Lecture 11 - Opaerational Amplifiers-3

Electronics –II
(308150)
Lecture : 11
Operational Amplifiers-3
11/16/2021 Engr Sardar Ahmad 2
Class Learning Objectives
 Introduction
 Ideal & Practical Operational Amplifiers
 OP-Amps : Input Signal Modes
 Op-Amps Parameters
 Op-Amps : Negative FeedBack
 Effects of Negative Feedback
 Open / Close loop Frequency & Phase Response
 Multi-Stage Op-Amps - Frequency & Phase response
 Op-Amps : Bandwidth
 Summary
3
Recap : Operational Amplifiers
Non-Inverting Op-Amp Inverting Op-Amp
Operational Amplifiers
 Attenuation of RC Lag Circuit :

 Midrange gain of Op-Amps reduced by negative feedback

 Close loop gain expressions for three Amplifiers are:
 Non-Inverting Amp
 Inverting Amp
 Voltage Follower Amp
Where ‘B’ is Feedback Attenuation

Selected Key Terms
Operational A type of amplifier that has very high voltage
amplifier gain, very high input impedance, very low
output impedance and good rejection of
common-mode signals.
Differential A mode of op-amp operation in which two

mode 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).
Common mode A condition characterized by the presence of

the same signal on both inputs
Selected Key Terms
Open-loop The voltage gain of an op-amp without
voltage gain external feedback.
Negative The process of returning a portion of the

feedback output signal to the input of an amplifier such
that it is out of phase with the input.
Closed-loop The voltage gain of an op-amp with external

voltage gain feedback.
Gain- A constant parameter which is always equal to

bandwidth the frequency at which the op-amp’s open-
product loop gain is unity (1).
Thanks
(Summary)
Summary
Operational Amplifers
 Operational amplifiers (op-amps) are very high gain dc

coupled amplifiers with differential inputs. One of the inputs
is called the inverting input (-); the other is called the
noninverting input. Usually there is a single output.
 Most op-amps operate from plus and minus
supply voltages, which may or may not be +V
shown on the schematic symbol.

–
+
8
1 1
8 8 20
1 1 –V
DIP DIP SMT SMT
Summary
Ideal Op-Amp
The ideal op-amp has characteristics that simplify analysis

of op-amp circuits. Ideally, op-amps have infinite voltage
gain, infinite bandwidth, and infinite input impedance. In
addition, the ideal op-amp has zero output impedance.
Vin Zin = ‘ AvVin Vout

Zout = 0
Av = ‘
+
Summary
Practical Op-Amp
Practical op-amps have characteristics that often can be

treated as ideal for certain situations, but can never actually
attain ideal characteristics. In addition to finite gain,
bandwidth, and input impedance, they have other
limitations.
–
Vin Zin AvVin Vout

Zout
+
Summary
Block Diagram
Internally, the typical op-amp has a differential input, a

voltage amplifier, and a push-pull output. Recall from the
discussion in Section 6-7 of the text that the differential
amplifier amplifies the difference in the two inputs.
+
Push-pull
Differential Voltage
amplifier
Vin amplifier amplifier(s) Vout
output
– input stage gain stage
stage
Summary
Signal modes
The input signal can be applied to an op-amp in differential-

mode or in common-mode. V in
Vout
Differential-mode signals are +
applied either as single-ended

(one side on ground) or
double-ended (opposite –
Vin
phases on the inputs). Vout
+
Differential signals
Summary
Signal modes
The input signal can be applied to an op-amp in differential-

mode or in common-mode. V in
–
Common-mode signals are
Vout
applied to both sides with the
+
same phase on both.
Vin
Usually, common-mode –
signals are from unwanted Vout
sources, and affect both +
inputs in the same way. The Vin
result is that they are Common-mode
essentially cancelled at the signals
output.
Summary
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).
Aol
CMRR is defined as CMRR 
Acm
where Aol is the open-loop differential-gain
and Acm is the common-mode gain.
 Aol 
CMRR can also be expressed in decibels as CMRR  20 log  
A
 cm 
Summary
Common-Mode Rejection Ratio
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.
 Aol 
CMRR  20 log  
A
 cm 
200, 000
 20 log  90 dB
6.3
(The minimum specified CMRR is 70 dB.)
Summar
y
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 I1  I 2
I BIAS 
average of the two dc currents 2
required to bias the differential
IOS: The input offset current is the
amplifier
I OS  I1  I 2
difference between the two dc bias
currents
Summar
y
Impedance Parameters –
ZIN(d)
ZIN(d) : The differential input impedance is
+
the total resistance between the inputs
–
ZIN(cm) : The common-mode input
ZIN(cm)
impedance is the resistance between
each input and ground +
–
Zout: The output impedance is the Zout
resistance viewed from the output of

