The operational amplifier (part 1)

Engr.Tehseen Ahsan

Lecturer, Electrical Engineering Department

EE-307 Electronic Systems Design

HITEC University Taxila Cantt, Pakistan

The Operational Amplifier (Part 1)

12-1 Introduction to Operational Amplifiers The devices, such as diode and the transistor, are separate

devices that are individually packaged and interconnected in a

circuit with other devices to form a complete, functional

unit- Discrete Components.

Linear integrated circuits (ICs), where many transistors, diodes,

resistors and capacitors are fabricated on a single tiny chip of

semiconductor material.

This means that we will be concerned with what the circuit

does from an external view point than from an internal,

component-level viewpoint.

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12-1 Introduction to Operational Amplifiers Continue…

Early operational Amplifiers ( i.e, used for +,-, ∫ and d/dx).

Early op-amp were constructed with vaccum tubes and

worked with high voltages.

Today’s op-amp are linear ICs that use relatively low dc

supply voltages and are reliable and inexpensive.

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12-1 Introduction to Operational Amplifiers Continue…

Symbol andTerminals

The standard op-amp symbol is shown in figure 12-1(a).

It has two terminals i.e, inverting(-) and non-inverting (+).

It has one output terminal.

The typical op-amp operates with two dc supply voltages, one

+ve and the other –ve as shown in figure 12-2(b).

Usually these dc voltage terminals are left off the schematic

symbol for simplicity but are understood to be there.

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12-1 Introduction to Operational Amplifiers Continue… The Ideal Op-Amp

It has infinite gain and infinite bandwidth.

It has infinite input impedance (open) so that it does not load

the driving source.

It has zero output impedance.

These characteristics are illustrated in figure 12-2 (a). The input

voltage ,Vin , appears b/w the two terminals, and the output

voltage is AvVin , as indicated by the internal voltage source

symbol.

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12-1 Introduction to Operational Amplifiers Continue… The Practical Op-Amp

Any device has limitations and IC op-amp is no exception.

Op-Amp has certain limitations but the two mentioned below

are major ones.

i. Slew Rate ( will be discussed in 12-2 Section).

ii. Output current is also limited by internal restrictions such as

power dissipation and component ratings.

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12-1 Introduction to Operational Amplifiers Continue… Internal Block Diagram of an Op-Amp

A typical op-amp is made up of three types of amplifier circuits.

A Differential Amplifier i.e, it is the input stage for the op-amp. It

provides the amplification of the difference voltage b/w the two

inputs.

A Voltage Amplifier(s) i.e, it is the second stage of op-amp and is

usually a class A amplifier that provides additional gain.

A Push-Pull Class B Amplifier i.e, it is typically used for output stage.

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12-1 Introduction to Operational Amplifiers Continue… The Differential Amplifier Input Stage

The differential amplifier forms the input stage of operational

amplifiers.

The term difference comes from the amplifier’s ability to

amplify the difference of two input signals applied to inputs.

Only the difference in the two signals is amplified; if there is no

difference, then the output is zero.

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12-1 Introduction to Operational Amplifiers Continue… The Differential Amplifier Input Stage Continue…

The transistors (Q1 and Q2) and the collector resistors (RC1 and

RC2) are carefully matched to have identical characteristics

(Refer to figure in previous slide).

Notice that the two transistors share a single RE.

Let us suppose both bases are connected to ground. The emitter

voltage will be -0.7V .

The emitter currents are equal (IE1= IE2).

The collector currents are both equal and are approximately

equal to the emitter currents.

Since collector currents are the same, the collector voltages are

also the same.

There is a zero difference in the input voltages ( both bases are

at 0V).9


If the base of Q1 is disconnected from ground and connected to

a small positive voltage, Q1 will conduct more current because

the positive voltage on its base causes the emitter voltage to

increase slightly.

The emitter current is now divided so that more of it is in Q1and less in Q2.

As a result the collector voltage of Q1 will decrease and the

collector voltage of Q2 will increase, reflecting the difference in

the input voltages (one is 0 V and the other at a small positive

value).

This condition is illustrated in figure next slide.

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12-2 OP-AMP Input Modes and Parameters

The differential amplifier exhibits three modes of

operation based on the type of input signals. These

modes are single-ended, differential and common. Since

the differential amplifier is the input stage of the op-

amp, the op-amp exhibits the same modes.

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12-2 OP-AMP Input Modes and Parameters Continue…

Single-Ended Mode

One input is grounded and a signal voltage is applied only to the

other input as shown in figure 12-4.

