Two common RC oscillator circuits

This article introduces two common circuits of RC oscillators, namely RC phase-shift oscillator circuit and RC bridge oscillator circuit, and introduces in detail the basic knowledge such as the characteristics and oscillation frequency of these two circuits.

RC Oscillator Circuit

The frequency selection network of the RC oscillator is an RC circuit. Their oscillation frequency is relatively low. There are two commonly used circuits: one is the RC phase-shift oscillation circuit, and the other is the RC bridge oscillation circuit.

1. RC phase shift oscillator circuit

Figure 4 (a) is an RC phase shift oscillator circuit. The three RC networks in the circuit play the role of frequency selection and positive feedback at the same time. From the AC equivalent circuit of Figure 4 (b), we can see that because it is a single-stage common emitter amplifier circuit, the output voltage Uo of the transistor VT is 180° out of phase with the output voltage Ui.

When the output voltage passes through the RC network and becomes the feedback voltage U f and is sent to the input terminal, since the RC network only produces a 180° phase shift for the voltage of a certain frequency f 0 , only the signal voltage with a frequency of f 0 is positive feedback and causes the circuit to oscillate.

It can be seen that the RC network is both a frequency selection network and a part of the positive feedback circuit. The characteristics of the RC phase shift oscillator circuit are: the circuit is simple and economical, but the stability is not high and the adjustment is not convenient. It is generally used as a fixed frequency oscillator and in occasions with less demanding requirements.

Its oscillation frequency is: When the parameters of the three RC networks are the same: f 0 = 1 2π 6RC . The frequency is generally tens of kilohertz.

2. RC bridge oscillator circuit

Figure 5 (a) is a common RC bridge oscillator circuit. The series- parallel circuit of R1C1 and R2C2 on the left side of the figure is its frequency selection network. This frequency selection network is also part of the positive feedback circuit. This frequency selection network has no phase shift (phase shift is 0°) for a signal voltage with a specific frequency of f0, and voltages of other frequencies have phase shifts of varying sizes.

Since the amplifier has 2 stages, the feedback voltage U f taken from the output terminal V2 is in phase with the amplifier input voltage (2-stage phase shift 360°=0°).

Therefore, when the feedback voltage is sent back to the input terminal of VT1 through the frequency selection network, only a voltage with a specific frequency of f0 can meet the phase balance condition and oscillate. It can be seen that the RC series-parallel circuit plays the role of frequency selection and positive feedback at the same time.

In fact, in order to improve the working quality of the oscillator, a series voltage negative feedback circuit composed of R t and RE1 is added to the circuit.

Rt is a thermistor with a negative temperature coefficient, which can stabilize the oscillation amplitude and reduce nonlinear distortion of the circuit.

From the equivalent circuit of Figure 5 (b), we can see that this oscillation circuit is a bridge circuit. R1C1, R2C2, Rt and RE1 are the four arms of the bridge, and the input and output of the amplifier are connected to the two diagonals of the bridge, so it is called an RC bridge oscillation circuit.

The performance of RC bridge oscillator circuit is better than that of RC phase shift oscillator circuit. It has high stability, small nonlinear distortion and convenient frequency adjustment. Its oscillation frequency is: when R1=R2=R, C1=C2=C, f0=12πRC. Its frequency range is from 1 Hz to 1 MHz.