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KKKL2174 Analogue Electronic
Operational Amplifiers (OP-AMP)
Anuar Mikdad Muad
Department of Electrical, Electronic and Systems Engineering
Faculty of Engineering and Built Environment
Universiti Kebangsaan Malaysia
Introduction
Output
Input 1
Input 2
Functionality: Change voltage amplitude and polarity.
Characteristics: High gain (), high input impedance (), low output impedance (0).
Applications: Oscillator, filter, regulator, comparator, and many more…
Op-Amp Schematic Diagram
Op-Amp – Symbol and Package
Single-Ended Input
Amplified output with the same polarity as the input signal.
Amplified output with the opposite polarity as the input signal.
Solve for the gain out
in
V
V
out OV A V V V Is grounded (0V)
0O inA V
O inA V
outO
in
VA
V Gain
Solve for the gain out
in
V
V
out OV A V V V Is grounded (0V)
0O inA V
O inA V
outO
in
VA
V Gain
Common-Mode Operation
• Same input signals at both inputs.
• Both of the input signals are equally amplified but in opposite polarity.
• These signals cancel out, making the output = 0 V.
• However, in practice, a very small output will be produced.
Differential and Common-Mode Operation
• For an op-amp with arbitrary input voltages, V1 and V2.
1 2dv V V Differential input signal,
Common input signal, 1 2
2c
V Vv
The output of the op-amp, o d d c cv A v A v
Common-mode rejection ratio (CMRR), CMRR d
c
A
A
10CMRR log 20log d
c
A
A
Common-Mode Rejection Ratio
The output voltage of the op-amp,
1
R1
1
CM R
o d d c c
cd d
d
cd d
d
c
d
A
v A v A v
vA v
A v
vA v
v
When CMRR is large, the output voltage will be influenced mostly
by the difference signal, while the effect of the common-mode
component is greatly reduced.
Common-Mode Rejection Ratio
• An op-amp has input voltages of and .
• The differential gain of the amplifier is
• Compare the output voltage with the component of differential input signals when the CMRR of the op-amp is
– 100.
– 105.
1150 ViV
2140 ViV
4000dA
Common-Mode Rejection Ratio
• CMRR indicates the op-amp’s ability to reject a signal applied simultaneously to both input.
• Higher CMRR, better op-amp.
• Typically, CMRR 80-120 dB.
• CMRR decreases when frequency increases.
AC Equivalent of Op-Amp Circuit
• The input signal applied between input terminals sees an input impedance Ri (typically very high) •The output voltage is shown to be the amplifier gain times the input signal taken through the output impedance Ro (typically very low). • An ideal op-amp circuit would have infinite input impedance, zero output impedance and infinite voltage gain.
(a) Practical
(b) Ideal
Op-amp gain
• Open-loop
– configuration where there is no feedback from output back to the input.
– In the open-loop configuration the gain can exceed 10,000.
• Closed-loop
– configuration reduces the gain.
– In order to control the gain of an op-amp it must have feedback.
– This feedback is a negative feedback.
– A negative feedback reduces the gain and improves many characteristics of the op-amp.
How to use op-amp
• We would like op-amp to amplify input signal by a factor of any arbitrary value.
• However, the open-loop gain is fixed and too large.
• We need to make the amplification system as close-loop using some external circuits.
• There are two configuration of using the external circuits: – Noninverting
– Inverting.
Inverting close-loop configuration
Basic op-amp
connection
Op-amp ac equivalent
circuit
AOL = open loop gain for the op-amp
Ri = input impedance
Ro = output impedance
o OL iV A V
Since AOL is large (), 0oi
OL
VV
A
We also know that iV V V
Because Vis grounded, therefore 0V V
It means that is also grounded. V
This concept is called the virtual ground.
Using KCL
1
1
i i i o
i f
V V V V V
R R R
Because 0iV , therefore 1
1
o
f
VV
R R
and
1 1
foRV
V R . Voltage gain is
1
f
v
RA
R
Circuit provides a voltage gain with an 180° phase inversion from the input signal
* You may use superposition theorem to derive the voltage gain (as explained in the Boylestad & Nashelsky).
The Golden Rules
1. Voltage difference between the inputs is zero.
2. The inputs draw no current.
Inverting Amplifier
• Input signal is at the inverting (–) input.
• Non-inverting (+) input terminal is grounded.
• Feedback resistor, Rf is placed between the output and
inverting input.
1
1
f
o
RV V
R
Non-inverting amplifier
o
f
VRR
RV
1
11
11
1
1
1R
R
R
RR
V
V ffo
The circuit’s output is in phase with its input signal
Using voltage divider
Rearrange
Non-inverting amplifier Equivalent circuit for
non-inverting amplifier
Unity Follower
• No amplification, and the output only follows its input.
• This circuit is useful for circuit-isolation applications (e.g. Buffer).
1
Gain 1oV
V
Summing Amplifier
Because the op-amp has a high input impedance, the multiple inputs
are treated as separate inputs.
Summing amplifier Virtual ground equivalent circuit
Summing Amplifier
31 21 2 3
1 2 3
, ,VV V
I I IR R R
1 2 3fI I I I
o f fV I R
1 2 3
1 2 3
f f f
o
R R RV V V V
R R R
Each input adds a voltage to the
output multiplied by its separate constant-gain multiplier.
Integrator
0V grounded. Since
Therefore 0V V
No current enters the input of op-amp.
Therefore
0in in inR
V V V VI
R R R
0out out outC
d V V d V dVdVI C C C C
dt dt dt dt
Integrator
0C RI I
•The input signal is integrated at the output.
• Used in low-pass filter circuits and sensor conditioning circuits.
1out indV V dt
RC
0out indV VC
dt R
0
10
t
out in outV t V t dt VRC
Integrator
Integration operation is one of summation, summing the area under a waveform or curve over a period of time.
Integrator
• At DC, capacitor becomes open
circuit (no feedback).
• The output voltage saturates.
• To provide closed-loop gain at DC,
an additional resistor is connected
parallel to the capacitor.
• R2 > R
Summing integrator
tv
CRtv
CRtv
CRtvo 3
3
2
2
1
1
111
Differentiator
0V
0V V
Grounded
No current enters the input of op-amp.
Therefore
0in in inC
d V V d V dVdVI C C C C
dt dt dt dt
0out out outR
V V V VI
R R R
Differentiator
0C RI I
inout
dVV RC
dt
•The input signal is differentiated at the output.
• Sensitive to noise due to the op-amp’s high ac gain.
• Used in high-pass filter circuits.
