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voltage follower question

Discussion in 'Electronic Basics' started by tempus fugit, Apr 17, 2004.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I don't know if there's a general answer for this question, but here goes.
    I'm reading up on voltage followers (noninverting opamp), and it says that
    the gain of a voltage follower is approximately 1. Is the gain a little less
    than 1 or a little more than 1, or is there no straight answer for this
    question?

    The reason I ask is that in the same textbook it says that for instability
    to occur, the gain must be greater than 1. I'm looking at using a high speed
    opamp to buffer the output of a signal generator, and I don't know if it
    will oscillate or become otherwise unstable if I configure it as a voltage
    follower.

    Thanks
     
  2. John Larkin

    John Larkin Guest


    Simplistically, the gain is G = g / (1+g)

    where g is the open-loop opamp gain, typically 20,000 to several
    million at low frequencies, rolling off as F goes up. So G is slightly
    less than 1. At very high frequencies, more complex stuff can happen,
    and G can possibly get above 1.
    Most opamps are unity-gain stable, so work as followers, but some
    minority are undercompensated; the datasheet has details. An external
    capacitive load - even a healthy hunk of cable - will make most opamps
    oscillate; a series 50 ohm or so resistor between the opamp output and
    the load will generally fix this. National has some cool "C-load" amps
    that tolerate any capacitive load.

    John
     
  3. tempus fugit

    tempus fugit Guest

    A buffer provides a very high impedance input and a low impedance output.
    This way, the opamp is not loaded by the incoming signal, and the outgoing
    signal doesn't load the next stage. In my application, I want to use a pot
    to control the output level, but using one large enough to do the job would
    surely load the next stage.
     
  4. electricked

    electricked Guest

    I'm a newbie and this is a question. What's the purpose of a buffer? I know
    it can delay the output for a while but other than that how is it used? What
    applications does a buffer have?

    Thanks!

    --Viktor
     
  5. tempus fugit

    tempus fugit Guest

    My recollection of the specifics of this stuff is a little weak, but I'll
    try my best.

    Generally speaking, you want to have a low impedance output going into a
    high impedance input. I don't really remember the math, but if you have the
    reverse, you end up with a voltage divider situation and your signal becomes
    weakened. This effect is also frequency dependent, in that lower frequencies
    (I think?) will be affected more than higher frequencies. This effect is
    known as loading. If you have a matched output to input Z, then you have
    maximum power transfer. The long and short of it is that if you are feeding
    a high impedance signal to a low impedance input, the signal will be
    degraded.

    I also recall something about wanting to have a signal feeding into an
    input that is 10x higher in impedance for audio, but, again, I don't remeber
    exactly why. I think it is because of the fact that impedance itself carries
    a frequency component, so if you have a range of frequencies (say 20 to
    20kHz, as in audio), the impedance will be different at each of the
    different frequencies, being higher at lower frequencies.
     
  6. tempus fugit

    tempus fugit Guest

    At very high frequencies, more complex stuff can happen,
    and G can possibly get above 1.


    Thanks John. By very high frequencies, do you mean 20MHz by any chance? This
    is the upper limit of my function generator. If I might end up with
    problems, I could always just go with a buffer amp, but they are so much
    more expensive than a normal high speed opamp.


    Thanks again
     
  7. electricked

    electricked Guest

    So how is the impedance important for input and output? What happens if you
    have low impedance? What about high impedance?

    If it's high impedance output, does that mean that you can drain a lot of
    current from the output? I see how high impedance input would help, being
    that the device requires less current and therefore less power dissipation
    and I would think with high impedance the device would work much easier with
    higher frequencies. Does that make sense?

    Any help is strongly appreciated!

    Thank you!

    --Viktor
     
  8. Rich Grise

    Rich Grise Guest

    You need to go to whoever or wherever you heard the word "impedance" and
    find out what it means.

