Twin-T Oscillator: Working Principle, Twin-T Network, Frequency Formula & Applications

Introduction

The Twin-T Oscillator is an RC oscillator that generates a sinusoidal output signal and is mainly used in fixed-frequency applications. Similar to the Wien Bridge Oscillator, it employs resistors and capacitors instead of inductors to determine the oscillation frequency.

The Twin-T Oscillator uses two “T-shaped” RC networks connected in parallel within the feedback loop of an inverting amplifier. Because of this configuration, it is also known as the Parallel-T Oscillator.

The oscillator works on the principle of positive feedback. A specific amplifier gain is required to sustain oscillations, and the Twin-T RC network provides the necessary frequency-selective feedback.

Working Principle of Twin-T Oscillator

In a Twin-T Oscillator, the RC feedback network introduces a phase shift of 180°. The inverting amplifier contributes an additional 180° phase shift. Together, these two phase shifts result in a total phase shift of 360°, which satisfies the phase condition for sustained oscillations.

Unlike the Wien Bridge Oscillator, which uses a bridge configuration, or the RC Phase Shift Oscillator, which uses a ladder network, the Twin-T Oscillator uses two interconnected “T” networks arranged in parallel.

Twin-T Network

The Twin-T network consists of two RC networks connected in parallel:

• A low-pass R-C-R network
• A high-pass C-R-C network

These two networks are configured with their resistive and capacitive elements in opposite arrangements. This structure is similar to a notch filter, where a specific frequency is rejected.

At frequencies above or below the tuned notch frequency, the Twin-T network provides negative feedback, preventing oscillations. However, at the tuned frequency, the negative feedback is minimized, allowing the amplifier’s positive feedback to dominate and generate oscillations at a single frequency.

Frequency-Selective Behavior

The depth, phase shift, and center frequency of the notch are determined by the component values used in the Twin-T network. The network produces a deep notch at the desired oscillation frequency.

Low-Pass R-C-R Network

This section allows low-frequency signals to pass while attenuating high-frequency signals.

High-Pass C-R-C Network

This section allows high-frequency signals to pass while attenuating low-frequency signals.

Frequency of Oscillation

The null or center frequency at which oscillations occur is given by:

f_c = 1 / (2πRC)

Where:

• f_c = Frequency of oscillation (Hz)
• R = Feedback resistance (Ohms)
• C = Feedback capacitance (Farads)
• π ≈ 3.142

Amplifier Requirement

After determining the Twin-T RC network that provides the required 180° phase shift, an amplifier is used to supply voltage gain and an additional 180° phase shift.

The phase shift of the Twin-T network occurs around the null frequency between −90° and +90°, unlike the Wien Bridge Oscillator, which operates between 0° and 180°.

Operational amplifiers are preferred for Twin-T Oscillator circuits due to their high input impedance and stable performance. They generally provide better results than transistor-based designs.

Twin-T Oscillator Circuit Operation

In the Twin-T Oscillator circuit, positive feedback is supplied through a voltage divider to the non-inverting input of the operational amplifier. Negative feedback is provided through the Twin-T RC network.

A resistor in the “T-leg” of the network, typically having a value of R/2, is often implemented using a trimmer potentiometer. This allows fine adjustment of the oscillation frequency and compensates for capacitor tolerances, ensuring reliable startup of oscillations.

Advantages of Twin-T Oscillator

• Simple RC-based design
• No inductors required
• Produces low-distortion sine wave output
• Suitable for fixed-frequency applications

Applications

• Audio signal generators
• Notch-filter-based oscillators
• Instrumentation circuits
• Test and measurement equipment