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First time poster and a beginner at electronics, please tell me if any formatting is wrong.

I implemented this amplifier and filter circuit and connected it with a function generator as input to see if it worked. It did, but I do not understand it's behaviour. At about 70k frequency, input 272mV gives me 1.56V as output. But the gain starts to be less than 1 when I put anything above 1V as input. For example, 4.8V gives me 280mV as output. All this was observed on an oscilloscope. Would anyone please explain how that happens?

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  • \$\begingroup\$ Try to workout the transfer function of your circuit, that will help you to understand the frequency response and the bandwidth of your circuit \$\endgroup\$ – learn design Dec 31 '20 at 7:20
  • \$\begingroup\$ Where did you connect the input? Please show details. \$\endgroup\$ – Andy aka Dec 31 '20 at 10:22
  • \$\begingroup\$ Voltage outputs of LM324 cannot be near its negative power supply if the amplifier is to operate normally, and the output 2N3904 requires an emitter pulldown to operate normally as a buffer amplifier. \$\endgroup\$ – Whit3rd Dec 31 '20 at 21:40
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Your 68k resistors are tied to ground, which is the negative supply of the op-amps. I suggest adding a 68k to 5V for each one. This will prevent the op-amps from saturating.

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  • \$\begingroup\$ I don't think that will be enough. There's no DC negative feedback path on the first op-amp. It's going to saturate. \$\endgroup\$ – Transistor Jan 1 at 15:30

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