5.2. Operational amplifiers Operational amplifiers play an important role that they amplify a weak signal and adjust
voltage or current in detecting circuit. An operation amplifier is an electronic device that
consists of plenty of transistors, resistors, and capacitors. Fully understanding its operation
requires that people have the knowledge of diodes and transistors. Circuit involving opera‐
tional amplifier forms the cornerstone of any bioinstrumentation, from amplifiers to filters.
Amplifiers used in biomedical applications have very high-input impedance to keep the
current down from the system being measured. Most body signals have small magnitudes.
For example, ECG has a magnitude in millivolts and the EEG has a magnitude in microvolt.
Analog filters are often used to remove noise from a signal, typically through frequency
domain analysis to design the filter.
Inverting input Noninverting input Differential input Circuit
Constitution
Requi rement:
R
2
=R
1
//R
F
Requirement:
R
2
=R
1
//R
F
Requirement:
R
1
=R
’
1,
R
F
=R
’
F
Voltage
amplification
factor
A uf =
u o u i = −
R F R 1
Output and input voltages are inverse
A uf =
u o u i =1−
R F R 1
Output and input voltages are
noninverse
A uf =
u o u i '
−u i =
R F R 1
When R
1
=R
’
1,
R
F
=R
’
F
R if R if = R 1
R if =
(
1 + A od F )
R id R if =2R 1
R o Low
Low
Low
Performance
characteristics
∎ Realize inverse proportional operation.
∎ Cite parallel voltage negative
feedback.
∎ Virtual ground, low common-mode
input voltage.
∎ Low input and output resistance.
∎ Realize noninverse proportional
operation.
∎ Cite series voltage negative
feedback.
∎ Virtual short, but not virtual
ground, high common-mode input
voltage.
∎ High input resistance and low
output resistance.
∎ Realize subtract operation.
∎ Virtual short, but not virtual ground,
high common-mode input voltage.
∎ Low input and output resistance.
∎ High requirement of component
symmetry.
