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How does this circuit work? Is it a pre-amp?

Discussion in 'Electronic Basics' started by MRW, Jul 3, 2007.

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  1. MRW

    MRW Guest


    Happy 4th of July to my American colleagues!

    I have this circuit:

    How does it work?

    I initially thought that the first opamp is a microphone pre-amplifier
    in an inverting configuration and the second one is used to invert the
    signal back. But when I looked at it again, the second opamp is
    configured as a voltage follower. Why is that?

    I think that C6 and C7 are just used as a bypass capacitor for any
    noise coming from VCC. R20 and C5 are just high pass filters that will
    roll off any signal below 8Hz. R15 and R17 are used to biased the
    input at half of VCC for single supply operation.

    Is that the right assumption?

    I also have a feeling that if the first opamp is indeed a pre-amp,
    then it's gain is frequency dependent. But how do I find out the
    equation for it? Is it just (-R13)/(reactance of C5) ?

    Any other insights into this circuit?

  2. Andrew Holme

    Andrew Holme Guest

    U6A is a current input amplifier, so the output voltage is -I_in * R13.

    C5 is a DC-blocker not a filter; but it needs to be much larger than C7 to
    ensure most of the signal current flows into the amplifier summing node.

    The unnecessary voltage follower U7A has a capacitor from its output to
    ground, which is a bit odd.

    C6 and C7 are decoupling capacitors, as you spotted.
  3. MRW

    MRW Guest

    Thanks, Andrew! How would I be able to tell in the future if an opamp
    is a current amplifier or not?
    On this site, , I don't spot
    the indication right away.

    So the value of C5 is not determined by any frequency at all?

    Actually, this is the rest of the circuit:
    (highlighted in the green box).
    I'm still not sure what U7A does.. I figured it's independent from the
    other part (highlighted in green).

  4. Eeyore

    Eeyore Guest

    It's rubbish. Probably 'engineered' by a monkey on an off day.

  5. MRW

    MRW Guest

    By the way, is a current input amplifier synonymous to a current
    feedback amplifier? If so, Opamps for Everyone has the following
    statement: "In general, VFAs (voltage feedback amplifiers) are used
    for precision and general purpose applications, while CFAs (current
    feedback amplifiers) are restricted to high frequency applications
    above 100 MHz."
  6. Eeyore

    Eeyore Guest

    It's shockingly incompetent..

    Whoever put this lot together need to be shot.

  7. Eeyore

    Eeyore Guest

    Current feedback is something else entirely.

  8. MRW

    MRW Guest

    Hi Graham,
    I'm still learning about circuit configurations, so I really could not
  9. MRW

    MRW Guest

    Believe it or not, it was another classmate's instructor.. I just
    copied the drawing on the computer because it was printed on paper. It
    is supposed to be an FM transmitter circuit, and he asked me for help
    in figuring it I came here to get some answers.

    The paper also says: "L2 is 112nH (this tunes to the middle of the FM
    band, 98 MHz), with C13 at its center value 33pF. L2 is 5 turns of 22
    swg enameled copper wire close-wound on a 5mm diameter former. You can
    also have a fixed 33pF capacitor instead of the variable C13 and have
    L2 as an adjustable molded coil."

    .... btw, what's a "former"?
  10. MRW

    MRW Guest

    Thanks, Graham! So how do I determine if the LM158 or any other ICs is
    a current amplifier?
  11. Andrew Holme

    Andrew Holme Guest

    The LM158 is not intrinsically a current amp. The circuit configuration in
    which it is used is a current amp. Current flowing into the summing node
    has nowhere else to go but through R13.

    It's fairly non-critical. It needs to be large enough to NOT be a high pass
    filter at audio frequencies.

    The circuit is an FM transmitter. U7A is an isolating amplifier. The 1nF
    capacitors are there to prevent RF feedback causing instability.
  12. MRW

    MRW Guest

    Thanks, Andrew! I understood most of what you said except for
    (regarding C5):
    If C5 does not become a high pass at audio frequencies, then how is
    the audio source getting thru past the opamp stages? I had the
    impression that C5 would short as the frequency gets higher. Isn't
    that what a DC block does?
  13. Andrew Holme

    Andrew Holme Guest

    I should have said the configuration is a current INPUT amp, or
    trans-resistance amp i.e. current input, voltage output.

    It works like this:

    The voltage at U6A pin 2 is held at VCC/2 by negative feedback. It never
    changes. The capacitance microphone produces a small AC speech current
    which flows through C5. Most of this current flows on through R13. Only a
    vanishingly small fraction of the current enters pin 2.

    The output voltage is therefore VCC/2 + I*R13
  14. Eeyore

    Eeyore Guest

    Nothing surprises me any more.

    Uh ? It is ?

    A former is something on which you can form a coil e.g.a piece of plastic

  15. Eeyore

    Eeyore Guest

    You're confusing 'current amplifier' with 'current feedback amplifier'. Not the
    same thing at all.

  16. Eeyore

    Eeyore Guest

    What kind of modulation ?

  17. Andrew Holme

    Andrew Holme Guest

    Correct. I meant that C5 needs to be large so it doesn't attenuate the
    lower audio frequencies. Strictly speaking, it is a high pass filter; but
    to call it one suggests that only high frequencies pass through; when in
    fact the cutoff frequency is so low that all audio frequencies can pass.
    Only sub-audio and DC are blocked.
  18. Bob Masta

    Bob Masta Guest

    I'm totally mystified by this circuit. If it really is an FM
    transmiter, then where is the carrier tuning? I have seen
    FM transmitters that used a condenser (capacitor) mic
    as the modulator for the tuning circuit, but that doesn't seem to
    be the case here. And if it is a transmitter, wouldn't C4 be
    trying to filter out the whole signal? (Assuming that's supposed
    to be the output, since there is no antenna connection marked,)

    BTW, if that really *is* supposed to be a condenser mic, good
    luck finding one at a reasonable price: These are typically only
    used as precision reference mics, at US$ 1000 and up. The
    cheap little electret mics that we see everywhere use the same
    underlying technology (sort of), but they have a permanently
    charged backing plate (the electret) and, unfortunately for
    tuning uses, an FET preamp that isolates the capacitance of
    the mic from the output and turns it into a voltage change.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  19. leo123

    leo123 Guest

  20. Andrew Holme

    Andrew Holme Guest

    Those bayimg URLs all look alike:
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