Questions about AB amps / output buffers

Discussion in 'General Electronics Chat' started by atferrari, Mar 20, 2012.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    I am about to build an output stage for a sine generator.

    I run across several circuits of AB class amplifiers but in spite of lot of reading I still have many basic doubts.

    My questions:

    1) A buffer output is expected to have gain by itself? In other words what defines the gain in a pushpull configuration?

    2) Several function generators seem to set their output impedance just by using a series resistor of 50 / 75 / 600 ohms at the output. Is that all is needed?

    3) Instead of building an AB stage, the LH0033 (or similar buffer), would be a better (if expensive) solution?

    4) When reading about AB amps it seems that those specifically for audio and those used as an output stage of signal generators share different necessities. Besides the obvious low impedance loads in audio, is anything that make them so different?

    5) Walter Jung in his IC Op-amp Cookbook says that the attached circuit has an output impedance of 10 Ohms. What determines that value? How could I change it, if possible?

    6) Read somewhere in a forum that diodes as shown above are old use; that a Vbe multiplier should be used instead. Can anyone elaborate?

    Gracias for any help to understand this a little more.
     
    Last edited: Mar 20, 2012
  2. studiot

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    Nov 9, 2007
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    1) Depends upon final max output required. To get more than a volt or two out of an oscillator at low distortion is a tall order. So a gain of up to 10x or 20 is good to obtain say up to 10 volts output.

    2) That's El cheapo way. A good generator will have a switched attenuator at the appropriate impedence. This will be a series of pi or T pads. This way the generator presents constant source impedence at all levels - vital for good measurements.

    3) You would still have the attenuator issue, but yes there are op amp solutions as well, though many will require an output pair.

    4) Sig generators access lower frequencies than standard audio amps which have low frequency roll off. Otherwise they are very similar so just remove this.

    5) Reduce R7 and R8.

    6) There would be no gain in a more sophisticated bias circuit for this amp. Note the diodes need to be in thermal contact with the output transistors to provide thermal compensation for the bias.
     
  3. bountyhunter

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    Sep 7, 2009
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    What frequency range does it have to have?

    This is a pretty good one I designed for headphone amplifier:
     
  4. atferrari

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    studiot,

    Thanks for your reply. Food for tought.


    5 Hz to 16 KHz
     
  5. studiot

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    If this is a serious project for building I can supply suitable attenuator designs from preferred values. there is no point reinventing the wheel or the 2pi!
    Since this is at audio frequencies I would stick to 600 ohms. Do you need db or volt/millivolt steps?
     
  6. atferrari

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    Have to be honest: never done anything serious in this matters, so have no hands-on experience. I would risk to say 50 and 600 ohms. Is it too much to ask?

    Again, no previous experience but I tend to feel better if dBs are not around, albeit I know what is all that about (almost). Volts / millivolts steps seems more intuitive to me...but that is me.

    BTW, my generator has an op amp as voltage follower at the output. How does it play regarding the calculation of the attenuator?

    All unknown waters for me.

    Gracias for your help and your time.
     
  7. studiot

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    I will see what I can dig out tomorrow.

    Meanwhile perhaps you can flesh out the oscillator specs a bit.

    Voltage level
    output impedance
    distortion

    That sort of thing

    And the final output

    Max voltage required
    do you wish to be able to produce useful output into low impedance (a loudspeaker). Perhaps uncalibrated?
     
  8. bountyhunter

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    Sep 7, 2009
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    The circuit I posted will probably drive any impedance down to about 4 Ohms and has a bandwidth of about 50 kHz (limited with feedback capacitor).

    If you need a sine wave generator just get an XR-2206.
     
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