Lecture 29: Oscillators
Conditions for Oscillations, Colpitts Oscillator, Hartley Oscillator, Armstrong Oscillator, Examples
Dr. Mohamed Bakr, ENGINEER 3N03, 2015 Feedback 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
AABclvoltage 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
AABclvoltage 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 oscillation 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