>Dwayne Reid wrote:
>
>> If the amplifier is expected to amplify AC and you are using a single
>> supply with the (+) input biased at 1/2 the supply voltage, you will
>> need a series coupling capacitor from the common point of the
>> resistors to the op-amp (-) input.
>
>> Also note that you should include a large value resistor from the
>> resistor side of the coupling capacitor to GND so that the capacitor
>> always has the correct voltage on it.
>
>  Err, ummph, well, it may not be as simple as that.  Obviously to bias
>the capacitor you want to have a permanent resistor to ground,
>corresponding to the minimum (or maximum if this happens to be in the
>negative feedback path) gain setting.
>
>  Whether you use the capacitor for a single-supply system, or use a
>split-supply for the op-amps, the use of such a circuit on AC means that
>an open switch could be subjected to a negative voltage.  If that
>voltage (corresponding to the resistor common point) exceeds -0.6V, the
>protection diode will kick in and things are no longer linear.

You are right if the input signal exceeds about 0.5V peak.  The op-amp (-)
input has to track the (+) input for the feedback loop to close.  But for
gains larger than 10 or so, there should be no problem (and, somehow, I
thought that was the case).

My feeble attempt at ASCII art: (monospace font)

                               |\
   input & bias----------------|+ \
                               |   >--------- out
                          -----|- /     |
                          |    |/       |
                          |             |
                          |----/\/\/\/--
                          |
      pic                ===  cap
    -------               |
    |     |---/\/\/\------|
    |     |---/\/\/\------|
    |     |---/\/\/\------|
    |     |---/\/\/\------|
    |     |---/\/\/\------|
    |     |               / this resistor sets minimum gain and ensures
    |     |               \ that the DC charge on the cap stays the same
    |     |               / even if all the pic pins are tristate.
    |     |               \ It can be eliminated if at least 1 pic pin
    -------               | is set to LO at all times.
                         gnd

dwayne







Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax