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Op-amp peak detector with gain

Discussion in 'Electronic Basics' started by SDC, Sep 9, 2006.

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  1. SDC

    SDC Guest

    I need a single-supply positive peak detector with a gain of 2. (Previously
    I used an op-amp with a gain of 2 followed by a peak detector, but would
    like to simplify the circuit.) The input voltage will range from 0V to about
    2.5V.
    Will the following circuit work? (Please excuse my ignorance - I don't have
    much experience with op-amps.)

    .... Steve

    IC=LM324

    +-[10K]-+----[10K]----+----------+
    | | | |
    === | | |\ |
    GND | |\ +-- |-\ |
    +--|-\ 1N4148 | >---+---o Vout = Vin(pk) x 2
    | >-->|--+----|+/
    Vin o-------|+/ | |/
    |/ 0.01uF---
     
  2. Eeyore

    Eeyore Guest

    You need the diode in the feedback loop to avoid seeing a significant effect
    from the forward voltage drop.

    Graham
     
  3. SDC

    SDC Guest

    Thanks for the reply Graham.
    In my circuit, won't the first op-amp's output just rise to about 0.6V
    higher and automatically compensate for the diode to balance the circuit, as
    it would do in a standard (unity gain) peak detector as shown below:-

    IC=LM324
    +-------------+---------+
    | | |
    | | |\ |
    | |\ +--|-\ |
    +--|-\ 1N4148 | >---+---o Vout = Vin(pk) x 2
    | >-->|--+---|+/
    Vin o-------|+/ | |/
    |/ 0.01uF---
     
  4. SDC

    SDC Guest

    A typo in my last post with the unity gain peak detector. Vout=Vin(pk), NOT
    Vout = Vin(pk) x 2

    .... Steve
     
  5. Eeyore

    Eeyore Guest

    Yes, silly me ! I haven't actually seen that configuration previously so I
    missed that point.

    You'll need to consider drift due to the input bias current of the 2nd op-amp
    btw.

    Please, please don't use am LM324 either. What are your specific requirements
    for bandwidth and precision ?

    Graham
     
  6. SDC

    SDC Guest

    This circuit will only be used for test purposes, and precision isn't too
    important. I'm developing a 2-quadrant 12V, <= 40A regenerative brush-motor
    controller and will eventually measure both driving and braking currents for
    current limiting. I'm using a PIC16F876 to run the show. Although I'm
    running the PWM at 20kHz, the motor's inductance smoothes this to a more
    constant voltage, so bandwidth isn't much of a problem either. The sensing
    element is a 10 milliohm, 3W (Ohmite) current-sensing resistor and the
    amplifier's front end is a precision op-amp rectifier with a (maximum) gain
    of 5, giving 50mV/A. I want a total gain of 10, so that the output is
    100mV/A, hence the extra gain in the peak detector stage.
    I need the input to be able to go right to 0V and wanted to keep the parts
    count and cost as low as possible. That's why I'm using an LM324. (Also,
    it's the only quad op-amp that I have on hand.)
    Why is this a bad choice?

    .... Steve
     
  7. Eeyore

    Eeyore Guest

    I'd have thought that the LM324's inpur bias current will cause quite a high
    rate of droop ( or rise ) in the peak detector voltage on the cap. Yes, it's pnp
    input. So, in the absence of an input, your output will float toward the supply
    rail.

    Are they really OK with inputs as low as ground on a single supply btw ?

    Graham
     
  8. SDC

    SDC Guest

    Thanks for the continued interest Graham. The LM324 datasheet claims
    that the common-mode input range extends to the negative supply rail, as
    does the output if it is not heavily loaded. It also claims fairly low input
    bias currents, about 100nA for the '324A. This is why I'm using the '324,
    (besides the prev mentioned fact that it's the only quad that I have on
    hand). However, I don't want the output to rise in the absence of inputs.
    This could potentially be a problem. I was probably going to add a
    high-value, (1M to 10M, yet to be determined), bleed resistor across the
    cap. Do you think the voltage across the capacitor would still rise due to
    the input bias current, even if I used a 10M resistor, allowing for the
    leakage of the cap?