+
the circuit.
Summary
Other Parameters
Slew rate: The slew rate is the maximum rate of change of

the output voltage in response to a step input voltage
Vout
Slew Rate 
t Vout (V)
Determine the slew rate for the output
13
response to a step input. 12
0 t
Vout  12 V    12 V 
Slew Rate  
t 4.0 μs –12
–13
4.0 m s
= 6 V/ms
Summary
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.
The advantage of negative
feedback is that precise values Vin +
Vout
of amplifier gain can be set. In
Vf –
addition, bandwidth and input
Internal inversion makes Vf
and output impedances can be 180° out of phase with Vin.
controlled. Negative
feedback
circuit
Summary
Noninverting Amplifier
A noninverting amplifier is a configuration in which the

signal is on the noninverting input and a portion of the
output is returned to the inverting input.
Feedback forces Vf to be equal
+
to Vin, hence Vin is across Ri. Vout
With basic algebra, you can Vin –
Rf
show that the closed-loop gain Vf Feedback
of the noninverting amplifier is circuit
Rf Ri
Acl (NI)  1 
Ri
Summary
Determine the gain of the noninverting amplifier shown.
Vin +
Rf Vout
Acl (NI)  1  –
Ri Rf
82 kW
82 k
 1
3.3 k Ri
3.3 kW
= 25.8
Summary
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. V ++
in
The input impedance of Vout
out
the voltage follower is – Rf
very high, producing an 82 kW
excellent circuit for

Ri
isolating one circuit from 3.3 kW
another, which avoids
"loading" effects.
Summary
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.
Feedback forces the inputs to Rf
be nearly identical; hence the
inverting input is very close to Ri
0 V. The closed-loop gain of –
the inverting amplifier is Vout
Vin
Rf +
Acl (I)  
Ri
0 V (virtual ground)
Summary
Inverting Amplifier
Determine the gain of the inverting amplifier shown.

Rf
Rf 82 kW
Acl (I)  
Ri Ri
–
82 k 3.3 kW
 Vout
3.3 k Vin +
= -24.8
The minus sign

indicates inversion.
Summary
Impedances
Noninverting amplifier:
Z in (NI)   1  Aol B  Zin Generally, assumed to be ∞
Z out
Z out (NI)  Generally, assumed to be 0
 1  Aol B 
Inverting amplifier:
Z in (I)  Ri Generally, assumed to be Ri
Z out
Z out (I)  Generally, assumed to be 0
 1  Aol B 
Note that the output impedance has the same form for both
amplifiers.
Summary
Bias Current Compensation
For op-amps with a BJT input stage, bias current can

create a small output error voltage. To compensate for
this, a resistor equal to Ri||Rf is added to one of the
inputs. R f R f
Ri
– –
Vout Vout
Ri + Vin +
Rc = Ri || Rf Rc = Ri || Rf
Vin
Noninverting Inverting
amplifier amplifier
Summary
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.
Aol (dB)
Midrange
Op-amps with this 106
100
characteristic are
called compensated 75
op-amps. The blue –20 dB/decade roll-off
line represents the 50
open-loop frequency
characteristic (Bode 25
Unity-gain frequency (f T)
Critical frequency
plot) for the op-amp.
0 f (Hz)
1 10 100 1k 10k 100k 1M
Summary
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))
Av
The closed-loop critical Open-loop gain
frequency is higher Aol(mid )
than the open-loop

critical frequency by
the factor (1 + BAol(mid)).
Closed-loop gain
This means that you Acl(mid )
can achieve a higher
BW by accepting less
gain. For a
f
compensated op-amp, 0 fc(ol) fc (cl )
Acl f(cl) = Aol fc(ol).

.
Summary
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.
Vin +
The fT for a 741C op amp is 1 MHz.
741C Vout
What is the BWcl for the amplifier? – Rf
82 kW
Rf
82 k
Acl (NI)  1   1
 25.8
Ri 3.3 k Ri
3.3 kW
f 1 MHz
BWcl  T   38.8 kHz
Acl 25.8
Thanks

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Electronics –II

 Attenuation of RC Lag Circuit :

 Midrange gain of Op-Amps reduced by negative feedback

 Voltage Follower Amp

Where ‘B’ is Feedback Attenuation

Differential A mode of op-amp operation in which two

Common mode A condition characterized by the presence of

Negative The process of returning a portion of the

Closed-loop The voltage gain of an op-amp with external

Gain- A constant parameter which is always equal to

 Operational amplifiers (op-amps) are very high gain dc

shown on the schematic symbol.

The ideal op-amp has characteristics that simplify analysis

Vin Zin = ‘ AvVin Vout

Practical op-amps have characteristics that often can be

Vin Zin AvVin Vout

Internally, the typical op-amp has a differential input, a

The input signal can be applied to an op-amp in differential-

applied either as single-ended

The input signal can be applied to an op-amp in differential-

Common-Mode Rejection Ratio

The ability of an amplifier to amplify differential signals and

Common-Mode Rejection Ratio

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

VO(p-p): The maximum output voltage swing is determined

resistance viewed from the output of

Slew rate: The slew rate is the maximum rate of change of

Negative feedback is the process of returning a portion of

A noninverting amplifier is a configuration in which the

Determine the gain of the noninverting amplifier shown.

A special case of the inverting amplifier is when Rf =0 and Ri

excellent circuit for

An inverting amplifier is a configuration in which the

Determine the gain of the inverting amplifier shown.

The minus sign

Bias Current Compensation

For op-amps with a BJT input stage, bias current can

Many op-amps have a roll off rate determined by a single

line represents the 50

For op-amps with a -20 dB/decade open-loop gain, the closed-loop

than the open-loop

Acl f(cl) = Aol fc(ol).

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