Use 1k and 100k instead of 10k and 1M in the feedback of first LM318. Use 1k and 47k instead of 10k and 470k in the feedback of second LM318. Use 22R + 100µF + 100nF in each supply voltage line to each LM318. Connect a 10k resistor from input of second LM318 to ground. Remove first diode, use 4k7 resistor instead. Remove 10k resistor at second diode, use 47k instead. Connect the cathode of second diode to inverting input of LM339, not to non-inverting input. Connect the poti to non-inverting input. Connect 1M resistor from non-inverting input to output of LM339. Remove 10k resistor at output, use 4k7 instead.


Sorry, but I have to state this: It's time that you have a look into a textbook about OPamps, because you ask this question over and over again and make the same mistakes. Designing a circuit needs two things to do, theoretical work and practical work. But you totally omit the theoretical work! For properly working with OPamps there's a minimum of theoretical knowledge needed!

Most Opamps have a maximum gain called 'open loop gain', that is frequency dependent. At unity gain bandwidth the maximum achievable gain is 1 and increases by about 20dB with decreasing frequency by one decade. So, LM324 showing a unity gain bandwidth of 1MHz has a maximum gain of 10 at 100kHz and 100 at 10kHz. At 40KHz the maximum gain is 25.

So, you can theoretically achieve a maximum gain of 25 at 40kHz. But, as most OPamps are very unlinear, which is especially true for LM324 you need a certain gain reserve called 'loop gain' to increase the linearity. A gain reserve of at least 10 is needed for most applications. So, this gives you a maximum gain of your circuit called 'closed loop gain' of 25/10 = 2.5 for LM324 at 40kHz!!

In applications, where the linearity of gain is not so demanding, especially if only a simple detection of a carrier is needed, then 'closed loop gain' can be increased a little bit, means 'loop gain' can be decreased a little bit. But take care, there's no much headroom for doing this, because there are many negative effects with this, like increased risk of instability and so forth.

Kai