OSCILLATORS Dr.S.SUJA Associate Professor Department oF Electrical and Electronics Engineering, Coimbatore Institute of Technology, Coimbatore-641014. 4/27/2017 OSCILLATOR

CONTENTS OSCILLATOR BASICS TYPES OF OSCILLATORS RC PHASE SHIFT OSCILLATORS WEIN BRIDGE OSCILLATORS LC OSCILLATOR COLPITTS OSCILLATOR HARTLEY OSCILLATOR 4/27/2017 OSCILLATOR

OSCILLATOR BASICS 4/27/2017 OSCILLATOR

OSCILLATOR A change that occurs repeatedly and regularly with reference to the mean value The parameters related to oscillators are wave-shape frequency amplitude distortion stability 4/27/2017 OSCILLATOR

THE OSCILLATOR Oscillators are electronic circuits that generate an output signal (oscillating in nature) without the necessity of an input signal . produces a periodic waveform (time domain) on its output with only the DC supply voltage as an input. They convert DC power into AC signal power. Signal generation implies production of self-sustained oscillations. The output voltage can be either sinusoidal or nonsinusoidal, depending on the type of oscillator. Different types of oscillators produce various types of outputs including sine waves, square waves, triangular waves, and sawtooth waves. 4/27/2017 OSCILLATOR

Simple Oscillator Circuit Oscillators are electronic circuits that generate an output signal (oscillating in nature) without the necessity of an input signal . produces a periodic waveform on its output with only the DC supply voltage as an input. 4/27/2017 OSCILLATOR

Basic elements of a feedback oscillator Feedback oscillator operation is based on the principle of positive feedback. Feedback oscillators are widely used to generate sinusoidal waveforms. A feedback oscillator consists of an amplifier for gain (A) (either a discrete transistor or an op-amp) and a positive feedback circuit (β) that produces phase shift and provides attenuation The voltage gain around the closed feedback loop, is the product of the amplifier gain, and the attenuation, of the feedback circuit. Loop gain A β After oscillations are started, the loop gain is maintained at 1.0 to maintain oscillations. 4/27/2017 OSCILLATOR

Positive Feedback In positive feedback, a portion of the output voltage of an amplifier is fed back to the input with no net phase shift, resulting in a strengthening of the output signal. The feedback oscillator, returns a fraction of the output signal to the input with no net phase shift, resulting in a reinforcement of the output signal. After oscillations are started, the loop gain is maintained at 1.0 to maintain oscillations. 4/27/2017 OSCILLATOR

OSCILLATION The in-phase feedback voltage is amplified to produce the output voltage, which in turn produces the feedback voltage. That is, a loop is created in which the signal maintains itself and a continuous sinusoidal output is produced. This phenomenon is called oscillation. In some types of amplifiers, the feedback circuit shifts the phase and an inverting amplifier is required to provide another phase shift so that there is no net phase shift. 4/27/2017 OSCILLATOR

Necessary Conditions for Oscillation Two conditions, are required for a sustained state of oscillation: 1. The phase shift around the feedback loop must be effectively Zero. 2. The voltage gain, around the closed feedback loop (loop gain) must be equal to 1 (unity). 4/27/2017 OSCILLATOR

Conditions for Oscillation If a sinusoidal wave is the desired output, a loop gain greater than 1 will rapidly cause the output to saturate at both peaks of the waveform, producing unacceptable distortion. To avoid this, some form of gain control must be used to keep the loop gain at exactly 1 once oscillations have started. For example, if the attenuation of the feedback circuit is 0.01, the amplifier must have a gain of exactly 100 to overcome this attenuation and not create unacceptable distortion . An amplifier gain of greater than 100 will cause the oscillator to limit both peaks of the waveform. 4/27/2017 OSCILLATOR

Vo-output voltage of Amplifier Vi-input voltage of Amplifier OSCILLATOR OPERATION Vo-output voltage of Amplifier Vi-input voltage of Amplifier Vf-output voltage of feedback Amplifier 4/27/2017 OSCILLATOR

OSCILLATOR CONDITIONS NOT MET If the feedback signal is not positive or the gain is less than one, then the oscillations will dampen out. If the overall gain is greater than one, then the oscillator will eventually saturate. 4/27/2017 OSCILLATOR