    Good Luck!
    Rich
     
  9. electricked

    electricked Guest

    I know ac impedance is the equivalent of dc resistance due to components
    such as caps and inductors and I know that the impedance changes with
    frequency. What I don't know is how this is used in everyday practical
    circuits. Please shed some light on my understanding.

    Thanks!

    --Viktor
     
  10. John Larkin

    John Larkin Guest


    Near the opamp's Ft (frequency where gain falls to 1) some opamps
    start to have additional phase shift, more than the ideal 90 degree
    lag. Around there, any small additional phase shift in the circuit can
    make trouble. Phase donations may also come from capacitive loads, or
    from the use of a resistor in the negative feedback path (don't do it,
    except for current-mode amps) or from non-ideal power rail impedances.

    A medium-fast, say 100-200 MHz, opamp in a tight layout with good
    power bypassing should be a good 20 MHz follower and not be too
    twitchey. Opamps like this are cheap nowadays. *Really* fast opamps
    (1-10 GHz GBW) are harder to work with.

    John
     
  11. Mantra

    Mantra Guest

    This is basically correct.

    The key thing is that everything has an "incident" impedance, that is,
    "looking" into a terminal of XYZ, you (or rather a connected circuit)
    will "see" an impedance and the resulting voltages and current will
    respond accordingly (via Ohms Law, Kirchhoff's Voltage and Current
    laws). This means impedances and impedance matches affect how your
    overall circuit behaves.

    Things like digital rise times, power efficiency and all depend on
    how the impedance of one circuit interacts with the impedance of
    another circuit.

    Typically an engineer reduces a bulk circuit down to a simple voltage
    or current source and an impedance also known as Thevenin or Norton
    sources, respectively (for example your stereo amplifer could be
    modeled thus). Similarly a complex circuit load can be reduced to a
    single "load" impedance (your loudspeakers would be the load). When
    you do this source model and load model are combined to form a circuit
    with just a source and two resistors: very easy to understand and to
    manipulate with just Ohms and Kirchoff's laws (in their AC forms).

    There are all sorts of things you can calculate with just this simple
    circuit. For example if your speaker if a subwoofer, then the
    impedance will "conjugate match" your source impedance at low
    frequencies so you'll get maximum power tranferred, which is why
    subwoofer do what they do. If you knew the impedances of the source
    and load you could predict both the frequency response of each
    component and the total response of your amplifier and speakers when
    connected.
     
  12. Mantra

    Mantra Guest

    The thing with voltage followers is that:

    1) They are low output impedance - damping of oscillations requires
    resistance which can imply high impedance, so there can be times when
    low impedance could pose a problem. This is the first clue.

    2) You do need *total loop* gain to be greater than one *and* a
    multiple of 360 degrees of phase shift to get oscillation. The
    voltage follower's "unity gain", which is actually <1, is only a
    *forward gain* but it pretty close. Further it nearly meets the other
    criteria due to *forward* phase by being 0 degrees shift. All you
    need is a little *reverse* feedback.

    3) They can have non-ideal, non-trivial output impedance. Not just a
    simple Thevenin source with a resistor, but an impedance. For
    example, emitter followers (bipolar transistors operating as voltage
    followers) can have net inductive output impedances at high frequency,
    which can cancel Miller capacitances that normally band-limit voltage
    feedback *and* if loaded improperly, can create an additional low
    impedance current feedback path. These two feedback paths could give
    enough total loop gain for oscillation.

    Op Amps, however, are a bit easier to deal with. To determine if your
    Op Amp will be unstable you need to look at its Bode Plot and see what
    the gain margin and phase margin are. That's the quick, cheap
    analytic method. The other is the empirical method - pick something,
    try it, if it oscillates add compensation to tweak the phase back to
    stability as per the manufacturer's app note.
     
  13. tempus fugit

    tempus fugit Guest

    Thanks again John. I'll order up a couple of 150MHz opamps and see how it
    goes.
     
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