When the signal is applied to the inverting input as in part(a),

an inverted , amplified signal voltage appears at the output.

When the signal is applied to the non-inverting input as in

part(b), an non-inverted , amplified signal voltage appears at the

output.

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DifferentialMode Two opposite-polarity (out-of-phase) signals are applied to the

inputs as shown in figure 12-5.

This type of operation is also referred to as double-ended.

The amplified difference between the two inputs appears on the

output.

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CommonMode Two signal voltages of the same phase, frequency and amplitude

are applied to the two inputs as shown is figure 12-6.

This results in a zero output voltage (as difference is 0V).

This action is called common-mode rejection.

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CommonMode Continue… It’s importance lies in the situation where an unwanted signal

appears commonly on both op-amp inputs.

Common-mode rejection means that this unwanted signal will

not appear on the output and distort the desired signal.

CommonMode Rejection Ratio

Desired signals can appear on only one input or with opposite

polarities on both input lines. These desired signals are

amplified and appear on the output.

Unwanted signals (noise) appearing with the same polarity on

both input lines are essentially cancelled by the op-amp and

don’t appear on the output. The measure of an amplifier’s ability

to reject common-mode signals is a parameter called the

CMRR (Common-Mode Rejection Ratio).16


CommonMode Rejection Ratio Continue... Ideally , an op-amp provides a very high gain for desired signals

(single-ended or differential) and zero gain for common-mode

signals.

However practical op-amps do exhibit a very small common-

mode gain (usually much less than 1), while providing a high

open-loop voltage gain (usually several thousands).

The higher the open-loop gain w.r.t the common-mode gain,

the better the performance of op-amp in terms of rejection of

common-mode signals.

This suggests:

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CommonMode Rejection Ratio Continue... The higher the CMRR, the better. A very high value of CMRR

means that the open-loop gain, Aol is high and common-mode

gain, Acm is low.

The CMRR is often expresses in decibels (dB) as

The open-loop voltage gain of an op-amp is the internal

voltage gain of the device and represents the ratio of the output

voltage to the input voltage when there are no external

components( no feedback circuit).

Open-loop voltage gain can range up to 200,000 and is not a

well tolerated parameter. Datasheets often refer to the open-

loop voltage gain as the large-signal voltage gain.18

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Input OffsetVoltage Ideal op-amp produces zero volts out for zero volts in.

However, in practical op-amp a small dc voltage,VOUT(error) ,

appears at the output when no differential input voltage is

applied. Its primary cause is a slight mismatch of the base-

emitter voltages of the differential amplifier input stage of an

op-amp.

Input offset voltage,VOS ,is the differential dc voltage required

between the inputs to force the output to zero volts.

Typical values of input offset voltage are in the range of 2mV or

less. It is 0V in ideal case.

Input offset voltage drift is a parameter related to,VOS ,that

specifies how much change occurs in the input offset voltage

for each degree change in temperature.20


Input Bias Current Input terminals of a differential amplifier are the transistor

bases and therefore , the input currents are the base currents.

The input bias current is the dc current required by the inputs

of an amplifier to properly operate the first stage. By definition

the input bias current is the average of both input currents as

follows:

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Input Impedance Two basic ways of specifying the input impedance

1. The Differential input impedance is the total resistance between

the inverting and non-inverting terminals as illustrated 12-8

(a) next slide. Differential impedance is measured by

determining the change in bias current for a given change in

differential input voltage.

2. The common-mode input impedance is the resistance between

each input and ground and is measured by determining the

change in bias current for a given change in common-mode

input voltage shown in figure 12-8(b) next slide .

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Input Impedance Continue…

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Input Offset Current Ideally, the two input bias currents are equal and thus their

difference is zero.

However, in a practical op-amp, the bias currents are not

exactly equal.

The input offset current , IOS is the difference of the input bias

currents expressed as an absolute value.

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Input Offset Current The actual magnitudes of offset current are usually at least ten

times less than the bias current.

In many applications offset current can be neglected.

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Output Impedance The output impedance is the resistance viewed from the output

terminal of the op-amp as shown in figure below

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Slew Rate The slew rate (SR) of an op-amp is the maximum rate at which

the output voltage can change in response to input voltage.

When the SR is too slow for the input, distortion results.

When a input sine wave is applied to a voltage follower it

produces a triangular output waveform.

The triangular waveform results because the op-amp simply

cannot move fast enough to follow the sine wave input.

This happens because voltage change in the second stage (

Voltage Amplifier(s) ) is limited by the charging and discharging

of capacitor.

