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Buffer stage

Discussion in 'Electronic Design' started by [email protected], Jan 10, 2009.

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  1. Guest

    Hi,
    with your help, i've designed an opamp folded cascode with a fixed
    capacitive load....now i want to make an output (mosfet) stage voltage
    follower-voltage follower....My output stage is composed by 2 voltage
    follower output transistor, each of them piloted by a mosfet current
    source (or sink) and a common drain...the gate of the 2 common drain
    are connected to folded cascode's capacitive load. I don't understand
    a strange problem that i've found.
    1)If, before to connect output stage to folded cascode,i connect an
    ideal ac source to buffer stage and an output load, i obtain the
    desidered output stage's pole frequency
    2)if i connect folded cascode's load (instead ac source), buffer pole
    has a lower frequency (folded cascode pole is unchanged)
    Why??
    Thanks
     
  2. MooseFET

    MooseFET Guest

    It sounds like the input capacitance of the buffer stage is big enough
    to matter.
     
  3. Guest

    It sounds like the input capacitance of the buffer stage is big enough
    How input capacitance can modifiy output pole?
     
  4. Guest

    It is presenting a capacitive load on the high impedance point of the
    folded cascode (output), shifting the pole. I assume you mean the Bode
    response of the amp+buffer is difference from the amp alone.
     
  5. Guest

    It is presenting a capacitive load on the high impedance point of
    the
    No, i have an output buffer with capacitive load: if this output
    buffer is not connected to folded cascode, it presents the right pole;
    if instead is connected to folded cascode with its capacitive load,
    the buffer's output pole has a lower value (the output pole of folded
    cascode is the same with or without buffer stage). I don't understand
    how input capacitance can vary buffer's output pole....
    thanks for the help
     
  6. MooseFET

    MooseFET Guest


    Have you tried driving the buffer with a high impedance of some other
    type? Your buffer stage likely has a feedback from the output node to
    the input. The situation may look something like this:

    Cfb
    -[Z]-----+----!!--------
    ! !
    ! ---- !
    ---!+ \ !
    ! GM OO---++---
    ---!- / ! !
    ! ---- ! --- Cload
    --------------- ---
    !
    GND

    With a zero value for Z, the pole position is controlled by the
    resistance of GM and Cload forming a simple time constant. With non-
    zero Z, the non-inverting input of the GM has a smaller signal on it
    and hence the GM appears to be a higher impedance.
     
  7. Guest

    Well, lets try to parse this.
    OK, just who exactly has this pole, i.e. the buffer itself? And if it
    is in the right hand plane, why bother attaching it to the cascode?

    This in itself seems unlikely since the buffer must present a bit of
    capacitance to the folded cascode, which should move the poles of the
    folded cascode a detectable but perhaps not significant amount.

    I don't understand
    Not seeing this buffer, I am assuming you go up a Pfet and down a
    Nfet. The fets have gate to source capacitance. Put them in a string
    (source of Pfet drives gate of Nfet) and connect the capacitive load,
    and you have capacitors in a string. CGS of P fet connects to CGS of
    Nfet connects to load capacitance. Hence the buffer has some inherent
    feedback path. Generally the buffer is of sufficient transconductance
    (at the output stage) that the feedback path is not significant, i.e.
    the dominant pole of the folded cascode would provide stability.
    Perhaps you need more current in the buffer output stage.
     
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