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Published byElisabeth Gordon Modified over 4 years ago

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**Lecture 3 Oscillator Introduction of Oscillator Linear Oscillator**

Wien Bridge Oscillator RC Phase-Shift Oscillator LC Oscillator Stability Ref: HKN EE3110 Oscillator

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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 Ref: HKN EE3110 Oscillator

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**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. Ref: HKN EE3110 Oscillator

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**Linear Oscillators Wien Bridge Oscillators RC Phase-Shift Oscillators**

LC Oscillators Stability Ref: HKN EE3110 Oscillator

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**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 Ref: HKN EE3110 Oscillator

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**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) Ref: HKN EE3110 Oscillator

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**Wien Bridge Oscillator**

Frequency Selection Network Let and Therefore, the feedback factor, Ref: HKN EE3110 Oscillator

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**For Barkhausen Criterion, imaginary part = 0, i.e.,**

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

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**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, Ref: HKN EE3110 Oscillator

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**RC Phase-Shift Oscillator**

Using an inverting amplifier The additional 180o phase shift is provided by an RC phase-shift network Ref: HKN EE3110 Oscillator

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**Applying KVL to the phase-shift network, we have**

Solve for I3, we get Or Ref: HKN EE3110 Oscillator

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**Hence the transfer function of the phase-shift network is given by,**

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, Ref: HKN EE3110 Oscillator

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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 Ref: HKN EE3110 Oscillator

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**For the equivalent circuit from the output**

Therefore, the amplifier gain is obtained, Ref: HKN EE3110 Oscillator

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**If the impedance are all pure reactances, i.e.,**

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, Ref: HKN EE3110 Oscillator

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**Hartley Oscillator Colpitts Oscillator Ref:06103104HKN**

EE3110 Oscillator

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**Colpitts Oscillator 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 Ref: HKN EE3110 Oscillator

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**Apply KCL at node 1, we have**

where, Apply KCL at node 1, we have For Oscillator V must not be zero, therefore it enforces, Ref: HKN EE3110 Oscillator

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**Imaginary part = 0, we have**

Real part = 0, yields Ref: HKN EE3110 Oscillator

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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. Ref: HKN EE3110 Oscillator

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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 Ref: HKN EE3110 Oscillator

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**Wien-bridge oscillator with bulb stabilization**

Ref: HKN EE3110 Oscillator

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**Wien-bridge oscillator with diode stabilization**

Ref: HKN EE3110 Oscillator

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Twin-T Oscillator Ref: HKN EE3110 Oscillator

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Bistable Circuit Ref: HKN EE3110 Oscillator

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**A Square-wave Oscillator**

Ref: HKN EE3110 Oscillator

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