1. Waveform-Shaping Circuits
Signal Generators and
Waveform-Shaping Circuits 1

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Figure The basic structure of a sinusoidal oscillator. A positive-feedback loop is formed by an amplifier and a frequency-selective network. In an actual oscillator circuit, no input signal will be present; here an input signal xs is employed to help explain the principle of operation.

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Figure Dependence of the oscillator-frequency stability on the slope of the phase response. A steep phase response (i.e., large df/dw) results in a samll Dw0 for a given change in phase Df (resulting from a change (due, for example, to temperature) in a circuit component).

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Figure (a) A popular limiter circuit. (b) Transfer characteristic of the limiter circuit; L- and L+ are given by Eqs. (13.8)
and (13.9), respectively. (c) When Rf is removed, the limiter turns into a comparator with the characteristic shown.

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Figure A Wien-bridge oscillator without amplitude stabilization.

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Figure A Wien-bridge oscillator with a limiter used for amplitude control.

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Figure A Wien-bridge oscillator with an alternative method for amplitude stabilization.

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Figure A phase-shift oscillator.

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Figure A practical phase-shift oscillator with a limiter for amplitude stabilization.

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Figure (a) A quadrature-oscillator circuit. (b) Equivalent circuit at the input of op amp 2.

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Figure Block diagram of the active-filter-tuned oscillator.

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Figure A practical implementation of the active-filter-tuned oscillator.

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Figure Two commonly used configurations of LC-tuned oscillators: (a) Colpitts and (b) Hartley.

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Figure Equivalent circuit of the Colpitts oscillator of Fig (a). To simplify the analysis, Cm and rp are neglected. We can consider Cp to be part of C2, and we can include ro in R.

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Figure Complete circuit for a Colpitts oscillator.

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Figure A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit. (c) Crystal reactance versus frequency [note that, neglecting the small resistance r, Zcrystal = jX(w)].

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Figure A Pierce crystal oscillator utilizing a CMOS inverter as an amplifier.

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Figure A positive-feedback loop capable of bistable operation.

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Figure A physical analogy for the operation of the bistable circuit. The ball cannot remain at the top of the hill for any length of time (a state of unstable equilibrium or metastability); the inevitably present disturbance will cause the ball to fall to one side or the other, where it can remain indefinitely (the two stable states).

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Figure (a) The bistable circuit of Fig with the negative input terminal of the op amp disconnected from ground and connected to an input signal vI. (b) The transfer characteristic of the circuit in (a) for increasing vI. (c) The transfer characteristic for decreasing vI. (d) The complete transfer characteristics.

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Figure (a) A bistable circuit derived from the positive-feedback loop of Fig by applying vI through R1. (b) The transfer characteristic of the circuit in (a) is noninverting. (Compare it to the inverting characteristic in Fig d.)

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Figure (a) Block diagram representation and transfer characteristic for a comparator having a reference, or threshold, voltage VR. (b) Comparator characteristic with hysteresis.

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Figure Illustrating the use of hysteresis in the comparator characteristics as a means of rejecting interference.

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Figure Limiter circuits are used to obtain more precise output levels for the bistable circuit. In both circuits the value of R should be chosen to yield the current required for the proper operation of the zener diodes. (a) For this circuit L+ = VZ1 + VD and L– = –(VZ2 + VD), where VD is the forward diode drop. (b) For this circuit L+ = VZ + VD1 + VD2 and L– = –(VZ + VD3 + VD4).

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Figure (a) Connecting a bistable multivibrator with inverting transfer characteristics in a feedback loop with an RC circuit results in a square-wave generator.

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Figure (Continued) (b) The circuit obtained when the bistable multivibrator is implemented with the circuit of Fig (a). (c) Waveforms at various nodes of the circuit in (b). This circuit is called an astable multivibrator.

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Figure A general scheme for generating triangular and square waveforms.

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Figure (a) An op-amp monostable circuit. (b) Signal waveforms in the circuit of (a).

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Figure A block diagram representation of the internal circuit of the 555 integrated-circuit timer.

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Figure (a) The 555 timer connected to implement a monostable multivibrator. (b) Waveforms of the circuit in (a).

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Figure (a) The 555 timer connected to implement an astable multivibrator. (b) Waveforms of the circuit in (a).

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Figure Using a nonlinear (sinusoidal) transfer characteristic to shape a triangular waveform into a sinusoid.

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Figure (a) A three-segment sine-wave shaper. (b) The input triangular waveform and the output approximately sinusoidal waveform.

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Figure A differential pair with an emitter degeneration resistance used to implement a triangular-wave to sine-wave converter. Operation of the circuit can be graphically described by Fig

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Figure (a) The “superdiode” precision half-wave rectifier and (b) its almost ideal transfer characteristic. Note that when vI > 0 and the diode conducts, the op amp supplies the load current, and the source is conveniently buffered, an added advantage.

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Figure (a) An improved version of the precision half-wave rectifier: Diode D2 is included to keep the feedback loop closed around the op amp during the off times of the rectifier diode D1, thus preventing the op amp from saturating. (b) The transfer characteristic for R2 = R1.

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Figure A simple ac voltmeter consisting of a precision half-wave rectifier followed by a first-order low-pass filter.

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Figure Principle of full-wave rectification.

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Figure (a) Precision full-wave rectifier based on the conceptual circuit of Fig (b) Transfer characteristic of the circuit in (a).

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Figure Use of the diode bridge in the design of an ac voltmeter.

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Figure A precision peak rectifier obtained by placing the diode in the feedback loop of an op amp.

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Figure A buffered precision peak rectifier.

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Figure A precision clamping circuit.

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Figure Example 13.1: Capture schematic of a Wien-bridge oscillator.

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Figure Start-up transient behavior of the Wien-bridge oscillator shown in Fig for various values of loop gain.

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Figure (Continued)

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Figure (Continued)

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Figure Example 13.2: Capture schematic of an active-filter-tuned oscillator for which the Q of the filter is adjustable by changing R1.

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Figure Output waveforms of the active-filter-tuned oscillator shown in Fig for Q = 5 (R1 = 50 kW).

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Figure E13.22

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Figure E13.28

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Figure E13.30

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Figure P13.8

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Figure P13.13

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Figure P13.14

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Figure P13.18

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Figure P13.21

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Figure P (Continued)

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Figure P13.22

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Figure P13.26

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Figure P13.33

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Figure P13.34

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Figure P13.41

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Figure P13.43

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Figure P13.44

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Figure P13.50

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Figure P13.51

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Figure P13.52