operational amplifier(it version)

SJTU Zhou Lingling 1

Chapter 7

Operational-Amplifier and its Applications


Outline

• Introduction • The 741 Op-Amp Circuit• The ideal Op Amp• The inverting configuration • The noninverting configuration• Integrator and differentiator• The antoniou Inductance-simulation Circuit• The Op Amp-RC Resonator• Bistable Circuit• Application of the bistable circuit as a comparator


Introduction

• Analog ICs include operational amplifiers, analog multipliers, A/D converters, D/A converters, PLL, etc.

• A complete op amp is realized by combining analog circuit building blocks.

• The bipolar op-amp has the general purpose variety and is designed to fit a wide range of specifications.

• The terminal characteristics is nearly ideal.


The 741 Op-Amp Circuit

• General description

• The input stage

• The intermediate stage

• The output stage

• The biasing circuits

• Device parameters


General Description

• 24 transistors, few resistors and only one capacitor

• Two power supplies

• Short-circuit protection


The Input Stage

• The input stage consists of transistors Q1 through Q7.

• Q1-Q4 is the differential version of CC and CB configuration.

• High input resistance.• Current source (Q5-Q7) is the active load of input

stage. It not only provides a high-resistance load but also converts the signal from differential to single-ended form with no loss in gain or common-mode rejection.


The Intermediate Stage

• The intermediate stage is composed of Q16, Q17 and Q13B.

• Common-collector configuration for Q16 gives this stage a high input resistance as well as reduces the load effect on the input stage.

• Common-emitter configuration for Q17 provides high voltage gain because of the active load Q13B.

• Capacitor Cc introduces the miller compensation to insure that the op amp has a very high unit-gain frequency.


The Output Stage

• The output stage is the efficient circuit called class AB output stage.

• Voltage source composed of Q18 and Q19 supplies the DC voltage for Q14 and Q20 in order to reduce the cross-over distortion.

• Q23 is the CC configuration to reduce the load effect on intermediate stage.

• Short-circuit protection circuitryForward protection is implemented by R6 and Q15.Reverse protection is implemented by R7, Q21, current

source(Q24, Q22) and intermediate stage.


The Output Stage

(a) The emitter follower is a class A output stage. (b) Class B output stage.


The Output Stage

Wave of a class B output stage fed with an input sinusoid.

Positive and negative cycles are unable to connect perfectly due to the turn-on voltage of the transistors.

This wave form has the nonlinear distortion called crossover distortion.

To reduce the crossover distortion can be implemented by supplying the constant DC voltage at the base terminals.


The Output Stage

QN and QP provides the voltage drop which equals to the summer of turn-on voltages of QN and QP.

This circuit is call Class AB output stage.


The Biasing Circuits

• Reference current is generated by Q12, Q11 and R5.• Wilder current provides biasing current in the

order of μA.• Double-collector transistor is similar to the two-

output current mirror. Q13B provides biasing current for intermediate stage, Q13A for output stage.

• Q5, Q6 and Q7 is composed of the current source to be an active load for input stage.


Device Parameters

• For npn transistors:

• For pnp transistors:

• Nonstandard devices:

Q14 and Q20 each has an area three times that of a standard device.

VVAI As 125,200,10 14

VVAI As 50,50,10 14

AISA141025.0 AISA

141075.0


The Ideal Op Amplifier

symbol for the op amp


The Ideal Op Amplifier

The op amp shown connected to dc power supplies.


Characteristics of the Ideal Op Amplifier

• Differential input resistance is infinite.• Differential voltage gain is infinite.• CMRR is infinite.• Bandwidth is infinite.• Output resistance is zero.• Offset voltage and current is zero.

a) No difference voltage between inverting and noninverting terminals.

b) No input currents.


Equivalent Circuit of the Ideal Op Amp


The Inverting Configuration

The inverting closed-loop configuration.

Virtual ground.



• Shunt-shunt negative feedback• Closed-loop gain depends entirely on passive

components and is independent of the op amplifier.

• Engineer can make the closed-loop gain as accurate as he wants as long as the passive components are accurate.


The Noninverting Configuration

The noninverting configuration.

Series-shunt negative feedback.


The Noninverting Configuration


The Voltage follower

(a) The unity-gain buffer or follower amplifier.

(b) Its equivalent circuit model.


The Weighted Summer


The Weighted Summer

)()())(())((4

43

32

21

1 R

Rv

R

Rv

R

R

R

Rv

R

R

R

Rvv cc

b

ca

b

cao


A Single Op-Amp Difference Amplifier

Linear amplifier.

Theorem of linear Superposition.



Application of superposition

Inverting configuration

11

21 Io v

R

Rv



Application of superposition.

Noninverting configuration.

234

4

1

22 )(1( Io v

RR

R

R

Rv ）


Integrators

The inverting configuration with general impedances in the feedback and the feed-in paths.


The Inverting Integrators

The Miller or inverting integrator.


Frequency Response of the integrator


The op-amp Differentiator


The op-amp Differentiator

Frequency response of a differentiator with a time-constant CR.


The Antoniou Inductance-Simulation Circuit


The Op amp-RC Resonator

An LCR second order resonator.



An op amp–RC resonator obtained by replacing the inductor L in the LCR resonator of a simulated inductance realized by the Antoniou circuit.



Implementation of the buffer amplifier K.



• Pole frequency

• Pole Q factor

25316460 11 RRRRCCLC

531

2

4

66660 RRR

R

C

CRRCQ


Bistable Circuit

• 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.

• 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

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


Generation of Triangle Waveforms

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