Electroincs, OP AMP

1. EE 2105: Electronics
Lecture 11
Operational Amplifier (Op-Amp)
Arefin Ahamed
Lecturer, EEE, KUET

2. Basic Op-Amp
Dept. of Electrical and Electronic Engineering, KUET 2
Operational amplifier or op-amp, is a very high gain differential amplifier with a high
input impedance (typically a few meg-Ohms) and low output impedance (less than 100 ).
Note the op-amp has two inputs and one output.
X
Voltage or
Current
X
1. Very High Gain (A = 𝟏𝟎𝟓 to 𝟏𝟎𝟔)
2. Differential Input
𝑉+
𝑉−
𝐕𝟎 = 𝐀(𝐕+- 𝐕−)

3. Basic Op-Amp
Dept. of Electrical and Electronic Engineering, KUET 3
For ideal Op-Amp:
1. Current i = 0
2. 𝐕𝟎 = 𝐀(𝐕+- 𝐕−)
i = 0

4. Op-Amp Gain
Dept. of Electrical and Electronic Engineering, KUET 4
Op-Amps have a very high gain. They can be connected open-loopor closed-loop.
• Open-loop refers to a 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.

5. Inverting Op-Amp
Dept. of Electrical and Electronic Engineering, KUET 5
• The signal input is applied to the inverting (–) input
• The non-inverting input (+) is grounded
• The resistor Rf is the feedback resistor. It is connected from the
output to the negative (inverting) input. This is negative feedback.

6. Virtual Ground
Dept. of Electrical and Electronic Engineering, KUET 6

7. Virtual Ground
Dept. of Electrical and Electronic Engineering, KUET 7
An understanding of the concept of virtual ground provides a better understanding of
how an op- amp operates.
The non-inverting input pin is at ground. The inverting input pin is also at 0 V for an AC
signal.
The op-amp has such high input impedance that even with a high gain there is no current
from inverting input pin, therefore there is no voltage from inverting pin to ground—all of the
current is through Rf.

8. Inverting Op-Amp Gain
Dept. of Electrical and Electronic Engineering, KUET 8

9. Practical Op-Amp Circuits
Dept. of Electrical and Electronic Engineering, KUET 9
Inverting amplifier
Noninverting amplifier
Unity follower
Summing amplifier
Integrator
Differentiator

10. Inverting/Noninverting Op-Amps
R1
Vo 
 Rf V1
InvertingAmplifier NoninvertingAmplifier
o 1
Dept. of Electrical and Electronic Engineering, KUET 10
R1
V  (1 
Rf )V

11. Noninverting Op-Amps
Dept. of Electrical and Electronic Engineering, KUET 11
𝑉− =
𝑅1
𝑅1 + 𝑅𝑓
𝑉0
𝑉0 = 𝐴(𝑉1 −
𝑅1
𝑅1+𝑅𝑓
𝑉0)
𝑉0 1 +
𝐴𝑅1
𝑅1+𝑅𝑓
= 𝐴𝑉1
𝑉0 =
𝐴𝑉1
1+
𝐴𝑅1
𝑅1+𝑅𝑓
=
𝐴𝑉1
𝐴𝑅1
𝑅1+𝑅𝑓
∴ 𝑉0 = 𝐴𝑉1(
𝑅1+𝑅𝑓
𝑅1
)

12. Math Problems
Dept. of Electrical and Electronic Engineering, KUET 12

13. Unity Follower
Dept. of Electrical and Electronic Engineering, KUET 13
Vo  V1

14. Summing Amplifier
Because the op-amp has a
high input impedance, the
multiple inputs are
treated as separate inputs.

Dept. of Electrical and Electronic Engineering, KUET 14

V3 

3
2
 1
o
Rf
V1 
R
V2 
R
R
 Rf Rf
V  

15. Math Problems
Dept. of Electrical and Electronic Engineering, KUET 15

16. Math Problems
Dept. of Electrical and Electronic Engineering, KUET 16

17. Integrator
Dept. of Electrical and Electronic Engineering, KUET 17
RC
The output is the integral
of the input. Integration
is the operation of
summing the area under
a waveform or curve over
a period of time. This
circuit is useful in low-
pass filter circuits and
sensor conditioning
circuits.
vo (t)  
1
v1 (t)dt

18. Differentiator
The differentiator
takes the derivative of
the input. This circuit
is useful in high-pass
filter circuits.
dt
Dept. of Electrical and Electronic Engineering, KUET 18
vo (t)  RC
dv1(t)

19. Op-Amp Applications
Dept. of Electrical and Electronic Engineering, KUET 19
Constant-gain multiplier
Voltage summing
Voltage buffer
Controlled sources
Instrumentation circuits
Active filters

20. Constant-Gain Amplifier
Inverting Version
Dept. of Electrical and Electronic Engineering, KUET 20
more…

21. Math Problems
Dept. of Electrical and Electronic Engineering, KUET 21

22. Constant-Gain Amplifier
Noninverting Version
Dept. of Electrical and Electronic Engineering, KUET 22

23. Math Problems
Dept. of Electrical and Electronic Engineering, KUET 23

24. Multiple-Stage Gains
Dept. of Electrical and Electronic Engineering, KUET 24

25. Multiple-Stage Gains
Dept. of Electrical and Electronic Engineering, KUET 25

26. Voltage Buffer
Dept. of Electrical and Electronic Engineering, KUET 26
Any amplifier with no gain or loss is called a unitygain
amplifier.
The advantages of using a unity gain amplifier:
• Very high input impedance
• Very low output impedance
Realistically these circuits
are designed using equal
resistors (R1 = Rf) to avoid
problems with offset
voltages.