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Table 3. Comparison of three types of scaling operation circuits
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
215

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The operational amplifier is an amplifier, but when it is combined with other circuit elements,
it may integrate, differentiate, product, divide, sum, and subtract. In circuit, there are three
basic types of proportional operation amplifiers: inverse scaling operation circuit, noninverse
scaling operation circuit, and differential scaling operation circuit. These three types of
operation circuit are compared as illustrated in table 3.
According to the information listed in table 3, we could know that operational amplifier is
drawn with the symbol in figure 32. The input terminals are labeled the no inverting input (+)
and inverting input (-). The power supply terminals are labeled 
V
+
and 
V
−
, which are frequently
omitted, since they do not affect the circuit behavior except in saturation conditions. Most
people shorten the name of operational amplifier to the “op amp”.
Figure 32. Circuit element symbol for the operational amplifier
Figure 33 shows an ideal mode of op amp, focusing on the internal behavior of input and
output terminals. The input-output relationship is the following:
Figure 33. An internal mode of the op amp
u
o
= A
(
V
p
−V
n
)
Advances in Bioengineering
216

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Since the terminal resistance is very large, we may replace it with an open circuit to simplify
analysis, leaving us with the op amp model shown in figure 34.
With the replacement of the internal resistance in an open circuit, the input current is zero
(
i
p
=i
n
=0A
). In addition, the output current (
i
A
) of the op amp is not zero. Because the output
current (
i
A
) is not known, seldom is KCL applied at the output junction. In solving the op amp
problems, KCL is always applied at input terminals.