Opamp

The 741 Op-Amp Circuit

• Schematic diagram of OP-AMP consists:

• The input stage

• The intermediate stage

• The output stage

• The biasing circuits

Schematic diagram of lm741

• 24 transistors, few resistors and only one capacitor

• Two power supplies

• Short-circuit protection

General Description

• 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 Input 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.

The Intermediate 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.

The Output Stage

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

(b) (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 turn-on voltages of QN and QP.

This circuit is call Class AB output stage.

The Output 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.

Short-circuit protection

• Reference current is generated by Q12, Q11 and R5.

• Wilder current provides biasing current in the order of μA.

• 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.

The Biasing Circuits

Ideal Opamp

Equivalent Circuit of the Ideal Op Amp

Characteristics of the Ideal Op AmplifierThe ideal OPAMP has the

following characteristic :

Differential Input resistance Ri=

Output resistance Ro = 0

Differential voltage gain Av=-

Bandwidth =

Offset voltage and current is zero.a) No difference voltage

between inverting and noninvertying terminals.

b) No input currents.

Perfect balance Vo=0 when V-= V+

A

Vo = (A V -A V ) = A (V - V )

+

+

--

OP AMP is a direct coupled high gain amplifier to which feedback is added to control its overall response characteristic

Operational Amplifier (OP AMP)

Basic and most common circuit building device. Ideally,

1. No current can enter terminals V+ or V-. Called infinite input impedance.

2. Vout=A(V+ - V-) with A →∞

3. In a circuit V+ is forced equal to V-. This is the virtual ground property

4. An opamp needs two voltages to power it Vcc and -Vee. These are called the rails.

A

Vo = (A V -A V ) = A (V - V )

+

+

--

OPAMP: COMPARATOR

Vout=A(Vin – Vref)

If Vin>Vref, Vout = +∞ but practically hits +ve power supply = Vcc

If Vin<Vref, Vout = -∞ but practically hits –ve power supply = -Vee

Compare the voltage of one input with the voltage with other input

Two types:

inverting comparator when the reference voltage apply to the inverting terminal

non inverting comparator when the reference voltage apply to the non inverting terminal

A (gain) very high

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

(b) Its equivalent circuit model.

V+ = VIN.

By virtual ground, V- = V+

Thus Vout = V- = V+ = VIN !!!!

OPAMP: VOLTAGE FOLLOWER

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The inverting closed-loop configuration.

Virtual ground.

OPAMP: The Inverting Configuration

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OPAMP: The Inverting Configuration

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OPAMP: The Inverting Configuration

OPAMP: INVERTING AMPLIFIER

1. V- = V+

2. As V+ = 0, V- = 0

3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2. 4. I1 = (VIN - V-)/R1 = VIN/R1

5. I2 = (V- - VOUT)/R2 = -VOUT/R2 => VOUT = -I2R2

6. From 3 and 5, VOUT = -I2R2 = -I1R2 = -VINR2/R1

7. Therefore VOUT = (-R2/R1)VIN

8. Gain = Vout / Vin = - R2 / R1

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The noninverting configuration.

Series-shunt negative feedback.

OPAMP: The Non Inverting Configuration

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OPAMP: The Non Inverting Configuration

OPAMP: NON – INVERTING AMPLIFIER

1. V- = V+

2. As V+ = VIN, V- = VIN

3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2. 4. I1 = VIN/R1

5. I2 = (VOUT - VIN)/R2 => VOUT = VIN + I2R2

6. VOUT = I1R1 + I2R2 = (R1+R2)I1 = (R1+R2)VIN/R1

7. Therefore VOUT = (1 + R2/R1)VIN

SUMMING AMPLIFIER

VOUT = -Rf (V1/R1 + V2/R2 + … + Vn/Rn)

If

Recall inverting amplifier and If = I1 + I2 + … + In

Summing amplifier is a good example of analog circuits serving as analog computing amplifiers (analog computers)!

Note: analog circuits can add, subtract, multiply/divide (using logarithmic components, differentiate and integrate – in real time and continuously.

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SUMMING AMPLIFIER

34

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

43

32

21

1 R

Rv

R

Rv

R

R

R

Rv

R

R

R

Rvv cc

b

ca

b

cao

SUMMING AMPLIFIER

Difference AMPLIFIER

•This type is of the same characteristic of the inverting and non inverting OPAMP.

•Vo is the differences between the two inputs

• Rin in both inputs must be equal, and equal to Rf

Vo = Rf (V1 –V2)/ Rin

Rf

Rin

Rin

V2

V1

Rf

Vo

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Linear amplifier.

Theorem of linear Superposition.

Difference AMPLIFIER

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Application of superposition

Inverting configuration

11

21 Io v

R

Rv

Difference AMPLIFIER

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Application of superposition.

Non inverting configuration.

234

4

1

22 )(1( Io v

RR

R

R

Rv ）

Difference AMPLIFIER

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

Integrators AMPLIFIER

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The Miller or inverting integrator.

The Inverting Integrators AMPLIFIER