Lecture 29: Oscillators

Conditions for , , , Armstrong Oscillator, Examples

Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Oscillators

A feedback oscillator is created by forming a closed loop consisting of an amplifier with voltage gain (Av ) and a feedback circuit with attenuation (B).

Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Conditions for Oscillations

Using Non-Inverting Amplifiers

AABclvoltage gain around the closed loop  v  1

feedback circuit introduces no phase shift

Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Conditions for Oscillations (Cont’d)

Using Inverting Amplifiers

AABclvoltage gain around the closed loop  v  1

feedback circuit introduces 180o phase shift

Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Starting Oscillations

the power supply turn-on transients generate all frequency components in the oscillator loop initially, the amplitude of the frequency component is weak requiring Acl  1 at start-up

after the desired level is reached, Acl should drop back to 1, for maintaining stable oscillations

Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Colpitts Oscillator the amplifier used is a common- emitter (CE) BJT amplifier the feedback circuit is a frequency selective ideal parallel resonance circuit, whose resonance frequency is: 1 f0  , 2 LCT

CC12 CT  total capacitance CC12

Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Colpitts Oscillator (Cont’d) attenuation (B) of the feedback circuit V IXX B attenuation f CC11   VIXX out C22 C 12  fC C  r 1   2 12 f C C  r 21 the -ve sign indicates that the feedback circuit introduces 180o phase shift knowing B, the gain of the amplifier can be obtained such that the condition of is satisfied: 1 C1 AABAcl v  1  v    BC2 Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Hartley Oscillator interchanging capacitors and inductors in a Colpitts oscillator results in a Hartley oscillator 1 f  , 0 2 LC T

LLLT total inductance 12  the attenuation (B) of the feedback circuit and the gain (Av) of the amplifier can be expressed as follows:

V f I X X2 f L L1 L BA L1  L 1  r 1  1,  2 V I X X2 f L Lv B L out L2 L 2 r 2 2 1

Dr. Mohamed Bakr, ENGINEER 3N03, 2015

Armstrong Oscillator a transformer is used as a feedback circuit the frequency of oscillation is: 1 f  0 2 LC pri 1 the attenuation (B) of the feedback circuit and the gain (Av) of the amplifier are:

V f Nsec 11 B     n, A v    Vout N pri B n

Dr. Mohamed Bakr, ENGINEER 3N03, 2015