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Lecture 3 Oscillator. Introduction of Oscillator Linear Oscillator Wien Bridge Oscillator RC Phase-Shift Oscillator LC Oscillator Stability. Oscillators. Oscillation: an effect that repeatedly and regularly fluctuates about the mean value Oscillator: circuit that produces oscillation

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Lecture 3 Oscillator


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    1. Lecture 3 Oscillator • Introduction of Oscillator • Linear Oscillator • Wien Bridge Oscillator • RC Phase-Shift Oscillator • LC Oscillator • Stability EE3110 Oscillator

    2. Oscillators Oscillation: an effect that repeatedly and regularly fluctuates about the mean value Oscillator: circuit that produces oscillation Characteristics: wave-shape, frequency, amplitude, distortion, stability EE3110 Oscillator

    3. Application of Oscillators • Oscillators are used to generate signals, e.g. • Used as a local oscillator to transform the RF signals to IF signals in a receiver; • Used to generate RF carrier in a transmitter • Used to generate clocks in digital systems; • Used as sweep circuits in TV sets and CRO. EE3110 Oscillator

    4. Linear Oscillators • Wien Bridge Oscillators • RC Phase-Shift Oscillators • LC Oscillators • Stability EE3110 Oscillator

    5. Integrant of Linear Oscillators For sinusoidal input is connected “Linear” because the output is approximately sinusoidal A linear oscillator contains: - a frequency selection feedback network - an amplifier to maintain the loop gain at unity EE3110 Oscillator

    6. Basic Linear Oscillator and If Vs = 0, the only way that Vo can be nonzero is that loop gain A=1 which implies that (Barkhausen Criterion) EE3110 Oscillator

    7. Wien Bridge Oscillator Frequency Selection Network Let and Therefore, the feedback factor, EE3110 Oscillator

    8.  can be rewritten as: For Barkhausen Criterion, imaginary part = 0, i.e., Supposing, R1=R2=R and XC1= XC2=XC, EE3110 Oscillator

    9. Example By setting , we get Imaginary part = 0 and Due to Barkhausen Criterion, Loop gain Av=1 where Av : Gain of the amplifier Wien Bridge Oscillator Therefore, EE3110 Oscillator

    10. RC Phase-Shift Oscillator • Using an inverting amplifier • The additional 180o phase shift is provided by an RC phase-shift network EE3110 Oscillator

    11. Applying KVL to the phase-shift network, we have Solve for I3, we get Or EE3110 Oscillator

    12. The output voltage, Hence the transfer function of the phase-shift network is given by, For 180o phase shift, the imaginary part = 0, i.e., Note: The –ve sign mean the phase inversion from the voltage and, EE3110 Oscillator

    13. LC Oscillators • The frequency selection network (Z1, Z2 and Z3) provides a phase shift of 180o • The amplifier provides an addition shift of 180o Two well-known Oscillators: • Colpitts Oscillator • Harley Oscillator EE3110 Oscillator

    14. For the equivalent circuit from the output Therefore, the amplifier gain is obtained, EE3110 Oscillator

    15. The loop gain, If the impedance are all pure reactances, i.e., The loop gain becomes, The imaginary part = 0 only when X1+ X2+ X3=0 • It indicates that at least one reactance must be –ve (capacitor) • X1 and X2 must be of same type and X3 must be of opposite type With imaginary part = 0, For Unit Gain & 180o Phase-shift, EE3110 Oscillator

    16. Hartley Oscillator Colpitts Oscillator EE3110 Oscillator

    17. Colpitts Oscillator Equivalent circuit In the equivalent circuit, it is assumed that: • Linear small signal model of transistor is used • The transistor capacitances are neglected • Input resistance of the transistor is large enough EE3110 Oscillator

    18. At node 1, where, Apply KCL at node 1, we have For Oscillator V must not be zero, therefore it enforces, EE3110 Oscillator

    19. Imaginary part = 0, we have Real part = 0, yields EE3110 Oscillator

    20. Frequency Stability • The frequency stability of an oscillator is defined as • Use high stability capacitors, e.g. silver mica, polystyrene, or teflon capacitors and low temperature coefficient inductors for high stable oscillators. EE3110 Oscillator

    21. Amplitude Stability • In order to start the oscillation, the loop gain is usually slightly greater than unity. • LC oscillators in general do not require amplitude stabilization circuits because of the selectivity of the LC circuits. • In RC oscillators, some non-linear devices, e.g. NTC/PTC resistors, FET or zener diodes can be used to stabilized the amplitude EE3110 Oscillator

    22. Wien-bridge oscillator with bulb stabilization EE3110 Oscillator

    23. Wien-bridge oscillator with diode stabilization EE3110 Oscillator

    24. Twin-T Oscillator EE3110 Oscillator

    25. Bistable Circuit EE3110 Oscillator

    26. A Square-wave Oscillator EE3110 Oscillator