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OP-Amp Configuration

Discussion in 'Electronic Repair' started by Michael Black, May 31, 2006.

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  1. What purpose does any capacitor have? To pass AC and not let DC through.
    SO when it's used as a coupling capacitor, it lets the AC signal pass
    between stages, but blocks the DC from the first stage from getting into
    the second stage where it would upset bias. When used as a bypass capacitor,
    it in effect shorts the AC signals to ground, without shorting out
    the DC voltage.

    Of course, a capacitor's ability to pass AC varies with frequency and
    capacitance. A small value capacitor won't do well at passing low
    frequencies, while a high value capacitor will.

    Now given that information, that should have been covered in the books,
    why do you think the capacitor is there? Think about why it might
    be different from having the resistor going directly to ground.

    Michael
     
  2. Rich Webb

    Rich Webb Guest

    Just a swag based on your description but the impedance of the series
    cap will fall as frequency rises, so less negative feedback for higher
    frequencies and correspondingly more gain at higher frequencies.
     
  3. M.Joshi

    M.Joshi Guest

    Hi,

    I was recently in a job interview and was asked to describe an OP-Am
    circuit.

    The OP-Amp configuration consisted of negative feedback with a resisto
    and capacitor connected in series to ground from the inverting input.

    I haven't seen this configuration before and I was asked the purpose o
    the capacitor
     
  4. Pooh Bear

    Pooh Bear Guest

    No DC gain.

    Graham
     
  5. M.Joshi

    M.Joshi Guest

    Just to clarify a few points:

    I have come across a capacitor on the input which is to block DC; an
    also in the feedback loop which gives an Integrator function I believ
    however, in this circuit, there was a resistor in the feedback loop an
    another resistor and capacitor in series to ground from the invertin
    input?

    Thinking about it now, could it form some sort of filter (Low pass)?
     
  6. How about drawing the circuit? :)

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  7. Franc Zabkar

    Franc Zabkar Guest

    Is this it?

    +---- Rfb -+
    | |\ |
    C | | \ |
    |---||-- Rin -+-|- \ |
    _|_ | \____|__ Vout
    = | /
    Vin ---|+ /
    | /
    |/

    Av = Vout / Vin
    = 1 + Zfb / Zin
    = 1 + Rfb / (Rin + 1/jwC)

    At DC, C=0 and Av = 1.
    At high frequencies, Av = 1 + Rfb / Rin

    At high frequencies the circuit behaves like a normal non-inverting
    amp, while at low frequencies the gain approaches unity.

    - Franc Zabkar
     
  8. Pooh Bear

    Pooh Bear Guest

    I don't think you've been listening.

    Graham
     
  9. Pooh Bear spake thus:
    Eggs-ackley: I think you missed this from the OP's posting:


    --
    Any system of knowledge that is capable of listing films in order of
    use of the word "****" is incapable of writing a good summary and analysis
    of the Philippine-American War. And vice-versa. This is an inviolable rule.

    - Matthew White, referring to Wikipedia on his WikiWatch site
    (http://users.erols.com/mwhite28/wikiwoo.htm)
     
  10. I've redrawn it here:

    R1
    +----/\/\----+
    | |
    | |\ |
    Input ---+-+---|- \ |
    / ??-|+ /
    R2 \ |/
    /
    \
    |
    _|_
    C1 ---
    |
    GND
    What about the + input? And the values of C1, R1, R2?

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  11. M.Joshi

    M.Joshi Guest

    I have attempted an ASCII drawing which can be downloaded from th
    following location:

    http://three.fsphost.com/Joshi

    Filename: Op-Amp Schematic.txt

    I hope it makes things a bit clearer - It is the purpose of th
    capacitor (C) in the diagram that I am not too sure of?
     
  12. CJT

    CJT Guest

    Isn't there a resistor on the input, too?

    Assuming there is, you ought to be able to do a typical op-amp
    analysis -- the sum of currents into the virtual ground at the
    inverting input will be zero.
     
  13. M.Joshi

    M.Joshi Guest

    I do not remember the values of all the components as I only saw th
    circuit for a brief moment in an interview but, as far as I ca
    remember, the positive input was connected to a potential divide
    consisting of two equal value resistors to +Supply and -Supply.

    The invering input was being fed from a mic pre-amp circuit which
    believe was capacitively coupled and there was another resistor i
    series with the input.

    *CJT* - Could you give me some pointers as to how I would go about a
    Op-Amp analysis as it has been some time since I have done one.

    Thanks.
     
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