Electrical – Op-amp envelope detector – capacitance load issue

analogoperational-amplifier

I have setup the below circuit; a basic envelope detector, however I believe that the op-amp is struggling with the load capacitance.

circuit

With the 100nF cap removed, then the output of the op-amp behaves as expected following the input voltage (AC around 1.6v DC). The output of the op-amp is driven higher to make up for the diode voltage drop. (Below scope trace, blue input, red output).

scope1

However when the 100nF is added, then the op-amp is not drving the output high enough. It just stays around 1.6V. In theory the output voltage should rise to around 1.9V.

enter image description here

I added the 2.2K output series resistor to try and get round the issue, but it didn't help. Are there any obvious errors or ways around the issue?

It may be that I need to use a different op-amp.

Thanks

Best Answer

The opamp will saturate at the negative rail unless you add some more components. The opamp can take a while to come out of saturation affecting the circuit operation. You may need to lower R1 if you are running at a high frequency.

Note that R2 and R3 will form a voltage divider, so if this is undesirable, make R2 very small. I try to avoid opamps hitting their current limit, which it will if R2 is small.

schematic

simulate this circuit – Schematic created using CircuitLab

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