Amplifier trouble! (non-inverting, voltage buffer)

Discussion in 'General Electronics Chat' started by Rogare, Jul 6, 2012.

  1. Rogare

    Thread Starter Member

    Mar 9, 2012
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    Hello! I've got a simple audio amp design (attached) which gives me the 20 dB boost I'm looking for... until I plug in my 50Ω load. Then the gain drops dramatically. I'm not sure if this is due to a) the load changing the feedback resistance, b) my power supply not being able to give the current I need, or c) something else. Any advice on how to fix this problem would be terrific. Thanks for reading.

    Specifics:
    - My input signal is a pulse that is high for ~100 μs and then low for ~100 ms (~0.1% duty cycle). The high level could be anywhere from 10s of mV up to a couple V; the low level is zero. The signal will never be negative.
    - The circuit gives 20 dB gain up to an output level of 20 V.
    - The load is 50 Ω (headphone). Therefore, the load could get up to 400 mA and 8 W—though for a very short amount of time.
    - The power supply is 24/0 V (two 9Vs in series, a 12 V regulator (7812), and a 7660 chip)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    If you are going to add a 50Ω load to the circuit, the output impedance of the circuit must be much less than 50Ω.

    Alternatively, design the circuit with the 50Ω load already in place.
     
  3. Rogare

    Thread Starter Member

    Mar 9, 2012
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    Thanks for the reply, MrChips.

    Based on @crutschow's initial suggestion here, I'm going to add on a complimentary emitter follower (PNP) and move my feedback resistor from the emitter of the NPN back to the op-amp output (see attached image). This should fix the output impedance problem you pointed out (which I'm guessing was about 400 kΩ?).

    I think my power supply may also be too weak for this application (7812 gives max output of 1A, and I'm not sure about the 7660), so I'm going to just get a AC adapter and see if that helps.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    An ICL7660 can boost a 9V battery to 15V with a current of only 7.5mA.
    I don't know if your "9Vs" are little 9V batteries that normally have a load of 10mA to 50mA.
    I don't know why you have a 12V regulator since your supply is 24V.
     
  5. Rogare

    Thread Starter Member

    Mar 9, 2012
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    The original setup was <two 9Vs in series> to <12 V regulator> to <7660>, which gave 24 V—but because of both the regulator and the 7660 (thanks for the number for this one) I couldn't get enough current.

    I've since built the circuit from the OP* and the same thing is happening: everything is great until the load is added. After a bit of investigation, I found that the output starts to die off as soon the output load drops below about 350 Ω. This test had a 1 V pulse at the output, so we're only dealing with a few mA and mW... not sure what the problem is here!


    *Now with a 12 V supply (AC adapter) and 1 MΩ resistors in the buffer stage in place of the 820 kΩ resistors.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    An ICL7660 might blow up with a 12V supply because its absolute maximum supply is 10.5V and a max of 10V is recommended.
    An ICL7660A has an absolute max supply of 13.5V and a max of 12V is recommended. A 7812 12V regulator might have an output of 12.5V.

    The voltage converter IC can double the voltage but only when there is no load current. It has an output resistance so load current (only a few mA) causes its output voltage to drop.

    Maybe adding a filter capacitor to the 24V will hold the voltage up during each short duration load current pulse.
     
  7. Rogare

    Thread Starter Member

    Mar 9, 2012
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    Thanks for the reply, Audioguru. The 7660 (and 9Vs and regulator) is out of the circuit altogether—it's now just a 12/0 V source (the AC adapter).

    The problem I'm having is that, despite this change, my output voltage drops substantially when I plug in a 50 Ω load. The circuit schematic is in the OP. Do you see any obvious problems that could be causing this?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The buffer circuit does not work because a 820k or 1M resistor cannot provide enough base bias current for the PN2222 transistor.

    The original circuit should work with a strong power supply.
     
    Rogare likes this.
  9. Rogare

    Thread Starter Member

    Mar 9, 2012
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    OK, just gave it a try and it does—wonderful, thank you. There's ~300 mV DC at the output when there's no input, but I'll put a ~100 uF capacitor in series with the headphones which should help (as in this schematic).

    One odd thing was that when I unplugged my input, the output shot up to a near-supply DC. That would likely damage the headphones. Would my 100 uF capacitor fix this problem?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    1) The circuit I posted has 0.0V on the output when the input is 0.0V, it does not have 300mV DC.
    2) You cannot put a capacitor in series with the output because the circuit has nothing to pull the output down like the audio amplifier has.
    3) Like I showed, the circuit does not has an input bias voltage like all audio amplifiers have so if its input is disconnected then it floats high then it amplifies the high which makes the output go high.
     
  11. Engr

    Member

    Mar 17, 2010
    114
    5
    you can use an opamp as your buffer stage and if you are concern with the output current of the opamp I attached an image of a circuit that might help with your concern.
     
  12. Rogare

    Thread Starter Member

    Mar 9, 2012
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    @Audioguru, here's where that 300 mV came from:

    After noticing a -150 mV baseline on my input signal (computer audio), I put a 1 uF capacitor between the input signal and the 24 Ω resistor. That brought my input baseline up to ~ 0 mV, which was what I'd wanted, but then the output baseline rose to 300 mV. (As a reminder, my input is a short, positive square pulse every couple ms.)

    The problem with the -150 mV baseline at the input was that only the positive portion was amplified. E.g., if I had a 500 mV ptp pulse at the input, only 350 mV was getting amplified... which meant 3.5 V at the output, and not the expected 5 V.

    Is there any simple way I'm missing to get baselines of ~0 mV at the input and the output? Thanks for your help on this, much appreciated!
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Since you added an input coupling capacitor then the input pin was floating. It floats to a positive voltage.

    The opamp input pin needs a resistor to ground so that it has a reference of 0VDC.
     
  14. Rogare

    Thread Starter Member

    Mar 9, 2012
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    First off, this really should be a new thread; however, my attempts to start one for the past week have been met with some kind of website error: a blank white screen whenever I try. No word back yet from support, so I've posted this as a reply to a previous post for now.


    Hello,

    I've got a basic volume control circuit that works wonderfully—with the exception of noise. Specifically, when my hand gets close to the enclosure (plastic), it really starts to buzz.

    The approximate schematic is attached—as I said, super simple. If anyone has any ideas of how to make this more noise proof, that would be great!

    Thanks for reading.
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    Audioguru will tell you a thing or two about that lousy LM358 opamp.
    Your impedances are too high. Try reducing the pot and load resistor to 1kΩ - 10kΩ.
     
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