In the block diagram of figure 1,
Vd = Vf + Vin
V0 = AVd
Vf = βV0
Using these relationships, the following equation is obtained:
However, Vin = 0 and V0 ≠ - implies that,
Expressed in polar form
Equation (i) & (ii) gives two requirements for oscillation:
The above conditions is known as Barkhausen criterion.
In figure 1, if the amplifier causes a phase shift of 180°, the feedback circuit must provide an additional phase shift of 180° so that the total phase shift around the loop is 360°. The type of waveform generated by an oscillator depends on the components in the circuit hence may be sinusoidal, square or triangular. In addition, the frequency of oscillation is determined by the components in the feedback circuit.
Oscillators can be of 2 types:
Because of its simplicity and stability, one of the most commonly used audio-frequency oscillators is the wein bridge. Figure 2 shows the wein bridge oscillator in which the wein bridge circuit is connected between the amplifier input terminals and the output terminal. The bridge has a series RC network in one arm and a parallel RC network in adjoining arm. In the remaining two arms of the bridge, resistors R1 and R2 are connected.
The phase angle criterion for oscillation is that the total phase shift around the circuit must be 0°. This condition occurs only when the bridge is balanced, that is at resonance. the frequency of oscillation f0 is exactly the resonant frequency of the balanced wein bridge and is given by:
Assuming that the resistors are equal in value, and capacitors are equal in value in the reactive leg of the wein bridge. At this frequency the gain required for sustained oscillation is given by
A = 1/β = 3
That is
or
Drawback of Colpitts oscillator
f0 is unstable due to the effect of internal capacitances.
net input capacitance Ci appears in parallel with C1 and net output capacitance C0 appears in parallel with C2. As a result.
where, C1 = C1 + Ci ; C2 = C2 + C0.
Since, Ci and C0 vary with time and temperature, f0 becomes unstable. To eliminate this drawback an additional capacitance C3 can be connected in series with inductor. Such circuit is called clapp oscillator.
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1. What is an oscillator in electronics and communication engineering? |
2. What are the different types of oscillators used in electronics and communication engineering? |
3. What is the importance of oscillators in electronics and communication engineering? |
4. How does an oscillator work? |
5. What factors affect the frequency stability of an oscillator? |
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