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I have been looking into positive clipping circuits using an op amp and found two circuits recommended here and various pages online like Microchip tips and tricks page 16 and nprcet.org page 22. Essentially, both setups use an op amp and a diode to clip positive voltage when the voltage source has reached above a certain reference. From what I see, the difference is which node of the op amp Vin is fed into. I have simulated the circuits using LTSpice to study any performance differences and it seems like Vin connecting to the negative node seems to follow the Vin more accurately. I am not sure why my other simulation has clipping at 1.8V, everything online shows that clipping at the lower end should not happen. If I fix that, perhaps the output follows the input a little more closely?

My question regarding these circuits is what makes a given configuration more preferable to the other? Is there is a specific reason for why there are these two topologies or do they exist because they perform relatively the same way?

In one earlier simulation I did on Falstad, I noted that when the Vin-to-non inverting node circuit is in the clipping mode, the excess current seen being sunk into the op amp so the series resistor (R1 in the first image) must be sized up to some maximum to limit how much current is being drawn, but the simulation suggests that if the resistor is too large then the output voltage sags over a 1k resistive load and doesn't follow Vin that closely, sagging by 0.1-3V near 3V.

Orientation 1

Orientation 2

LT Spice simulation

Falstad

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It has to do with the sinking capability of the AD712 with the given supply rails.

If you use +/- 5V for the supply rails it works fine with R = 10K.

If you use the original single-sided +5V supply you get closer to the expected using R = 1Meg.

enter image description here

Here's some more insight into what's going on... In circuit 1 the lowest voltage the op amp needs to output is about 2.5V. In circuit 2 the op amp needs to output voltages from 2.5V down to 1V in order to follow the trough.

The AD712 doesn't have rail-to-rail output and it can't get down that low.

As you can see from the graphs, v(op2) is clipping in the troughs of the input signal where as v(op1) isn't.

enter image description here

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