That gain can be tweaked to set the output precisely at +/- 5 volts. This would then mean that the 2nd stage filter will need a gain of 2 to obtain +/- 5 volts peak to peak. I might also consider that to give a reasonably constant output amplitude, the output from the oscillator is limited by precision voltage shunts (possibly LT1389 or SPX4040) to restrict the output to +/- 2.5 volts. Those two components are there to remove any DC offset from the oscillator because that sort of oscillator is going to produce an offset that is generally undesireable due to asymmetries in the output limiting. Next I would ensure that the output from the first stage (the relaxation oscillator) fed the 2nd stage (a 2nd order low pass filter) via a decoupling capacitor and resistor to ground. R4 would also need to connect to the negative rail. The split bipolar supply also naturally gives you a centre point of 0 volts (desireable). This gives enough supply overhead to produce a sensible output level of a 5 volt peak sine wave. I'd start by powering both op-amps with bipolar supplies, something like +/- 9 volts. I was wondering what changes to the circuit would be needed to change
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