Double T frequency selection network sine wave generator

Double T frequency selection network sine wave generator

The twin-T network is connected in parallel in the negative feedback loop of an operational amplifier with positive feedback to form a twin-T frequency selection network sine wave generator, and its circuit is shown in Figure 1. As can be seen from the figure, the two T-type networks are composed of R, R, 2C and C, C, R/2 respectively. By using the method of converting between star circuit and triangle circuit, the twin-T network can be simplified into the equivalent circuit shown in Figure 2.

in,

Its amplitude-frequency characteristics and phase-frequency characteristics are

In the circuit shown in Figure 1, the double-T network is connected in parallel between the inverting input and output of op amp A, generating strong negative feedback for other frequency components except ω=ω0, which is enough to offset the positive feedback introduced by the positive feedback network Rf1 and Rf2; while for the frequency component of ω=ω0, the negative feedback is extremely weak, and the positive feedback introduced by Rf1 and Rf2 is enough to make the circuit oscillate on ω0, and output a sinusoidal signal with a frequency of fo=1/2πRC. In order to stabilize the amplitude of the oscillation signal, Rf1 or Rf2 should be selected as a nonlinear element with appropriate characteristics.

It should be pointed out that the above analysis results are obtained under the condition that the R and C parameter values ​​are completely symmetrical. If the parameter values ​​are asymmetrical, for example, only the value of the resistor R/2 is reduced, the phase-frequency characteristics of the twin-T network will change significantly, reaching -180° at ω=ω0. At this time, the twin-T network connected in parallel to the negative feedback loop will form positive feedback on the ω=ω0 frequency component, while other frequency components are still strong negative feedback. Therefore, the sinusoidal signal generator can be constructed by canceling the positive feedback network. Its practical circuit is shown in Figure 3.