[sdiy] Spare op-amp useful in gain stage?

[Andrew Simper]

Really it depends on what the circuit is in the first place to decide
what would be useful to add with a spare op-amp.

I find a lot of the time it's useful to have gain staging to adjust
the signal level prior to hitting a non-linearity, so for example if
you have a filter circuit and you have two op-amps spare then being
able to increase the input level while decreasing the output level is
really useful. Even just being able to increase the input level is
useful, but then you can hit a later stage too hard.

In some filter designs it's useful to use a spare op-amp to buffer an
input to the base of one of the caps in the filter to generate a new
filter response, or to sum the outputs to generate another filter
response.

You could add a distortion / wave shaper section to a circuit similar
to the TS-9 using a single opamp, a couple of diodes, and a few of
resistors and caps.
http://www.geofex.com/article_folders/tstech/tsxtech.gif

If it's a modulation source you could use a spare opamp to provide an
inverted and possibly offset output.

Andy

On 28 January 2016 at 20:03, Mattias Rickardsson <mr at analogue.org> wrote:
> Hi!
>
> I was in a discussion about what to do with a spare OP-amp. These
> fellows come in duals and quads, and sometimes the quantities don't
> end up perfectly and you stand there with an extra unused op-amp.
>
> Is there any clever way of using two op-amps instead of one in, say, a
> noisy high-gain stage in order to reach lower noise or something like
> that?
>
> Douglas Self's small signal book suggests paralleling two gain stages
> ("multipath amplifiers"), having their outputs tied together with 10
> ohms to the destination, winning 3 dB SNR. Are there other clever ways
> of using a spare op-amp, particularly in a stage with high gain?
>
> /mr
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