Microelectronic Circuits SJTU Yang Hua Chapter 12 Signal generators and waveform-shaping circuits...

Microelectronic Circuits SJTU Yang Hua

Chapter 12 Signal generators and waveform-shaping circuits

Introduction12.1 Basic principles of sinusoidal oscillators12.2 RC oscillator circuits12.3 LC and crystal oscillators12.4 Bistable Multivibrators12.5 Generation of a standardized pulse-The monostable multivibrator

SJTU Yang HuaMicroelectronic Circuits

Introduction

The twoThe two different different approachesapproaches

(1)(1)linear oscillators:linear oscillators:

employs a positive-feedback loop consisting of an employs a positive-feedback loop consisting of an amplifier and an RC or LC frequency-selective netamplifier and an RC or LC frequency-selective network.work. (Section 13.1-3)(Section 13.1-3)

((2)nonlinear oscillators or function gener2)nonlinear oscillators or function generators:ators:

• The bistable multivibrator(Section 13.4)The bistable multivibrator(Section 13.4)

• the astable multivibratorthe astable multivibrator (Section 13.5)(Section 13.5)

• the monostablethe monostable multivibrator(Section 13.6)multivibrator(Section 13.6)


The basic structure of sinusoidaloscillators

The basic The basic

structurestructure

Amplifier circuitAmplifier circuit ：： realize the energy controrealize the energy controll

Frequency-selective networkFrequency-selective network ：： oscillator freoscillator frequency is determinedquency is determined

Positive feedback loopPositive feedback loop ：：

amplitude controlamplitude control ：： implementation of the implementation of the nonlinear amplitude-stabilization mechanismnonlinear amplitude-stabilization mechanism

fi xx


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.



Feedback signal xf is summed with a positive sign

The gain-with-feedback is

The oscillation criterion: Barkhausen criterion.

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Nonlinear amplitude control To ensure that oscillations will start, the Aβ 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.


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.


A Popular Limiter Circuit for Amplitude Control

54

5

54

4

32

2

32

3

121 off, is,

RR

Rv

RR

RVv

RR

Rv

RR

RVv

vR

RvDD

oB

oA

If

o

When vi is close to zero:



3 3

2 2

4 4

5 5

1

on the contrary:

1

D

D

R RL V V

R R

R RL V V

R R

When vi goes positive,D1 is on, D2 is off



Transfer characteristic of the limiter circuit;

When Rf is removed, the limiter turns into a comparator with the characteristic shown.

5

4

5

4

2

3

2

3

1

1

R

RV

R

RVL

R

RV

R

RVL

D

D


Oscillator Circuits

Op Amp-RC Oscillator Circuits The Wien-Bridge Oscillator The phase-Shift Oscillator

LC-Tuned Oscillator Colpitts oscillator Hareley oscillator

Crystal Oscillator


The Wien-Bridge Oscillator

A Wien-bridge oscillator without amplitude stabilization.


Analysis of frequency-selectivenetwork for Wien-bridge oscillator

(b) Low frequency: 1/wc>>R(c) high frequency: 1/wc<<R



The loop gain transfer function

Oscillating frequency

To obtain sustained oscillation

sCRsCR

RRsL

13

11)( 2

RC1

0

21

2 RR



A Wien-bridge oscillator

with a limiter used for

amplitude control.


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 1800


The Phase-Shift Oscillator

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


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.

)(121

210 CC

CCL


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.

)(121

210 CC

CCL


Crystal Oscillators

A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit.


Crystal Oscillators

Crystal reactance versus frequency (neglecting the small resistance r, ).

A series resonance at

A parallel resonance at ss LC1

)(1ps

psp CC

CCL


Homework:

June 12th, 2008

12.13;12.14


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.


Bistable Circuit

The bistable circuit (positive feedback loop)

The negative input terminal of the op amp connected to an input signal vI.

oo vRR

Rvv

21

1


Bistable Circuit

The transfer characteristic of the circuit in (a) for increasing vI.

Positive saturation L+ and negative saturation L-

LVTH


Bistable Circuit

The transfer characteristic for decreasing vI.

LVTL


Bistable Circuit

The complete transfer characteristics.


A Bistable Circuit with Noninverting Transfer Characteristics

21

1

21

2

RR

Rv

RR

Rvv oI


A Bistable Circuit with Noninverting Transfer Characteristics

The transfer characteristic is noninverting.

21

21

21

RR

RRLV

RRLV

TL

TH

）（）（


Application of Bistable Circuit as a Comparator

Comparator is an analog-circuit building block used in a variety applications.

To detect the level of an input signal relative to a preset threshold value.

To design A/D converter. Include single threshold value and two

threshold values. Hysteresis comparator can reject the

interference.



Block diagram representation and transfer characteristic for a comparator having a reference, or threshold, voltage VR.

Comparator characteristic with hysteresis.



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


Making the Output Level More Precise

For this circuit L+ = VZ1 + VD and L– = –(VZ2

+ VD), where VD is the forward

diode drop.


Making the Output Level More Precise

For this circuit L+ = VZ + VD1 + VD2

and L– = –(VZ + VD3 + VD4

).


Generation of Square Waveforms

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


Generation of Square Waveforms

The circuit obtained when the bistable multivibrator is implemented with the positive feedback loop circuit.


Waveforms at various nodes of the circuit in (b).

This circuit is called an astable multivibrator.

Time period T = T1+T2

1

)1ln1

LLRCT

（

1

)1ln2

LLRCT

（

1

1ln2RCT

/

/

)(

)(t

t

eLLLv

eLLLv


Generation of Triangle Waveforms


Generation of Triangle Waveforms

L

VVRCT

CR

L

T

VV

TLTH

TLTH

2

2

L

VVRCT

CR

L

T

VV

TLTH

TLTH

1

1


Homework:

June 19th,2008

12.28; 12.32; 12.33

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Microelectronic Circuits SJTU Yang Hua Chapter 12 Signal generators and waveform-shaping circuits...

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