Lecture 38 Oscillators Amit Kumar Mishra ECE, IIT G

Amplitude Stabilization Loop gain of oscillator changes due to power supply voltage, component value or temperature changes. If loop gain is too small, desired oscillation decays and if it is too large, waveform is distorted. Amplitude stabilization or gain control is used to automatically control loop gain and place poles exactly on jw axis. At power on, loop gain is larger than that required for oscillation.As oscillation builds up, gain is reduced to minimum required to sustain oscillations.

Amplitude Stabilization in RC Oscillators: Method 1 R 1 is replaced by a lamp. Small-signal resistance of lamp depends on temperature of bulb filament. If amplitude is large, current is large, resistance of lamp increases, gain is reduced. If amplitude is small, lamp cools, resistance decreases, loop gain increases. Thermal time constant of bulb averages signal current and amplitude is stabilized.

Active LC oscillator Higher range Higher Q factor (=> ??)

Hartely (b) and Colpitt (a) oscillators

LC Oscillators: Colpitts Oscillator  =0, collect real and imaginary parts and set them to zero. At  0 Generally more gain is used to ensure oscillation with amplitude stabilization.

LC Oscillators: Hartley Oscillator  =0, collect real and imaginary parts and set them to zero. At  0 Generally more gain is used to ensure oscillation with amplitude stabilization. G-S and G-D capacitances are neglected, assume no mutual coupling between inductors.

Another practical Colpitt Osc.

Crystal oscillator In its heart is a piezoelectric crystal Pizo crystal have opposite faces plated with electrodes. 3 major advantages: Very high Q (10s to 100s of thousands) Stable with temp. and time Can give freq. upto several MHz Q and res. Freq. depends on the size, orientation of faces, and mount

Crystal Oscillators Crystal: A piezoelectric device that vibrates is response to electrical stimulus, can be modeled electrically by a very high Q (>100,000) resonant circuit. L, C S, R represent intrinsic series resonance path through crystal. C P is package capacitance. Equivalent impedance has series resonance where C S resonates with L and parallel resonance where L resonates with series combination of C S and C P. Below  S and above  P, crystal appears capacitive, between  S and  P it exhibits inductive reactance. Used to replace L in Colpitt

Crystal

Crystal Oscillators: Example Problem: Find equivalent circuit elements for crystal with given parameters. Given data: f S =5 MHz, Q=20,000 R =50 W, C P =5 pF Analysis:

Pierce crystal oscillator configuration

Crystal Oscillators: Topologies Colpitts Crystal OscillatorCrystal Oscillator using BJT Crystal Oscillator using JFET Crystal Oscillator using CMOS inverter as gain element.

The classic 555 timer circuit Since 1972 (by Signetics Co.) called “IC Time Machine”! Numerous clones available Low-cost, accurate and easy to design with (>1B units per year) ~23 Transistors; 2 diodes; ~16 resistors (DIP- 8) Can work in monostable, astable and bistable configurations

From the SE555 datasheet

Schemtics

Block diagram S=R=0; Q=Q’ S=1;R=0; Q=1 S=0; R=1; Q=0 Vcc ~ 5V Vth = 2/3Vcc Vtl = 1/3Vcc Why 555? Transistor ~ switch

Monostable configuration

Astable configuration

Many Thanks