Here is a picture of the chopped waveform.
Hi there,
I have to wonder where you are coming up with these values and why you biased the second stage at 2v instead of 1/2 Vcc like the previous stage. I also have to wonder why you added that extra cap, is that really meant to go across the power supply lines?
To get this right you have to follow a certain set of rules.
I'm starting to think maybe you would be better off with a completely redesigned circuit one of us can recommend for you? Or are you trying to learn the procedures for doing these kinds of circuits?
You might get 15v peak to peak from two 9v supplies (plus and minus 9v) but you may only end up with 12v peak to peak, which will clip a sine if the circuit is designed to output a 15v peak to peak sine (using the TL082). Perhaps you should set the target output to 12v peak to peak instead. You'll also have to analyze the second stage input to make sure the input is within the common mode input range for the IC.
Adding C6 may destroy the bandpass feature of that op amp filter, but we'd have to do an analysis to be sure. As audioguru suggested, it's better to leave it out and go with the tried and proven bandpass section the way it was before the change. That is a well known bandpass configuration that usually works ok, but it's operation with the extra cap would have to be looked at carefully so it's better to leave out C6.
As i said before too, you'll have to change the other values to tune near to 40kHz.
LATER:
Took a quick look at the addition of the C6 capacitor. It's effect appears to be that it does not detune the bandpass center frequency, but it flattens out the response (makes it less sharp) and also reduces gain. At 0.01uf it probably reduces gain quite a bit, possibly actually makes the filter an attenuator rather than an amplifier. It should be left out unless you need a simple way to widen the bandwidth quickly and even then it has to be rather low in value, but probably leaving it out is the best way to go in any case.
Perhaps you would like some suggested values...
R11=66.3k
R10=1k
R9=400k
C4=200pf
C5=200pf
C6 (remove from the circuit)
The nice thing about these values is you can change R11 up or down to reduce or increase gain, while not having to change anything else.
For example, with R11 equal to 200k the pass band gain is reduced to 1/3 of what it is with the value above. With R11 equal to 50k the
gain is slightly increased.
This design can also be impedance scaled quite easily without too much trouble. For example if you change both caps to 400pf instead of 200pf then reduce ALL the resistor values to one half of what they are above. This could help if the output seems noisy or you want a lower input impedance.
These designs could also suffer from a slight lowering of the center frequency when you go from the theoretical circuit (perfect op amp which the design component values usually come from) to a real life circuit (op amp with frequency limitations). To help get around this if needed, lower R2 and R3 by a small amount, each one by the same percentage, to make up for the op amp frequency limitations. For example, lowering R2 and R3 by 5 percent could be just enough to make up for a real life op amp. The effect is to increase the center frequency without changing anything else, which makes up for the op amp tendency to lower the center frequency because of internal limitations.
The TL082 could affect the design by as much as 10 percent making the values:
R11=55k
R10=900
R9=360k
C4=200pf
C5=200pf
where R11 was decreased slightly to make up for some gain loss.