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Using the Analog Devices Filter Design Tool, I have created a band pass filter for a 1MHz input with 20dB of gain. I have implemented the circuit in LTSpice with the expected behaviour, however when I implement it on a breadboard I am not getting the correct amount of gain (but the filtering seems to work when I vary the input frequency).

For my voltage reference I am using a MAX6001 1.25V reference. I have tested this with different bypass capacitors, finding little difference.

My thoughts are that perhaps I have wired the circuit wrong?

Simulated output and circuit: enter image description here enter image description here

mycircuit

Output from Picoscope (Red input signal, blue output signal): enter image description here

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    \$\begingroup\$ Do you have any idea how much capacitance sits on that wireless protoboard, between each row? Try wiring all that as dead-bug or manhattan style and see, for starters. Or choose a design that isn't depending upon 47 pF caps. Maybe something at least 10 times larger, maybe more? \$\endgroup\$ Commented Jul 17, 2023 at 9:34
  • \$\begingroup\$ @periblepsis, Interesting, thank you for your reply. Due to the nature of the design, I wish to keep the capacitance low. From what you are saying, the lower than expected gain is from the use of the breadboard, rather than the circuit itself? \$\endgroup\$
    – LK26
    Commented Jul 17, 2023 at 9:38
  • \$\begingroup\$ @LK26 what's that requirement that keeps the capacitances low? Note how that also makes the resistors low-ohmic (i.e., "strong"), meaning you're significantly loading the outputs of the opamps, which probably is a bad idea, as well. \$\endgroup\$ Commented Jul 17, 2023 at 9:39
  • \$\begingroup\$ @MarcusMüller I need my design (whole design, not just this circuit) to be intrinsically safe, so wish to keep capacitance low \$\endgroup\$
    – LK26
    Commented Jul 17, 2023 at 9:41
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    \$\begingroup\$ 180 MHz op-amps "on breadboard" likely explains it. You need a proper PCB which is designed very carefully to avoid parasitic capacitances and inductances from affecting the performance. \$\endgroup\$
    – Justme
    Commented Jul 17, 2023 at 9:42

1 Answer 1

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The midband-gain of the 1st stage for the shown circuit for two equal capacitors is (assuming ideal opamp properties): -

Gmax,1=R5/2R4=4.16 (12.4 dB).

Similarly, for the 2nd stage we get

Gmax,2=22k/5.4k=4.075 (12.2 dB)

Question 1: Did you use the filter tool correctly (because your design goal was 20dB gain) ?

Question 2: What did you measure for each stage (separately)?

Question 3: Did you check the slew rate of the opamp used?

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