A twin-T oscillator is a RC oscillator that employs a twin-T-type of notch filter network as the frequency selective component in the feedback network as shown in the figure below. It employs both positive as well as negative feedback. The positive feedback necessary to produce oscillations is provided by a voltage divider network of R1 and R2. The negative feedback is through the frequency selective twin-T network.
What is a twin-T oscillator?
Twin-T network is a parallel connection of a lag-type T-network (lower T-network in figure of Q28) and a lead-type T-network (upper T-network in figure of Q28). The lag-type T-network causes the magnitude of transfer function to fall and the lagging phase shift angle to increase with increase in frequency. On the other hand, the lead-type T-network causes the magnitude of transfer function to increase and the leading phase shift angle to decrease with increase in frequency. At ɷ = 1/RC, the two T-networks counter-balance each other with the result that both the magnitude as well as the phase of the transfer function tend to become zero. Also, while lagging phase angle tends to become -90o, the leading phase angle tends to become +90o. Figure below shows the amplitude and phase response of the twin-T network as a function of normalized frequency. Oscillation frequency ɷc equals 1/RC.
(a)
(b)
(a) Magnitude; (b) phase response of twin-T network
The magnitude of T(s) is
The phase angle of T(s) is
Substituting ɷ = 1/RC in the above equation, we get the magnitude of transfer function as zero and the phase angle as either -90o or +90o. Zero amplitude for the transfer function implies zero negative feedback at ɷ = 1/RC. At all other frequencies, there will be very high negative feedback thus allowing the circuit to oscillate only very close to the notch frequency.
When the current through R2 is less initially, a low value of R2 produces a large amount of positive feedback to initiate oscillations. Once the oscillations build up, the current flowing through R2 heats it and thus raises its temperature. Increase in temperature increases the value of R2 thus reducing the positive feedback to stabilize the magnitude of oscillations. An incandescent lamp may also be used in place of thermistor. The positive temperature coefficient of resistance of tungsten filament used in incandescent lamp is used for the purpose.
Twin-T oscillator produces a low distortion sinusoidal output because of the following two reasons.
• First, harmonics are subjected to a very high level
of negative feedback, thus severely attenuating them.
• Second, the operating point of the oscillator is very delicately balanced between positive and negative feedback. This necessitates a very small amount of non-linearity to stabilize the amplitude.