LF353 op amp oscillation circuit

The oscillation circuit is shown in Figure 1. It consists of a bandpass filter and an inverting amplifier. The bandpass filter composed of R2, C2, R1, and C1 constitutes the positive feedback circuit of the amplifier.

When R1=R2=R and C1=C2=C are set, the phase of the bandpass filter is zero at the frequency fo. Therefore, when the feedback resistors R3 and R4 of the operational amplifier are set to R3=2R4, the gain of the in-phase amplifier is 3 times, and the loop gain at fo is exactly 1 times, the circuit generates oscillation and can just maintain the oscillation.

The oscillation frequency can be calculated using fo = 1/2πRC. The method is as follows:

1. Given the oscillation frequency fo and the value of the resistor R, calculate the value of the capacitor C.

For example, if fo=1000Hz, R=10kΩ, then C=0.0159μF can be obtained.

2. Given the oscillation frequency fo and the value of the capacitor C, calculate the value of the resistor R.

Since 0.0159μF is not the nominal value of the capacitor, it can only be achieved by combining several capacitors in series and parallel. When making the circuit, it is not only costly but also troublesome to operate. Therefore, the capacitance can be determined first, and then the resistance value can be calculated. For example, the capacitance is set to C=0.0159μF, that is, R=1/2πfoC=10.6kΩ.

For this oscillation circuit, even if the loop gain is only slightly less than 1, the circuit will not oscillate. Therefore, in order to ensure oscillation when designing the actual circuit, R3 and R4 are set to the relationship of R3=2.2R4. At this time, oscillation can be ensured. However, the oscillation will continue to increase over time, and finally the top of the waveform will be limited. For this reason, a voltage regulator diode needs to be connected in the feedback circuit to limit the increase in amplitude.

Figure 2 is an example of an oscillation circuit with an output voltage of 2Vrms and an oscillation frequency of 1kHz. The output impedance of the circuit is 600Ω. The amount of negative feedback reduces the loop gain of the amplifier. The harmonic distortion rate of the oscillation circuit is about 0.5%. After turning on the power, the amplitude will continue to increase and it will take about 5ms to stabilize.

The amplitude of this oscillation circuit will change due to the error of R1 and R2 or the error of C1 and C2, which is the shortcoming of this oscillation circuit. For example, in the circuit of Figure 3, if R=2.2kΩ, and C2 is changed from 0.015μF to 0.016μF, the amplitude of the oscillation circuit will change from 7Vrms to 9.3Vms.

This oscillator circuit can oscillate in a wide frequency range, from a few hertz to hundreds of hertz.