Mumbai University > EXTC > Sem 3 > Analog Electronics I

Marks: 7 M

Year: May 2014

• Twin T-oscillator is an RC oscillator consists of Twin T-network and an op-amp.
• Twin-T circuit is also known as twin-t filter it is basically lead-lag network whose phase changes according to the frequency. It is parallel connection of lead-T network and lag T network.
• Twin-T is actually combination of low pass and high pass filter combined parallel response provides a band-stop filter with a center frequency equal to $$fr=\frac{1}{2πRC}$$
• This is resonant frequency, Oscillations can occur only at this frequency as the filter provides significant negative feedback at frequencies below or above the resonant frequency which is not good for oscillation.

$$\text{Fig1 (a) Twin-T oscillator circuit (b) Frequency response curve}$$

• At resonant frequency the negative feedback is negligible thus positive feedback through $R_1$ and $R_2$ allows circuit to oscillate.
• In lag network the phase angle tends to -90º and in lead network the phase angle tends to +90º and at resonant frequency the phase angle of lag network is balanced by phase angle of lead network and resultant wave have phase angle of zero.
• The gain of twin-T oscillator is 1 at high and low frequencies and at f=fr, the gain reduces to zero. So it notches out frequencies near the resonant frequency and hence it is also known as “notch filter”
• The positive feedback is provided to circuit through voltage divider of $R_1$ and $R_2$
• The resistance $R_2$ is lamp resistance whose resistance is proportional to current at start the current is low hence the resistance of $R_2$ is minimum and maximum current will pass through $R_2$ in other words positive feedback is maximum. This helps to build the oscillations. As oscillation grows the resistance of $R_2$ increases thus current passing through $R_2$ decreases thus decreasing the positive feedback. This controls the growing oscillations and makes them as sustained oscillation. Thus lamp helps in amplitude stabilization
• This oscillator provides good frequency stability
• But this oscillator is rarely used because it is operated only at one frequency