Electronics 2
Lecture 7
Sine-wave generators
Literature- U. Tiecze, Ch. Schenk: Analogue and digital electronic circuits- B Carter, TR Brown: Handbook of Operational Amplifier Applications, TI, 2001- Ron Mancini (ed): Op Amps for Everyone, Texas Instruments, 2002
Oscillator principle of operation
Oscillator circuit
Amplitude condition:
Phase condition (positive feedback):
LC oscillator
General oscillation condition:
Open loop gain:
Oscillators with operational amplifiers
Autonomous oscillations emerge when the denominator is 0.
The large gain of the op amps ensures oscillation (component noise is sufficient to provide output signal). Available frequency: several kHz (at higher frequencies the phase lag of the op amp is too large). Current feedback op amps have significantly larger bandwidth, but are sensitive to capacitive feedback.
General complex oscillation condition:
Amplitude condition:
Phase condition (negative feedback):
Typical oscillator characteristics
Meissner oscillator
Alexander Meißner, 1913Feedback with transformer in the collector circuit.
Tuned collector circuit, common base implementation
Tuned collector circuit, common emitter implementation
Armstrong oscillator
Edwin Howard Armstrong, 1914
Tuning in the gate circuit
Hartley oscillator
Ralph Hartley, 1915
inductive three-point oscillator
With FET amplifier
With common emitter bipolar transistor
k: magnetic coupling coefficient
Colpitts oscillator
Edwin Henry Colpitts, 1919
capacitive three-point oscillator
Common emitter Common base Emitter coupled push-pull
Clapp oscillator
James Kilton Clapp, 1948
C1, C2: voltage divider, determines the measure of positive feedback;C0: tuning capacitor to adjust the frequency.
Vackár oscillator
Jiri Vackár, 1949
L1, Ca and C0: the resonant circuit of the Colpitts oscillator, C0 is the tuning capacitor. Cg and Cv: gate voltage divider.
Can be tuned in a wider frequency range than the Colpitts or Clapp oscillators.
Wien bridge oscillators
Av 3 Lin = R2C
William Reddington Hewlett, 1939
= − R
HP200A – the first Hewlett-Packard product: precision sinewave generator
Wien bridge oscillators
Wien – Robinson oscillator
Wien – Robinson bridge
Accurate values R1 larger by 5%
Wien-bridge oscillator with gain control
Regulating resistor with negative temperature coefficient
Regulating resistor with positive temperature coefficient
A =3
Tunable Wien-bridge oscillator with FET gain control
The channel resistance of the FET increases with the current amplitude.
Double T filter oscillator
More difficult to tune than the Wien bridge oscillators, but works with lower distortion.
Double T is a stopband filter. Cancels the negative feedback at resonance frequency.
Push-pull oscillators
With inductive coupling With capacitive coupling
Used at high powers, exploit the higher efficiency of B class amplifiers.
Relaxation oscillators
Pearson-Anson oscillator
When the voltage on the capacitor reaches the ignition value the neon lamp flashes, the capacitor is discharged and the charging process starts again.
Hysteresis-oscillator
Quartz crystal oscillator
Tuning with capacitor in series connection:
Series resonance frequency
Equivalent scheme of a quartz crystal:
Typical parameters of a 4MHz quartz:
Parallel resonance frequency:
Quartz crystal oscillator
George W. Pierce, 1923
Common emitter stage With CMOS inverter Emitter-coupled multivibrator