    .... Steve
     
  9. SDC

    SDC Guest

    It appears that mt ISP is having problems. 10 minutes and my reply hasn't
    appeared, so here it is again:-

    Thanks for the continued interest Graham. The LM324 datasheet claims
    that the common-mode input range extends to the negative supply rail, as
    does the output if it is not heavily loaded. It also claims fairly low input
    bias currents, about 100nA for the '324A. This is why I'm using the '324,
    (besides the prev mentioned fact that it's the only quad that I have on
    hand). However, I don't want the output to rise in the absence of inputs.
    This could potentially be a problem. I was probably going to add a
    high-value, (1M to 10M, yet to be determined), bleed resistor across the
    cap. Do you think the voltage across the capacitor would still rise due to
    the input bias current, even if I used a 10M resistor, allowing for the
    leakage of the cap?

    .... Steve
     
  10. Eeyore

    Eeyore Guest

    The 10M will limit the effect to 100.10^-9 x 10.10^ 6 = 1000.10^-3 or 1V !

    You need a fet input op-amp.

    Graham
     
  11. SDC

    SDC Guest

    When you put it like that:- (10^-9 x 10.10^ 6 = 1000.10^-3 or 1V !)
    Incidentally, I hadn't realised that the input stage of the '324 was PNP.
    I don't have room for 4 single FET input op-amps. I could use a pair of
    CA3230's, if I can find them. Off the top of your head, do you know of a
    quad FET input op-amp with a common mode input range and output that extends
    to 0V, (in a 5V single-supply circuit)?
    I'll also do a quick Google
    .... Steve
     
  12. Eeyore

    Eeyore Guest

    You specifically need 4 ?

    Unless DC precision is an a big issue, I'd look at TI's offerings. I'm sorry but
    I'm not that familiar with the single supply types.

    Graham
     
  13. SDC

    SDC Guest

    Yes, I do need four - two for the precision rectifier and two for the peak
    detector. In my prototype, I used separate CA3130E's. I just checked out a
    few of the LFxxx series and the LT1058, but their input/output ranges aren't
    suitable. Thanks for the tip and for all of your help. I'm grateful. I'll
    check out the TI site in the morning. I'll also check back here in case
    someone has made any suggestions. It's getting late here, (NSW Australia),
    so I'd better hit the sack.

    .... Steve
     
  14. SDC

    SDC Guest

    We've already covered this point in the earlier posts. The diode will not
    introduce any error.
    .... Steve
     
  15. SDC

    SDC Guest

    I should have added that a fast, low-leakage would make a difference,
    though.
    .... Steve
     
  16. SDC

    SDC Guest

    I left out the "diode". I'm getting tired.
    .... Steve
     
  17. John  Larkin

    John Larkin Guest

    You'll eventually need to discharge that cap, too. Some opamps, like
    the 324, have positive input bias current so will charge the cap until
    it hits the positive rail.

    John
     
  18. SDC

    SDC Guest

    Yep, that's the biggest drawback. (We also covered this earlier). I've spent
    the last hour or two searching for a suitable rail-to-rail input and output
    FET-input quad op-amp. The AD8544 is looking good so far, with the AD844
    running a close second.

    .... Steve
     
  19. Fred Bartoli

    Fred Bartoli Guest

    SDC a écrit :
    No need for a 'fancy' opamp.
    You can easily make it like that:


    +-[10K]-+----[10K]----------------+----------+
    | | | |
    === | + | |\ |
    GND | |\ | `-- |-\ |
    +--|-\ 1N4148 |/ | >---+---o Vout = Vin(pk) x 2
    | >-->|--+---+---|\ .---|+/
    Vin o-------|+/ | | v | |/
    |/ | | +---'
    0.01uF--- RL |
    --- | 1M
    | | |
    === === ===
    GND GND GND



    Or better if you have enough supply:


    +-[10K]-+----[10K]-----------------+----------+
    | | | |
    === | 2N7000 + | |\ |
    GND | |\ | `---|-\ |
    +--|-\ 1N4148 |- | >---+---o Vout = Vin(pk) x 2
    | >-->|--+---+---||<. .---|+/
    Vin o-------|+/ | | |-| | |/
    |/ | | +--'
    0.01uF--- RL |
    --- | 10K
    | | |
    === === ===
    GND GND GND
     
  20. SDC

    SDC Guest

    "Fred Bartoli"
    Hi Fred, I only have a 5V supply for this circuit, not high enough for a
    MOSFET, so the first of your suggestions is more suitable. It looks good to
    me. I don't know why I didn't think of it. It's almost 2am here, so I'd
    better get to bed and continue tomorrow. (I said this a couple of hours ago)
    Thank you for your help.
    .... Steve
     
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