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I have a 7 stage passive Chebyshev filter designed to attenuate signals outside of a ~1MHz bandwidth centered around 50MHz in order to protect a clock input from frequencies that are too high or low. After filtering out these signals I am aiming to force the input wave to a square wave saturated at the desired amplitude of the clock frequency.

I have done a good bit of testing of the filter schematic, but once I added the amplification my outputs became bizarre. Inside the range of about 49-50MHz the amplifier diminishes the input signal. Outside this band, it amplifies the signal but the amplification doesn't appear to be proportional to the gain of the circuit. I've attached the final stage of the filter + amplification stage and the plot of the Vin and Vout of the amp at 50MHz.

I've tried adjusting the slew, GWB, open loop gain, and double checking the rail limits but still can't come up with anything that's causing this.

enter image description here

Ignore K’s in the feedback resistors, they were for testing and forgot to remove, the output was independent of their magnitude

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  • \$\begingroup\$ @brhans It’s currently just an ideal op amp as modeled in LTSpice \$\endgroup\$
    – John B
    Commented May 26, 2022 at 21:59

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Oh man that feedback resistor (R3) and gain resistor (R4) are massively too high in value for operating with decent gain at 50 MHz. Check the data sheet and see what it recommends for the op-amp you have chosen.

Just think about the feedback resistor of 150 kΩ. If there is 1 pF of stray capacitance across it then the cut-off frequency will be about 1.06 MHz and far, far too low for properly amplifying 50 MHz signals.

After filtering out these signals I am aiming to force the input wave to a square wave saturated at the desired amplitude of the clock frequency.

If you are expecting the op-amp to do the clipping-to-square then you might easily get even more problems with the op-amp because recovery from saturation can take several microseconds for most op-amps.


Maybe you should consider using a high-speed comparator like the MAX999 after the parallel tuned circuit. It's quick and will nicely square-up even quite small signal levels.

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  • \$\begingroup\$ The thing is right now it’s just using the LTSpice ideal amp, forgot to get rid of the k at the end of either resistor, the weird output right now is independent of the gain for some reason. However using the Max999 is a good idea! \$\endgroup\$
    – John B
    Commented May 26, 2022 at 21:59

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