RE:Output clamping
RE:Output clamping
(OP)
I have a diff-opamp that runs on +/-9vdc and would like to limit the output rail at +/-5vdc.What kind of zener or other component arrrangement will work, please help fast because I need something soon.





RE: RE:Output clamping
<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032
RE: RE:Output clamping
RE: RE:Output clamping
RE: RE:Output clamping
TTFN
RE: RE:Output clamping
RE: RE:Output clamping
You should bias the zeners individually, giving them say 5mA current each. Then you can use this as a clamp point for a low leakage diode. We are talking about one zener, one bias resistor, and one low leakage clamp diode for the +5V clamp and this same circuit again for the -5V clamp. If you have +5V and -5V power rails you could of course just use schottky diodes to these power rails. You still need the series protection resistor from the opamp in all schemes.
schemes.
Note that putting zeners across the feedback network does not limit the amplifier output as desired. It drops the gain into the non-inverting input from 8.333 to 1, but that still means that increasing the non-inverting input will increase the output.
Whilst we are on the subject, your basic circuit bias is wrong. You have a non-inverting signal of +3.1V to +1.9V, that is nicely centred about the +2.5V reference level. When you put in +2.5V to the non-inverting input, given that the inverting input resistor is also at +2.5V, how much are you expecting at the output? If you think the answer is 0V then you flunked opamps 101.
Try again using nodal analysis, following the currents.
RE: RE:Output clamping
The last way would to be to use a comparator.
If cost is an issue, then go with the diode approach. You might want to consider it if the bandwidth of your amplifier is large. Diodes work great at high frequencies.
Good luck.
RE: RE:Output clamping
RE: RE:Output clamping