An oscillator consists of an amplifier and a feedback network
1)Active device ie opamp is used as an amplifier
2) Passive components such as R-C or L-C combinations are used as feed back network
To start the oscillation with the constant amplitude positive feedback is not the only sufficient condition Oscillator circuit must satisfy the following two conditions known as Barkhausen conditions
I The fiest condition is that the magnitude of the loop gain (AB) = 1
A = Amplifier gain and B = Feedback gain
Ii The second condition is that the phase shift around the loop must be 360 or 0 The feedback signal does not produce any phase shift This is the basic principle of a Wien bridge oscillator
Lead-Lag circuit
The given circuit shows the Rc combination used in Wien bridge oscillator This circuit is also known as lead-lag circuit Here resistor R1 and capacitor C1 are connected in the series while resistor R2 and capacitor C2 are connected in parallel
Woeking of lead-lag circuit
At high frequencies the reactance of capacitor C1 and C2 approaches zero This causes C1 and C2 appears short Here capacitor C2 shorts the resistor R2 Hence the output voltage Vo will be zero since output is taken across R2 and C2 combination So at high frequencies circuit acts as a lag circuit
At low frequencies both capacitors act as open because capacitor offers very high reactance Again output voltage will be zero because the input signal is dropped across the R1 and C1 combination Here the circuit acts like a lead circuit
But at one particular frequency between the two extreames the output voltage reaches to the maximum value At this frequency only resistance value becomes equal to capacitive reactance and gives maximum output Hence this particular frequency is known as resonant frequency or oscillator frequency