What is a Wien Bridge Oscillator?

A Wien Bridge network (Figure 1a), which consists of four arms connected in a bridge fashion, is the foundation of the type of phase-shift oscillator known as a Wien Bridge oscillator. Here, two of the arms are solely resistive while the other two are capacitors and resistors combined. In particular, the series connection of R1 and C1 on one arm and the parallel connection of R2 and C2 on the other arm.

This shows that the behavior of these two network arms is the same as that of a high pass filter or low pass filter, mimicking the circuit’s behavior as shown in Figure 1b.

The reactance of the capacitors C1 and C2 in this circuit will be much lower at high frequencies, which will cause the voltage V0 to become zero as R2 is shorted. The reactance of the capacitors C1 and C2 will then increase significantly at low frequencies. The output voltage V0 will still be zero even in this scenario because the capacitor C1 would be acting as an open circuit. The Wien-Bridge network behaves in a way that, for low and high frequencies, respectively, makes it a lead-lag circuit.

Wien Bridge Oscillator Frequency Calculation

However, between these two high and low frequencies, there is a specific frequency at which the resistance and capacitive reactance values will equalize, resulting in the highest output voltage.

Resonant frequency is the name given to this frequency. The following formula is used to determine a Wein Bridge Oscillator’s resonant frequency: Additionally, at this frequency, the magnitude of the output voltage will be equal to one-third of the input value, and the phase-shift between the input and the output will become zero. Additionally, it is evident that only at this specific frequency will the Wien-Bridge be balanced.

The Wien-Bridge network from Figure 1 will be used in the feedback path for the Wien-Bridge oscillator, as shown in Figure 2. Below is a schematic for a Wein oscillator that uses a BJT (Bi-polar Junction Transistor):

Circuit Diagram using BJT

The amplifier section of these oscillators will be a two-stage amplifier made up of the transistors Q1 and Q2, with the output of Q2 being fed back into Q1 as an input via a Wien-Bridge network (shown in the blue enclosure in the figure). Here, the noise in the circuit will alter Q1’s base current,. Which will then change at its collector point after being amplified with a 180-degree phase shift. This appears with an additional 180o phase shift after being amplified and fed as an input to Q2 via C4.

As a result, the signal fed back to the Wien-Bridge network has a net phase difference of 360 degrees,. Satisfying the phase-shift requirement for sustained oscillations. The Wien-Bridge oscillators will be extremely frequency selective as a result, resulting in a frequency-stabilized design. However, this condition will only be met in the case of resonant frequency. Figure 3 illustrates how Op-Amps can be used in the amplifier section of Wien-bridge oscillators. However, it should be noted that because the Wien-Bridge network offers no phase-shift in this situation,. The Op-Amp must function as a non-inverting amplifier.

Furthermore, it is clear from the circuit that both inverting and non-inverting input terminals receive a feedback signal from the output voltage. The voltages applied to the inverting and non-inverting terminals will be equal. And in phase with one another at resonant frequency. However, even in this case, oscillations must begin with a voltage gain of the amplifier greater than 3. And must continue with a gain of 3 or greater. These Op-Amp-based Wien Bridge Oscillators typically have open-loop gain restrictions that prevent them from operating above 1 MHz.

Circuit using op-amp

Low frequency oscillators called Wien-Bridge networks are used to produce audio and sub-audio frequencies between 20 Hz and 20 KHz. Additionally, they offer a wide range of frequencies of stabilized, minimally distorted sinusoidal output. That can be chosen using decade resistance boxes. Additionally, by simply changing the values of capacitors C1 and C2,. It is possible to change the oscillation frequency in this type of circuit quite easily. However, these oscillators can only be used up to a certain maximum frequency. And require a substantial number of circuit components.