A new type of RC oscillator

Generally a frequency-determining network and a feedback network in sine wave oscillators are not independent but are coupled together. Because of this it is hard to adjust this type of oscillators.

The circuit diagram of the RC oscillator shown in figure 1 using two independent network. The transfer function of this op-amp based circuit from the terminal A to the terminal B is described by formula

UB = (1+p(C1+C2)R2)/(1+p(C1+C2)R2+p2*C1C2R1R2) * UA = F1(p)UA ,

and from the terminal B to the terminal A is described by formula UA = (pC3R3)/(1+pC3R3) * UB = F2(p)UB.

If the formula F1(p)F2(p) is equal to 1, then the circuit will produce sine waves oscillations. For the frequency f = 1/(2π(R1R2C1C2)1/2) it is necessary to make the equation true that (C1+C2)R2=C3R3.

RC oscillator based on op-amps circuit diagram

Fig. 1

Download LTSpice model of the RC sine wave oscillator.

From the last two expressions, it is evident that by changing the value of the resistor R3 it is possible to change the amplitude of the oscillation, it is doesn't affect the frequency of the oscillator. The frequency of the oscillator is determined by the value of R1. Another advantage of this oscillator is that it uses two op-amps as voltage-follower. In this configuration op-amps yields the widest bandwidth.

Waveforms of the RC oscillator

Fig.2. Waveforms of the oscillator. R1=R2=10K, R3=20K; C1=C2=C3=10nF; DA1, DA2 - LT1013.

Experimental testing of this oscillator with the two op amp the μA747 provided the frequency up to 150 kHz. Using a supply voltage of +-15 V indicate a capability to deliver an output amplitude up to 25 V at the frequency of 5 kHz. At the higher frequencies the output amplitude decreases.

RC oscillator with swapped components based on op-amps circuit diagram

Fig. 3

Download LTSpice model of the RC sine wave oscillator with swapped components.

If swap the resistors with the capacitors, and swap the capacitors with the resistors, then the oscillator will work as well (see Fig. 3). In this case the condition for the oscillation is R3C3=C2R1R2/(R1+R2).

Novy typ oscilatoru RC. - Sdelovaci technika, 1986, № 4, p. 156.