0
\$\begingroup\$

I have a dual-supply op-amp (±12V) that outputs a signal in the range of ±3V. I cannot neither change the op-amp configuration nor add another one. Please not the schematic below is just an example. The actual configuration is more complex, but it doesn't matter about the question.

The output goes to a voltage divider and I need to clip the negative voltages.

The common way is to add a diode on the output:

schematic

simulate this circuit – Schematic created using CircuitLab

But when the output swings below the threshold voltage nothing limits the current.

Where should I put a limiting resistor?

  • in series to the diode? I think it will affect the clipping
  • on the op-amp output, after the fb node? I think it will affect the resister divider
  • on the op-amp output, before the fb node? I'm afraid it will affect the feedback gain

Is there a better way to achieve this?

\$\endgroup\$
4
\$\begingroup\$

Where should I put a limiting resistor?

The usual place to put this is directly in series with the op-amp output. However, you need to choose a value that isn't too high else you won't get the positive swing you need. You could also use a precision half wave rectifier circuit - it will only produce positive going signals. Here's one that is non-inverting and has a gain of unity: -

enter image description here

Should you go for the precision rectifier approach take note that you should use a diode with fast reverse recovery in order to obtain least unwanted distortion. A BAS16 or 1N4148 is always a good choice.

If you add only the diode (as shown in your question), the TL081 will limit current at about 20 mA to 30 mA. Maybe this is enough - maybe if you do calcs on heat dissipation you'll find you don't need a resistor.

\$\endgroup\$
  • \$\begingroup\$ you could add R1 and R2 in your schematic to achieve the same gain as well. \$\endgroup\$ – Sclrx May 12 '17 at 10:54
  • \$\begingroup\$ Would you please explain a little more what do you mean with "you need to choose a value that isn't too high else you won't get the positive swing you need" ? What "value" are you referring to? \$\endgroup\$ – Mark May 12 '17 at 11:31
  • \$\begingroup\$ Your first question is "Where should I put a limiting resistor?" Value refers to ohmic value. If too high (kohm) you won't get the full positive peaks across your load (R3 and R4). \$\endgroup\$ – Andy aka May 12 '17 at 11:38
0
\$\begingroup\$

The solution here is not to add a limiting resistor anywhere, but to move the diode, and put it in parallele with R4. This way, the op-amp can still output a negative voltage, but the voltage on your output node will effectively be clipped, and the diode current will be limited by R3.

\$\endgroup\$
  • \$\begingroup\$ I'm sorry, I didn't say the load is placed on another board, and I cannot change it. I've updated the question. \$\endgroup\$ – Mark May 12 '17 at 11:30
  • \$\begingroup\$ Well then @andy aka 's answer is the way to go :) \$\endgroup\$ – Sclrx May 12 '17 at 11:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.