In the figure we have a typical Wien Bridge oscillator with AGC. The negative waveform is sampled by D1 and stored in C1. R1 and R2 must be chosen to center the polarization of Q1 in such a way that (RG + Rqq1) = Rf / 2 at the desired output voltage. When the output voltage moves to a higher value, Q1 increases its resistance thereby reducing the gain. In the oscillator shown in the figure, the 0.833 source is applied to the positive input of the operational amplifier in order to centralize the quiescent voltage of output at Vcc / 2 = 2.5 V.