The slew rate is expressed as : SR = ∆VO/∆t

The unit of SR is volts per microseconds.27

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12-3 Negative Feedback Negative feedback is the process whereby a portion of the

output voltage of an amplifier is returned to the input with a

phase angle that opposes the input signal.

Negative feedback is illustrated in figure 12-14. The inverting

(-) input effectively makes the feedback signal 180˚ out of

phase with the input signal.

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12-3 Negative Feedback Why Use Negative Feedback?

The inherent open-loop voltage gain of a typical op-amp is very

high ( usually greater than 100,000). Therefore , an extremely

small input voltage drives the op-amp into its saturated output

states. In fact , even the input offset voltage of an op-amp can

drive it into saturation. For example, assume VIN= 1 mV and Aol= 100,000.Then, Vout= VIN Aol= 100V

Since the output level of an op-amp can never reach 100 V, it is

driven deep into saturation and the output is limited to its

maximum output levels as illustrated in figure 12-15 next slide

for both a positive and negative input voltage of 1 mV.

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12-3 Negative Feedback Why Use Negative Feedback? (continue…)

The usefulness of an op-amp operated without negative

feedback is severely restricted and is generally limited to

comparator and other applications.

With negative feedback, the closed-loop voltage gain (Acl) can

be reduced and controlled so that op-amp can function as a

linear amplifier.

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12-4 OP-AMPS With Negative Feedback The extremely high open-loop gain of an op-amp creates an

unstable situation because a small noise voltage on the input

can be amplified to a point where the amplifier is driven out

of its linear region.

Negative feedback takes a portion of the output and applies it

back out of phase with the input, creating an effective

reduction in gain.

This closed-loop gain is usually much less than the open-loop

gain.

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12-4 OP-AMPS With Negative Feedback Continue… Closed-LoopVoltage Gain, Acl The closed-loop gain is the voltage gain of an op-amp with

external feedback. The amplifier configuration consists of the

op-amp and an external feedback circuit that connects the

output to inverting input.

The closed-loop voltage gain is determined by the external

component values and can be precisely controlled by them.

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12-4 OP-AMPS With Negative Feedback Continue… Non-inverting Amplifier

An op-amp in a closed-loop configuration as a non-inverting

amplifier with a controlled amount of voltage gain is shown in

figure 12-16 next slide.

The input signal is applied to the non-inverting (+) input.

The output is applied back to the inverting(-) input through the

feedback circuit (closed loop) formed by the input resistor Riand the feedback resistor Rf.

Resistors Ri and Rf form a voltage-divider circuit which reduces

Vout and connects the reduced voltage Vf to the inverting input.

The feedback voltage is expressed as

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12-4 OP-AMPS With Negative Feedback Continue… Non-inverting Amplifier Continue…

The difference of the input voltage Vin , and the feedback

voltage Vf , is the differential input to the op-amp as shown in

figure 12-17 next slide. This differential voltage is amplified by

the open-loop voltage gain of the op-amp(Aol) and produces an

output voltage expressed as

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12-4 OP-AMPS With Negative Feedback Continue… Voltage Follower

It is a special case of the non-inverting amplifier where all of the

output voltage is fed back to the inverting (-) input by a straight

connection as shown in figure 12-19.

The straight feedback connection has a voltage gain of 1.

The closed-loop voltage gain of a non-inverting amplifier is

1/B.

Since B= 1 for a voltage follower case, the closed-loop voltage

gain of the voltage follower is Acl(VF) = 1/1= 1

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12-4 OP-AMPS With Negative Feedback Continue… Inverting Amplifier

An op-amp connected as an inverting amplifier with a

controlled amount of voltage gain as shown in figure 12-20.

The input signal is applied through a series input resistor Ri to

the inverting (-) input.

The output is fed back through Rf to the same input. The non-

inverting input is grounded.

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12-4 OP-AMPS With Negative Feedback Continue… Inverting Amplifier Continue…

At this point, the ideal op-amp parameters mentioned earlier

are useful in simplifying the analysis of this circuit.

Particularly the concept of infinite input impedance is of great

value.

An infinite input impedance implies zero current at the

inverting input.

If there is zero current through the input impedance, then there

must be no voltage drop between the inverting and non-

inverting terminals.

This means that the voltage at the inverting (-) input is zero.

This zero voltage at the inverting input terminal is referred to as

virtual ground. This condition is illustrated in figure 12-21(a)

next slide.42


Since there is no current at the inverting input, the current

through Ri and the current through Rf are equal as shown in

figure 12-21 (b). i.e, Iin= If

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