Twin-T is one of several method of build a sine wave oscillator. Here it is shown how to make a twin-T oscillator with LM741 or uA741 operational amplifier. Any oscillators needs two parts- an amplifying part and feedback part. These two parts of oscillators are connected in positive feedback path. Here the amplifying part is made using LM741 operational amplifier. And the feedback part is made using the twin-T notch filter.
The following shows the circuit diagram showing how to make the twin-T oscillator with LM741 operational amplifier.
In the above circuit, the amplifying part of the oscillator is the LM741 op-amp, RV2 and resistor R6. The feedback part, the twin-T filter is made up of the R1,R2,C3 and C1,C2,R3 and RV1. In twin-T filter, the value of resistor R3 is half of the value of resistors R1 and R2. So here R1=R2=10Kohm so the value of R3 is ideally 5KOhm but since practically 5KOhm resistor is not available the value resistor RV1 is used to set the value of 5KOhm. Similarly, the value of C3 is equal to half of value of C1 and C2. Since the value of C1=C2=10nF the value of C3 is 20nF. The output signal from the operational amplifier enters the twin-T filter and reenters the operational amplifier via the positive terminal. So this is a positive feedback scheme as required for oscillator. The frequency of oscillation of the twin-T oscillator is set by these resistors and capacitors which is given by,
\[f_{r} = \frac{1}{2 \pi R C}\]
where, \(f_r\) is the resonant frequency of the oscillator, R = R1=R2 and C=C1=C2.
The following is twin-T oscillator calculator that calculates the resistor value for given resonant frequency and capacitor.
One must note that one of the disadvantage of using twin-T oscillator is that it oscillates only at a specific frequency.
If the gain of the operational amplifier is A and the feedback fraction is \(\beta\) then the condition for oscillation is,
\[A \beta \geq 1\]
The gain is set by the resistor R6 andRV2. Since this operational amplifier configuration used here is inverting amplifier the gain equation is,
\[A = -\frac{RV_1}{R_6}\]
Here the LM741 is operated with single power supply of 5V. The equal 10KOhm resistors R4 and R5 creates a bias of 2.5V at the negative terminal of the operational amplifier.
The following shows animation of how the twin-T oscillator works.
Here twin-T oscillator design with LM741 op-amp was illustrated. Other sine wave oscillator are as follows.
See other oscillator tutorials below.
- 2N2222 Phase Shift Oscillator
- Practical Colpitts Oscillator on Breadboard
- Colpitts Oscillator Design and Experiment on Breadboard
Colpitts Oscillator Simulation in Proteus