An Oscillator is a circuit which produces a periodic waveform at its output with only the dc supply voltage at the input. The output voltage can be

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2 An Oscillator is a circuit which produces a periodic waveform at its output with only the dc supply voltage at the input. The output voltage can be either sinusoidal or non sinusoidal depending upon the type of oscillator. Two major classifications of oscillators are Feedback and Relaxation oscillators.

4 Feedback Amplifier consists of an amplifier for gain and a feedback circuit for attenuation and phase shift.

5 Relaxation oscillator uses an RC timing circuit and a device that changes states to generate a periodic waveform A Triangular Wave Oscillator

6 Conditions for Oscillation Phase shift around the feedback loop must effectively be 0 degree. Loop gain must equal unity.

8 Start up Conditions Unity gain condition must be maintain to obtain sustained oscillations. Loop gain in the beginning must be greater than unity to acquire the desired level of oscillation. Once the desired level is achieved, the gain should reduce to unity for sustained oscillations. This reduction in gain is achieved by automatic gain control circuits (AGC). Initial voltage for positive feedback is provided by thermal noise produced in resistors or from other components or from power supply turn on transients.

9 Feedback circuit permits only that frequency which equals the resonant frequency to appear in phase at the input.

10 Oscillators with RC Feedback RC Feedback Oscillators are generally used for frequencies up to 1 MHz. Wien-Bridge, Phase-Shift and Twin-T Oscillators are the examples Wien-Bridge is the most commonly used one among these

11 The Wien-Bridge Oscillator

12 Fundamental part of Wien-Bridge Oscillator is Lead-Lag circuit. R 1 and C 1 together form the lag circuit. R 2 and C 2 form the lead circuit. At low frequencies, the lead circuit dominates due to the high reactance of C 2 As the frequency increases,xc 2 decreases,thus allowing the output voltage to increase. At some specified frequency, the lag circuit takes over, and the decreasing value of Xc 1 causes the output voltage to decrease.

13 RC Lag Circuit An RC lag circuit is a phase shift circuit in which the output voltage lags the input voltage by a specified amount OR

14 RC Lead Circuit An RC lead circuit is a phase shift circuit in which the output voltage leads the input voltage by a specified amount

15 At the resonant frequency If R 1 =R 2 and Xc 1 =Xc 2 then Below f r, the lead circuit dominates and output leads the input Above f r, the lag circuit dominates and output lags the input Lead Lag circuit is used in the positive feedback loop of an op amp

16 The two conditions for sustained oscillation are met as shown on the next slide

17 To achieve a closed loop gain of 3

18 Start up Condition

Achieving Sustained Oscillation Initially zener diode appear as open when dc supply is first applied R3 now comes in series with R1,increasing the closed loop gain as When the output signal

19 Achieving Sustained Oscillation Initially zener diode appear as open when dc supply is first applied R3 now comes in series with R1,increasing the closed loop gain as When the output signal level reaches the zener breakdown voltage, zeners start conducting while shorting R3 and hence reducing the closed loop gain to 3. Loop gain is now unity and oscillations are thus sustained

20 This circuit is not very efficient since it suffers from nonlinearities present in zener diodes It is difficult to achieve undistorted sinusoidal output using the above circuit. Another method of AGC uses JFET in the negative feedback loop as a voltage controlled resistor.

21 This method can produce an excellent sinusoidal waveform that is stable A JFET operating with a small or zero V DS operates in the ohmic region As the gate voltage increases, the drain to source resistance increases This change in resistance is used for automatic gain control (AGC) With no output signal, gate is at zero potential and rds is minimum and loop gain is greater than 1 Negative excursions of the output forward bias the diode causing the capacitor C 3 to charge to a negative voltage This increases the source to drain resistance and reduces the gain and hence the output. Try Example 16.1 of Floyd

The Phase Shift Oscillator Each of the three RC circuits in the feedback loop can provide a maximum of 90 degree phase shift, but each is designed to provide 60 degree phase shift Total

22 The Phase Shift Oscillator Each of the three RC circuits in the feedback loop can provide a maximum of 90 degree phase shift, but each is designed to provide 60 degree phase shift Total phase shift provided by the three RC circuits is thus 180 degree. Another 180 degree is provided by the inversion of the op amp; hence making it 360 degree(0 degree) in total around the loop.

23 A series RC circuit shifts the phase of the output voltage by an amount that depends upon the values of R and C and the frequency of the signal. Generally the requirement is to provide a phase shift of 180 degrees in the feedback path A single RC lag circuit is limited to phase shifts that are smaller than 90 degree

24 The attenuation provided by the three section RC feedback circuit is B=1/29 Where B=R 3 /R f (Derivation given in Appendix-B, Equation 16-3) When

25 Twin-T Oscillator Two T-type RC filters used in the feedback loop One has the low pass response and the other has the high pass response The combined parallel filters produce a band stop or notch response with a center frequency equal to f r

27 Oscillation cannot occur at frequencies above or below f r because of the negative feedback through the filters At f r,negative feedback is negligible and the positive feedback provides oscillation The Band Stop circuit can also be implemented as shown on the next slide

28 At the resonant frequency, the impedance is maximum

Lecture # 11 Oscillators (RC Circuits)

December 2014 Benha University Faculty of Engineering at Shoubra ECE-312 Electronic Circuits (A) Lecture # 11 Oscillators (RC Circuits) Instructor: Dr. Ahmad El-Banna Agenda Introduction Feedback Oscillators