# [sdiy] Circuit purpose?

Ove Ridé nitro2k01 at gmail.com
Tue Mar 18 02:45:12 CET 2014

```On 18 March 2014 02:23, David Ingebretsen <dingebre at 3dphysics.net> wrote:
> I've run across a circuit bit a couple of times now that follows the output
> of an op-amp, and I don't understand its purpose. It consists of two 10uF
> electrolytic capacitors connected negative to negative with a 680 ohm
> resistor between the two negative sides of the capacitors. There is then a
> 100k resistor on the positive side of the output capacitor to ground.
>
>                          +              680R             +
> OP-AMP  >--------| |------/\/\/\/------| |--------------->output
>                                                                  |
>                                                                   \
>                                                                   /
>                                                                   \    100k
>                                                                   /
>                                                                  |
>                                                                  |
>                                                                ----
>                                                                 --    GND
>
> Why have this? What is it doing for the circuit?
>
> David

The capacitors are used as a low frequency high-pass filter, or more
precisely for DC blocking, ie preventing that there is a constant DC
offset in the output signal. The 680 ohm resistor is likely there for
current limiting in case the output is shorted to something, or
connected output to output by accident, so you don't exceed the op
amp's output current limit. The 100k resistor is there to discharge in
theory to form the second link the RC high-pass filter (so you're not
just relying on the output of the input the other end to determine the
HP cutoff) and also in practice to discharge the capacitors when
turning the power off.

So to the real interesting part, why the back to back to back
configuration? This is because a polarized capacitor shouldn't be
reverse biased. Doing so will blow it up, with a pop or a wheeze
depending on the amount of current. Putting them back to back makes
sure none of them are permanently reverse biased, even though it
halves the equivalent value, as putting capacitors in series does. The
back to back configuration is common, but I've even seen an even more
serious version of it where they are connected (say) positive to
positive, and then a weak pull-up resistor to the positive rail, so
both capacitors are, not only not backward biased, but permanently
forward biased. This may be an even slightly more reliable, and
theoretically more correct version of the common back to back
connection.

--
/Ove

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