times more loop gain at 100 Hz and will reduce
    the ripple at the output about 10 times better.
    Here is a repost of what I recommend, with one improvement,
    a capacitor in parallel with the base drive resistor, to
    improve the phase margin:

    If you added a 15k emitter resistor to the -15 volt supply,
    eliminate the base diode and 330k base resistor and replace
    them with a 12k resistor opamp output to base, paralleled
    with 100 pf and 10k from base to -15 V supply, the stability
    and noise of the high voltage stage improves dramatically.

    I would still use a high frequency roll off capacitor opamp
    output to - input, but it could be reduced to something like
    47 pF to extend the closed loop response out to almost 10
    kHz.
     
  18. MooseFET

    MooseFET Guest

    You can also place a resistor in series with that capacitor to add a
    zero to the system. This lets you push the gain crossover higher than
    you otherwise can because it takes out some of the phase shift at
    higher frequencies. Normally, you would place the zero almost at the
    gain crossover frequency.
     
  19. Guest

    I would have to have the transistor type to figure out if that
    resistor would be helpful for not. As it is, it is cancelling the
    base capacitance, approximately, including the Miller capacitance.
     
  20. MooseFET

    MooseFET Guest


    Since I didn't specify the value, I maintain that it can be optimized
    for any transistor that may be there. If you had some mythical
    transistor with no capacitance, a quite large value would be used.
    For any real transistor, you can move the zero to line it up with a
    pole. I can't think of a case where zero would be the best value.
     
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