analog electronics

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th editionThomas L. Floyd

Analog Electronics

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+V

–V

Lecture 5



Operational Amplifers

Op-amp is an electronic device that amplify the difference of voltage at its two inputs.

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+V

–V

Very high gain dc coupled amplifiers with differential inputs.

One of the inputs is called the inverting input (-); the other is called the non-inverting input. Usually there is a single output.

Most op-amps operate from plus and minus supply voltages, which may or may not be shown on the

schematic symbol.



The Ideal Op-Amp

Ideally, op-amps have characteristics (used in circuit analysis):o Infinite voltage gain

o Infinite input impedance (does not load the driving sources)

o Zero output impedance (drive any load)

o Infinite bandwidth (flat magnitude response, zero phase shift)

o Zero input offset voltage.

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+

Zin = ‘Vin VoutZout = 0

AvVin

Av = ‘

The ideal op-amp has characteristics that simplify analysis of op-amp circuits.

The concept of infinite input impedance is particularly a valuable analysis tool for several op-amp configurations.



The Practical Op-Amp

Real op-amps differ from the ideal model in various respects. In addition to finite gain, bandwidth, and input impedance, they have other limitations.

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+

ZinVin Vout

Zout

AvVin

o Finite open loop gain.o Finite input impedence.o Non-zero output impedence.o Input current.o Input offset voltage.o Temperature effects.



Input and Output Impedances



Input Signal modes

The input signal can be applied to an op-amp in differential-mode or in common-mode.

Differential-mode signals are applied either as single-ended (one side on ground) or double-ended (opposite phases on the inputs). Vin

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+

Vout

Vin

–

+

Vout



Signal modes

Common-mode signals are applied to both sides with the same phase on both.

Vin

Vin

–

+

Vout

Vin

–

+

Vout

Usually, common-mode signals are from unwanted sources, and affect both inputs in the same way. The result is that they are essentially cancelled at the output.

Common-mode signals



Common-Mode Rejection Ratio

The ability of an amplifier to amplify differential signals and reject common-mode signals is called the common-mode rejection ratio (CMRR).

CMRR is defined as ol

cm

A

ACMRR where Aol is the open-loop

differential-gain and Acm is the common-mode gain.

CMRR can also be expressed in decibels as 20log ol

cm

A

A

CMRR

Acm is zero in ideal op-amp and much less than 1 is practical op-amps.

Aol ranges up to 200,000 (106dB)

CMRR = 100,000 means that desired signal is amplified 100,000 times more than un wanted noise signal.



Common-Mode Rejection Ratio

Example

What is CMRR in decibels for a typical 741C op-amp?

The typical open-loop differential gain for the 741C is 200,000 and the typical common-mode gain is 6.3.

90 dB

20log ol

cm

A

A

CMRR

200,00020log

6.3

(The minimum specified CMRR is 70 dB.)



Current Offsets

IBIAS: The input bias current is the average of the two dc currents required to bias the differential amplifier

IOS: The input offset current is the difference between the two dc bias currents

1 2BIAS 2

I II

OS 1 2I I I

Vos = Ios.RinVos = Ios.Rin

VOUT(ERR) = AvIos.RinVOUT(ERR) = AvIos.Rin



Schematic



Voltage and Current Parameters

VO(p-p): The maximum output voltage swing is determined by the op-amp and the power supply voltages

VOS: The input offset voltage is the differential dc voltage required between the inputs to force the output to zero volts

IBIAS: The input bias current is the average of the two dc currents required to bias the differential amplifier

IOS: The input offset current is the difference between the two dc bias currents

1 2BIAS 2

I II

OS 1 2I I I



Impedance Parameters

ZIN(d) : The differential input impedance is the total resistance between the inputs

ZIN(cm) : The common-mode input impedance is the resistance between each input and ground

ZIN(d)

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+

ZIN(cm)

–

+

Zout: The output impedance is the resistance viewed from the output of the circuit.

Zout

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+



Slew Rate

Slew rate: The slew rate is the maximum rate of change of the output voltage in response to a step input voltage

Slew Rate outV

t

Determine the slew rate for the output response to a step input.

12 V 12 VSlew Rate

4.0 μs outV

t

Vout (V)

t

–13–12

1312

0

4.0 sm= 6 V/ms



Extremely High Gain

Open loop gain of an op-amp is in order of 100,000.

Even an extremely small input such as input offset voltage can saturate the out put.

Vin * Aol= (1mv * 100,1000) = 100V

It limits the use to comparator.

It is not well-controlled parameter.



Negative Feedback

Negative feedback is the process of returning a portion of the output signal to the input with a phase angle that opposes the input signal.

Vout

+

–

Vin

Vf

Internal inversion makes Vf

180° out of phase with Vin.

Negativefeedback

circuit

One of the most useful concepts in electronics

The advantage of negative feedback is that precise values of amplifier gain can be set. In addition, bandwidth and input and output impedances can be controlled.The gain with external feedback is called closed loop gain, Acl



Noninverting Amplifier

A noninverting amplifier is a configuration in which the signal is given on the noninverting input and a portion of the output is returned to the inverting input.

The closed-loop gain of the noninverting amplifier is

Rf

Ri

Vf

Vin

+

–

Feedbackcircuit

Vout

(NI) 1 fcl

i

RA

R

The difference of input voltage, Vin and the feedback voltage Vf is the differential input to op-amp .