Start-Up Conditions The requirements for the oscillation to start when the dc supply voltage is first turned on. the unity-gain condition must be met for oscillation to be maintained. For oscillation to begin, the voltage gain around the positive feedback loop must be greater than 1 so that the amplitude of the output can build up to a desired level. The gain must then decrease to 1 so that the output stays at the desired level and oscillation is sustained. 4/27/2017 OSCILLATOR

TYPES OF OSCILLATORS 4/27/2017 OSCILLATOR

Types of Oscillator in communication systems Most electronic communication systems operate with sources of sinusoidal electrical waves. Many classes of oscillator circuits are used to produce these sinusoids. 4/27/2017 OSCILLATOR

Types of Oscillator-Waveform Generated According to the types of waveforms produced oscillators can be classified into one of four generic types: Harmonic oscillators: used for sine-wave generation. Saw tooth oscillators: used for the generation of exponential or linear saw tooth waves. Relaxation oscillators: used for current or voltage pulse generation with negative resistance devices. Astable multivibrators: used for the generation of rectangular or square waves. 4/27/2017 OSCILLATOR

Harmonic oscillator in S Domain The Positive Feedback Approach for a Harmonic Oscillator With positive feedback, the harmonic oscillator can be represented in block diagram form of HO, where G(s) is the Laplace transform of the open-loop voltage-gain function of the amplifier stage and H(s) is the transfer function of the passive feedback network. 4/27/2017 OSCILLATOR

Relaxation Oscillators A second type of oscillator is the relaxation oscillator. Instead of feedback, a relaxation oscillator uses an RC timing circuit to generate a waveform that is generally a square wave or other nonsinusoidal waveform. Typically, a relaxation oscillator uses a Schmitt trigger or other device that changes states to alternately charge and discharge a capacitor through a resistor. 4/27/2017 OSCILLATOR

RC PHASE OSCILLATORS 4/27/2017 OSCILLATOR

Integrant of Linear Oscillators A linear oscillator contains: - a frequency selection feedback network - an amplifier to maintain the loop gain at unity 4/27/2017 OSCILLATOR

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) 4/27/2017 OSCILLATOR

RC Phase-Shift Oscillator BJT Amplifier-3 RC cells- Each cell with a phase shift of 60 deg Using an inverting amplifier The additional 180o phase shift is provided by an RC phase-shift network 4/27/2017 OSCILLATOR

Equivalent Circuit of RC phase Shift Oscillator BJT – current Amplifier Loop gain-A(s)=Ib/Ib’ 4/27/2017 OSCILLATOR

Applying KVL to the phase-shift network, we have Solve for I3, we get Or 4/27/2017 OSCILLATOR

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, 4/27/2017 OSCILLATOR

Wien Bridge Oscillators 4/27/2017 OSCILLATOR

Wien Bridge Oscillator Frequency Selection Network Let and Therefore, the feedback factor, 4/27/2017 OSCILLATOR

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, 4/27/2017 OSCILLATOR

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, 4/27/2017 OSCILLATOR

LC OSCILLATORS 4/27/2017 OSCILLATOR

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: are Colpitts Oscillator Harley Oscillator 4/27/2017 OSCILLATOR

For the equivalent circuit from the output Therefore, the amplifier gain is obtained, 4/27/2017 OSCILLATOR

It indicates that at least one reactance must be –ve (capacitor) 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, 4/27/2017 OSCILLATOR

HARTLEY OSCILLATOR and COLPITTS OSCILLATOR 4/27/2017 OSCILLATOR

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 4/27/2017 OSCILLATOR

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, 4/27/2017 OSCILLATOR

Imaginary part = 0, we have Real part = 0, yields 4/27/2017 OSCILLATOR

Hartley Oscillator Colpitts Oscillator 4/27/2017 OSCILLATOR

HARTLEY OSCILLATOR 4/27/2017 OSCILLATOR

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. 4/27/2017 OSCILLATOR

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 4/27/2017 OSCILLATOR

RC PHASE OSCILLATOR 4/27/2017 OSCILLATOR

A Square-wave Oscillator 4/27/2017 OSCILLATOR

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. 4/27/2017 OSCILLATOR

THANK YOU 4/27/2017 OSCILLATOR