Twin-T Oscillator

The Twin-T Oscillator is another RC oscillator circuit that uses two parallel connected RC networks to produce the sinusoidal output waveform of a single frequency.

Twin-T Oscillators are another type of RC oscillator that produces sinus wave output for use in fixed frequency applications similar to wein-bridge oscillator. The Twin-T oscilator uses two "T" shaped RC networks in the feedback loop (hence the name) between the output and input of an inverter amplifier.

As we can see, oscillators are basically a positive feedback amplifier with a constant voltage gain required to maintain oscillations, in the same way that the twin-T oscillator is made to perform this task. Feedback is provided by the Twin-T configured RC network, which allows part of the output signal to be fed back to the amplifier's input terminal. Thus the twin-T RC network provides 180o phase shifts, and the amplifier provides another 180o phase shift. These two conditions create a total of 360 o phase shiftsthat allow continuous oscillations.

Unlike the typical RC Phase Shift Oscillator, which configures feedback resistors and capacitors into a stair network, or the standard Wien-bridge Oscillator that uses resistors and capacitors in a bridge configuration, the twin-T oscillator (sometimes known as parallel) uses a passive resistance-capacitance network (RC) that is connected to each other in parallel (where R and C elements have opposite formation).

twin-t oscillator
Twin-T Network

Clearly, we can see that one of the RC passive networks has a low migration response, while the other has a high migration response, and we have seen this RC network editing before in our tutorial on Notch Filter. The difference this time is that we use unified parallel RC T-structured networks to produce a notch type response with a center frequency equal to the desired empty oscillation frequency.

As a result, oscillations cannot occur at frequencies above or below the set notch frequency due to the negative feedback path created through the Twin-T network. However, any negative feedback at the set frequency becomes negligible, so it allows the positive feedback path created by the amplifier to dominate the creation of oscillations at a single frequency (unlike the Wien bridge oscillator can be adjusted in a wide frequency range).

The frequency-selector twin-T network of the Twin-T oscillator produces an output transfer function determined by the frequency, depth and phase shifting of the notch. Thus, the individual twin-T networks that make up the RC network are defined by the following equations:

For a Low Pass R-C-R network:

twin-t oscillator

For the High Pass C-R-C network:

twin-t oscillator

Combining these two groups of equations will give us the final equation for the zero or central frequency of the notch, which results in oscillations for a twin-T network.

twin-t oscillator

εC is the frequency of oscillations in Hertz.
R is feedback resistance in Ohm C is
feedback capacitance in Persian
π (pi) is a constant with a value of approximately 3,142

After determining the twin-T network for the oscillatorproducing the required 180 o phase shifts consisting of -90oto +90 o at the empty frequency (as opposed to zero to 180o for the Wien-bridge oscillator), it is best applied by combining the RC feedback network with a transactional amplifier to achieve a voltage gain, since op-amps tend to work better with this type of oscillator compared to transistors due to their high input impedance properties.

Twin-T Amplification

Standard transactional amplifiers can provide high voltage gain, high input impedance as well as low output impedance, and therefore are excellent amplifiers for twin-T oscillators. At the oscillation frequency, the εc feedback gain drops to almost zero, so we need an amplifier with a voltage gain much larger than one (union).

The positive feedback required for oscillation is provided by the feedback resistance R1, while the resistance allows R2 initiation. As a general rule, the ratio of these two resistances must be more than a hundred (>100) to ensure that the circuit oscillates as close to the required frequency as possible.

To achieve the necessary positive gain in the oscillation frequency, we can use an inverted amplifier configuration in which a small part of the output voltage signal is applied directly to the inverted ( + ) input terminal via an appropriate voltage dividing network. The negative feedback generated by the Twin-T oscillator circuit is connected to the inverted ( – ) input terminal. This closed loop configuration produces a non-inverted oscillator circuit with very good stable, very high input impedance and low output impedance, as shown.

twin-t oscillator
Twin-T Oscillator Circuit

Next, we can see that the twin-T oscillator receives positive feedback to the non-inverted input through the voltage dividing network and negative feedback via the twin-T RC network. To allow the circuit to vibrate at the required single frequency, the "T-leg" resistance R/2 may be an adjustable trimmer pocinciometer, but it must also be adjusted to compensate for capacitor tolerances for the circuit to oscillate at startup.

Twin-T Oscilator Question Example 1

A twin-T oscillator circuit is required to produce a 1kHz sinusoidal output signal in an electronic circuit. If a transactional amplifier with an earnings ratio of 200 is used, calculate the values of frequency determination components R and C and the values of gain resistances.

The oscillation frequency will be 1kHz, if we choose a reasonable value for the two feedback resistances, we can calculate the required capacitor value using the formula for the frequency R 10kΩ (note that these two resistances must have the same values).

twin-t oscillator

Thus R = 10kΩ and C = 16nF. The central T-leg capacitor is 2C = 2 x 16nF = 32nF, so the nearest preferred value of 33nF is used.

Since the value of the high transition capacitor is 33nF and therefore does not exactly equal 2C (2 x 16nF), we can adjust this variation and adjust the low-pass capacitor to ensure that oscillations are started correctly. Thus, the exact value of the R(leg) will be 10kΩ/2 = 5kΩ, but the calculated value of this resistance is given as follows: R(leg) = R/(33nF/16nF) = 4.85kΩ. Then the use of 5kΩ trim-pot will meet our requirements in this example.

The cycle gain of the process amplifier must be 200, so if we choose 1kΩ for R2, the R1 resistance will be 200kΩ, as shown.

twin-t oscillator
Final Twin-T Oscillator Circuit


In this course, we found that twin-T Oscillator circuits can be easily created using some passive components and a transactional amplifier. The twin-T oscillator circuit uses an RC network set for the feedback circuit to produce the required sinusoidal output waveform. As two T-networks connected in parallel, they work in the anti-phase to each other, creating zero output at zero frequency, but a finous output at all other frequencies.

As a result, the circuit will not be released at frequencies above or below the set frequency due to negative feedback over the twin-T RC network. Therefore, at the empty frequency, the voltage at the non-inverted inlet of the op-amp is in the same phase as the output voltage, causing continuous oscillations at the desired frequency.

To ensure that the oscillation frequency is as close to the empty frequency as possible, a trim-pot can be used in the T-leg resistance of the low transition stage to balance the RC network for initiation and the purity of the output waveform as one of the following. The biggest disadvantage of the "Twin-T oscillator" is that the oscillation frequency and quality of the output waveform depends greatly on the interaction of resistors and capacitors in the twin-T network, in which case the values and selection of these components should be done correctly.