Difference is amplified with Aol



Noninverting Amplifier

Example

(NI) 1 fcl

i

RA

R

Determine the gain of the noninverting amplifier shown.

Rf82 kW

Vin +

–

Vout

Ri3.3 kW

82 k1

3.3 k

= 25.8



Voltage Follower

A special case of the inverting amplifier is when Rf =0 and Ri = ∞. This forms a voltage follower or unity gain buffer with a gain of 1.

Rf82 kW

Vin +

–

Vout

Ri3.3 kW

It produces an excellent circuit for isolating one circuit stage from another, which avoids "loading" effects.

Vin +

–

Vout

This configuration offers very high input impedance and its very low output impedance.

These features make it a nearly ideal buffer amplifier for interfacing high-impedance sources and low-impedance loads.



Inverting Amplifier

An inverting amplifier is a configuration in which the noninverting input is grounded and the signal is applied through a resistor to the inverting input.

The closed-loop gain of the inverting amplifier is

(I)f

cli

RA

R

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+

Rf

Vout

Ri

Vin

0 V (virtual ground)

Concepts of infinite open loop gain and infinite input resistance extends to ’virtual ground’ at inverting input.

Current through Ri and through Rf is equal as no current to the inverting input.

Iin

If

I1=0



Example

Determine the gain of the inverting amplifier shown.

82 k

3.3 k

= -24.8

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+

Rf

Vout

Ri

Vin

82 kW

3.3 kW

(I)f

cli

RA

R

The minus sign indicates phase inversion.

Inverting Amplifier



Noninverting amplifier:

Impedances

(NI) 1in ol inZ A B Z

(NI) 1out

outol

ZZ

A B



Impedances

(I) in iZ R

(I) 1out

outol

ZZ

A B

Inverting amplifier:

Note that the output impedance has the same form for both amplifiers.



Noninverting amplifier:

Impedances

(NI) 1in ol inZ A B Z

(NI) 1out

outol

ZZ

A B

Generally, assumed to be ∞

Generally, assumed to be 0

(I) in iZ R

(I) 1out

outol

ZZ

A B

Generally, assumed to be Ri

Generally, assumed to be 0

Inverting amplifier:

Note that the output impedance has the same form for both amplifiers.



Bias Current and Offset voltage with compensation techniques

Transistors within op-amp need bias current.

Practical op-amp has small input bias currents.

Small imbalances in transistors produce a small offset voltage between the inputs.



Bias Current and Offset voltage with compensation techniques



Bias Current Compensation

To compensate for input offset current, a resistor equal to Ri||Rf is added to one of the inputs.

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Rf

Vout

Ri

Vin

Rc = Ri || Rf

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Rf

Vout

Ri

Vin

Rc = Ri || Rf

Noninverting amplifier

Inverting amplifier



Input Offset Voltage Compensation

Most ICs provide a mean of compensation.

An external potentiometer to the offset null pins of IC package



Bandwidth Limitations

Many op-amps have a roll off rate determined by a single low-pass RC circuit, giving a constant -20 dB/decade down to unity gain.

–20 dB/decade roll-off

Unity-gain frequency (fT)Critical frequency

101 100 1k 10k 100k 1Mf (Hz)

106100

75

50

25

0

Aol (dB)MidrangeThe blue line

represents the open-loop frequency characteristic (Bode plot) for the op-amp.




For op-amps with a -20 dB/decade open-loop gain, the closed-loop critical frequency is given by fc(cl) = fc(ol)(1 + BAol(mid))

The closed-loop critical frequency is higher than the open-loop critical frequency by the factor (1 + BAol(mid)). This means that you can achieve a higher BW by accepting less gain. For a compensated op-amp, Acl f(cl) = Aol fc(ol).

.

Av

f

Aol(mid )

0 fc(ol) fc (cl )

Acl(mid )

Closed-loop gain

Open-loop gain



Example


The equation, Acl f(cl) = Aol fc(ol) shows that the product of the gain and bandwidth are constant. The gain-bandwidth product is also equal to the unity gain frequency. That is fT = Acl fc(cl), where fT is the unity-gain bandwidth.

The fT for a 741C op amp is 1 MHz. What is the BWcl for the amplifier?

Rf82 kW

Vin +

–

Vout

Ri3.3 kW

741C

(NI)

82 k1 1 25.8

3.3 kf

cli

RA

R

1 MHz

25.8T

clcl

fBW

A 38.8 kHz



Selected Key Terms

Operational amplifier

Differential mode

Common mode

A type of amplifier that has very high voltage gain, very high input impedance, very low output impedance and good rejection of common-mode signals.

A mode of op-amp operation in which two opposite-polarity signals voltages are applied to the two inputs (double-ended) or in which a signal is applied to one input and ground to the other input (single-ended).

A condition characterized by the presence of the same signal on both inputs



Selected Key Terms

Open-loop voltage gain

Negative feedback

Closed-loop voltage gain

Gain-bandwidth

product

The voltage gain of an op-amp without external feedback.

The process of returning a portion of the output signal to the input of an amplifier such that it is out of phase with the input.

The voltage gain of an op-amp with external feedback.

A constant parameter which is always equal to the frequency at which the op-amp’s open-loop gain is unity (1).

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