The op amp as an oscillator

The op amp voltage comparator may be used as an oscillator if positive feedback is used. The circuit and waveforms are shown in Figures 1 and 2.

Suppose that the output voltage is positive at a particular instant. A certain fraction b of the output voltage (V_o) is fed back to the non-inverting input, and V_o is fed back to the inverting input via R₁.

The feedback fraction β is given by the equation:





V₁ rises exponentially as the capacitor C is charged.

After a certain time that depends upon the time constant CR₁, the voltage at P exceeds that at Q, that is, V₁ > V₂. The op amp therefore switches to negative saturation (V_o = - V_S) and as a result of the positive feedback Q becomes negative.

The capacitor (C) now charges in the opposite direction, and the voltage at P falls until V₁ < V₂, at which point the op amp switches to positive saturation and the cycle repeats itself. The output voltage is therefore a square wave, as can be seen from the graphs in Figure 2.

The equation for the frequency (f) of such a system is:

Oscillator output frequency (f) = 1/[2R₁C ln(2R₂/R₃ +1)]

Amplifier with a varying input voltage

If the input to the non-inverting input is a sine wave, then the op amp will saturate every time the difference between the voltages at the inverting input and the non-inverting input exceeds about 150 μV.

The saturation will swing from positive to negative as this voltage difference becomes positive or negative.
This will result in a square wave output, as shown in Figure 3.