THE OPERATIONAL AMPLIFIER

1
THE OPERATIONAL AMPLIFIER

• OSMAN PARLAKTUNA
• OSMANGAZI UNIVERSITY
• ESKISEHIR, TURKEY
• www.ogu.edu.tr/oparlak

2
THE OP AMP

• In 1968, Fairchild Semiconductor introduced an op
  amp ?A741.
• It is referred to as operational amplifier
  because it was used to implement mathematical
  operations such as addition, subtraction,
  integration, differentiation, sign changing, and
  scaling.

3
DIP PACKAGE AND SYMBOL
1
8
Offset null
NC
7
Inverting input
2
V
Noninverting input
3
6
Output
5
4
V-
Offset null
Positive power supply V
Noninverting input
Output
Inverting input
Negative power supply V-
4
TERMINAL VOLTAGES AND CURRENTS
The terminal behavior of the op amp as a linear
circuit element is characterized by constraints
on the input voltages and the input currents.
Positive saturation
Vo
Vcc
Vcc
Linear region
Vp
Vo
Vn
Vp-Vn
(Vcc/A)
(-Vcc/A)
-Vcc
-Vcc
Negative saturation
5
Virtual Short (virtual ground)
When the op amp operates in the linear region, a
constraint is imposed on the input voltages Vp
and Vn. For a typical op amp Vcc seldomly exceed
20V, and the gain A is rarely less than 10000.
Thus, in the linear region, the magnitude of the
input voltage difference (Vp-Vn) must be less
than 2mV. A voltage difference less than 2mV
means the two voltages are essentially equal.
Then, in the linear region operation we will
assume that VpVn. This constraint on the input
voltages is known as virtual short concept. If
one of the input voltages is connected to the
ground, then VpVn0 (Virtual ground)
6
Current Constraint
ip
ic
The equivalent resistance seen by the input
terminals of the op amp is very large, typically
1M? or more. Ideally, the input resistance is
infinite, resulting in the current constraint
ipin0.
io
in
ic-
ipinicic-io0. Then,
io-(icic-)
7
EXAMPLE
100k?
The op amp in the circuit is idea a) Calculate
Vo if Va1V and Vb0V b) Repeat a) for Va1V and
Vb2V c)If Va1.5V, specify the range of Vb that
avoids saturation
i100
10V
25k?
i25


Va
-10V
Vo
Vb
VpVb0Vn
i25i1000 i25(Va-Vn)/251/25 mA
i100(Vo-Vn)/100Vo/100 mA 1/25Vo/1000
Vo-4V
8
b)
9
c)
10
THE INVERTING-AMPLIFIER CIRCUIT
Rf
if
Vcc
Rs
in

is

Vs
-Vcc
Vo
For a linear region operation, VoltVcc. Then
11
THE SUMMING-AMPLIFIER CIRCUIT
Rf
Ra
Va
if
Vcc
Rb
Vb
in
Rc


Vc
-Vcc
Vo
12
THE NONINVERTING-AMPLIFIER CIRCUIT
Rf
if
Because ipin0, VpVgVn
Vcc
Rs
in


-Vcc
Rg
ip
Vo
Vg
13
THE DIFFERENCE-AMPLIFIER CIRCUIT
Rb
Vcc
Ra
in


Va
-Vcc
Rc
ip
Vo
Vb
Rd
14
EXAMPLE
i
1k?
Vp
1k?
Vo
i

6k?
3k?
3k?
12V
6k?
12V
Calculate i for both circuits, compare the
results.
15
EXAMPLE
5?
Find Vo in terms of Vs
V1
Vs
4?
2?
Vo

Vx
2?
3?
3?