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1 .Chapter 12 Signal generators and waveform-shaping circuits Introduction 12.1 Basic principles of sinusoidal oscillators 12.2 RC oscillator circuits 12.3 LC and crystal oscillators 12.4 Bistable Multivibrators 12.5 Generation of a standardized pulse-The monostable multivibrator
2 .Introduction The two different approaches linear oscillators: employs a positive-feedback loop consisting of an amplifier and an RC or LC frequency-selective network. (Section 13.1-3) ( 2)nonlinear oscillators or function generators: The bistable multivibrator(Section 13.4) the astable multivibrator (Section 13.5) the monostable multivibrator(Section 13.6)
3 .The basic structure of sinusoidal oscillators The basic structure Amplifier circuit ： realize the energy control Frequency-selective network ： oscillator frequency is determined Positive feedback loop ： amplitude control ： implementation of the nonlinear amplitude-stabilization mechanism
4 .Basic Principles of Sinusoidal Oscillator The oscillator feedback loop The basic structure of a sinusoidal oscillator. A positive-feedback loop is formed by an amplifier and a frequency-selective network.
5 .Basic Principles of Sinusoidal Oscillator Feedback signal x f is summed with a positive sign The gain-with-feedback is The oscillation criterion: Barkhausen criterion.
6 .Basic Principles of Sinusoidal Oscillator Nonlinear amplitude control To ensure that oscillations will start, the A β is slightly greater than unity. As the power supply is turned on, oscillation will grown in amplitude. When the amplitude reaches the desired level, the nonlinear network comes into action and cause the Aβ to exactly unity.
7 .The implementation of the nonlinear amplitude-stabilization mechanism The first approach makes use of a limiter circuit The other mechanism for amplitude control utilizes an element whose resistance can be controlled by the amplitude of the output sinusoid.
8 .A Popular Limiter Circuit for Amplitude Control When vi is close to zero:
9 .A Popular Limiter Circuit for Amplitude Control When vi goes positive,D1 is on, D2 is off
10 .A Popular Limiter Circuit for Amplitude Control Transfer characteristic of the limiter circuit; When R f is removed, the limiter turns into a comparator with the characteristic shown.
11 .Oscillator Circuits Op Amp-RC Oscillator Circuits The Wien-Bridge Oscillator The phase-Shift Oscillator LC-Tuned Oscillator Colpitts oscillator Hareley oscillator Crystal Oscillator
12 .The Wien-Bridge Oscillator A Wien-bridge oscillator without amplitude stabilization.
13 .Analysis of frequency-selective network for Wien-bridge oscillator (b) Low frequency: 1/wc>>R (c) high frequency: 1/wc<<R
14 .The Wien-Bridge Oscillator The loop gain transfer function Oscillating frequency To obtain sustained oscillation
15 .The Wien-Bridge Oscillator A Wien-bridge oscillator with a limiter used for amplitude control.
16 .The Phase-Shift Oscillator The circuit consists of a negative-gain amplifier and three-section RC ladder network. Oscillating frequency is the one that the phase shift of the RC network is 180 0
17 .The Phase-Shift Oscillator A practical phase-shift oscillator with a limiter for amplitude stabilization.
18 .The LC-Tuned oscillator Colpitts Oscillator A parallel LC resonator connected between collector and base. Feedback is achieved by way of a capacitive divider Oscillating frequency is determined by the resonance frequency.
19 .The LC-Tuned oscillator Hartley Oscillator A parallel LC resonator connected between collector and base. Feedback is achieved by way of an inductive divider. Oscillating frequency is determined by the resonance frequency.
20 .Crystal Oscillators A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit.
21 .Crystal Oscillators Crystal reactance versus frequency (neglecting the small resistance r , ). A series resonance at A parallel resonance at
22 .Homework: June 12th, 2008 12.13;12.14
23 .Bistable Circuit -- three basic factors The output signal only has two states: positive saturation(L + ) and negative saturation(L - ). The circuit can remain in either state indefinitely and move to the other state only when appropriate triggered .(threshold voltage) The direction of one stage moving to the other stage. A positive feedback loop capable of bistable operation.
24 .Bistable Circuit The bistable circuit (positive feedback loop) The negative input terminal of the op amp connected to an input signal v I .
25 .Bistable Circuit The transfer characteristic of the circuit in (a) for increasing v I . Positive saturation L + and negative saturation L -
26 .Bistable Circuit The transfer characteristic for decreasing v I .
27 .Bistable Circuit The complete transfer characteristics.
28 .A Bistable Circuit with Noninverting Transfer Characteristics
29 .A Bistable Circuit with Noninverting Transfer Characteristics The transfer characteristic is noninverting .