Adjustment of simple high input impedance dual wave linear detection circuit (RC4558DN)

As shown in the figure, it is a simple high input impedance dual wave linear detection circuit. This circuit compensates for the nonlinearity of the feedback level Vf movement by adding a diode to the negative feedback loop of the operational amplifier. The circuit does not need to use precision resistors. Its characteristic is that the gain difference between positive and negative inputs can be simply compensated by adjusting VR1. Since the signal is input from the in-phase terminal of A1, the input impedance is high and no nonlinear distortion occurs.

In the positive half cycle of the input AC signal, the output of A1 is positive, D1 is forward biased and turned on, so the input of A2 is also positive, and the output of A2 is also positive. A1 is negatively fed back with β=1 by R2, VR1, and R1. In general, it becomes a non-inverting amplifier with a gain of 1. In this case, the gain remains unchanged when VR1 is adjusted.

When the input signal is in the negative half cycle, the output of A1 is also negative, D1 is reverse biased, and the input of A2 is 0, then A2 becomes an inverting amplifier. At this time, A1 performs negative feedback through D2, and actually becomes a voltage follower. The inverting input of A2 is the same as the AC input voltage, and A2 works as an inverting amplifier with a gain of 1. In this case, adjusting VR1 can change the gain of A2, and adjusting VR1 makes the overall gain 1.