Lecture 38Oscillators

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

R1 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

)/(1 orSRG GS

CCC 23

L

CC

sL

Gmg

GCGCGDCsCC

GDCCCs

s

s

GmgCCs

sC

mgCs

sLGDCCs

)31

(3

)3

()31

(31

2

)(sV

)(gV

)31

(3

)3

(

/1)3

(

0

0

=0, collect real and imaginary parts and set them to zero.

TCLCo

131

31CC

CCGDC

TCC

At 0

13CC

Rmg

Generally more gain is used to ensure oscillation with amplitude stabilization.

LC Oscillators: Hartley Oscillator

2

1

1

1

212

1

2

)(sV

)(gV

)2

/1()1

/1(2

/1

)2

/1(2

/1

0

0

LLC

LLssLmg

ogmgsC

s

s

ogmgsLsL

sL

mgsL

sLsC

=0, collect real and imaginary parts and set them to zero.

)21

(1LLCo

At 0

21LL

f

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, CS, R represent intrinsic series resonance path through crystal. CP is package capacitance. Equivalent impedance has series resonance where CS resonates with L and parallel resonance where L resonates with series combination of CS and CP.

SC

PC

SCPC

TC

TLCL

RssS

LCLRss

PsC

SZ

PZ

SZPZ

CZ

12

12

1

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: fS=5 MHz, Q=20,000 R =50 W, CP =5 pF

Analysis:

5.02MHz

fF)6.31mH)(8.31(21

2

1

fF8.31

)0318.0(2710

12

1

mH8.31)6105(2

)000,20(50

SC

PC

SCPC

LPf

LS

SC

S

RQL

Pierce crystal oscillator configuration

Crystal Oscillators: Topologies

Colpitts Crystal Oscillator Crystal 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