Earlier we have studied about oscillator and its types. Here we are going to cover Wein Bridge Oscillator, its circuit with and without using op-amp. In our previous articles, we have explained LC oscillators i.e. Hartley oscillator and Colpitts oscillator and Crystal oscillator in detail.
What is Wein Bridge Oscillator: Definition
According to Wikipedia:
A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies.
Now let us learn definition in detail:
A Wein bridge oscillator is an oscillator that is based on bridge circuit basically used to generate sine waves at a large range of frequencies. It is a two-stage RC coupled amplifier circuit that has low distortion and good stability at its resonant frequency. For the measurement of impedances, it was developed by Max Wein in 1891. It is one of the most demandable Oscillators used in an audio or sub-audio frequency ranging from 20-20KHz.
Wein Bridge Oscillator Circuit and Working
As shown in the circuit diagram above, it is essentially a two-stage amplifier with an RC bridge circuit. In this bridge circuit, R1 connected in series to C1 form one of the four arms and R2, R3, R4 connected in parallel to C2 form the other 3 arms of the bridge circuit.
In the above circuit; transistor Q1 serves as an oscillator and an amplifier while transistor Q2 acts as an inverter to cause a phase shift of 180o. Resistors R3 and R4 are used to stabilize the amplitude of the output. At the input of the transistor Q1, the negative feedback is applied through the voltage divider and positive feedback is through R1, C1, R2, and C2.
In this circuit, the Wein bridge feedback network is added to make the oscillator sensitive to a signal of only one particular frequency. If this feedback network is not employed than for producing oscillations the output of transistor Q2 is fed back to transistor Q1 for providing regeneration.
The transistor Q1 will amplify signals over a wide range of frequencies and so the direct coupling will result in poor frequency stability. Thus by employing a Wein bridge feedback network frequency stability is increased.
It comprises of Feedback circuit consisting of a series RC circuit connected with a parallel RC of the same component value producing a phase delay depending upon the frequency. It employs two transistors each producing a phase shift of 180o, causes a total phase shift of 360 o or 0 o and ensures proper positive feedback. To ensure a constant output over a range of frequencies, negative feedback is provided in the circuit.
This can be achieved by taking resistor R4 in the form of a temperature-sensitive lamp whose resistance increase with the increase in current.
As shown in the figure, the amplifier voltage gain is given as:
A = (R3 + R4) / R4 = 3
= (R3 / R4) + 1 = 3
= R3 / R4 = 2
Since R3 = 2 R4 corresponds with a feedback network attenuation of 1/3 indicates that the voltage gain of the amplifier circuit must be equal to or greater than three for oscillations to start.
Wein Bridge oscillator using op-amp:
In the above section, we have learned about simple Wein bridge oscillator circuit. This section will cover the Wein bridge circuit using op-amp. Here op-amp works as a non-inverting amplifier.
As shown in Figure below, the Wein bridge circuit consists of a series RC network in one arm and parallel RC network in the adjoining arm, Ri & Rf are connected in the remaining arm. The op-amp used in this oscillator circuit is working as a non-inverting amplifier mode.
Components required for this circuit are a capacitor, potentiometer, resistor, and an operational amplifier.
In this oscillator circuit, the feedback signal is connected to the non-inverting input terminal so that the op-amp works as a non-inverting amplifier. For sustained oscillations, zero phase shift is essential across the circuit which is achieved by balancing the bridge.
At the resonance frequency, the inverting and non-inverting voltages will be equal and in phase so that the negative feedback signal will be canceled out by the positive feedback causing the circuit to oscillate. The resonant frequency of the balanced bridge represents the frequency of oscillations given as:
fo = 1 / 2∏ RC
For an inverting amplifier, the gain is set by the feedback resistor network Ri & Rf and is given by the ratio of -Rf / Ri and also represents that for sustained oscillations amplifier must have a gain of 3 so that the loop gain become unity.
Advantages and Disadvantages
Wein bridge oscillator is an electronic oscillator that is used in many applications. Advantages of Wein Bridge Oscillator are
- By using decade resistance boxes, the frequency range can be selected easily.
- Over a wide range of frequency, it provides a stable low distortion sinusoidal output.
- By varying capacitances C1 and C2 simultaneously, the frequency of oscillation can be easily varied.
Like a coin has two sides, similarly, there are some problems that occur while using these type of oscillators. Disadvantages of Wein Bridge Oscillator are
- The maximum frequency output is limited because of the amplitude and phase shift characteristics of the amplifier.
- The design is bulky because circuit requires two transistors and a large number of other electronic components.
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