Page 297 - Mechatronics with Experiments

October 28, 2014 11:15 254mm×178mm
Printer: Yet to Come
JWST499-c05
JWST499-Cetinkunt
ELECTRONIC COMPONENTS FOR MECHATRONIC SYSTEMS 283
_
+

_ Z out
+
V Z kV i FIGURE 5.24: Model of an op-amp:
i in single-ended output op-amp. Idealized
_ +
model assumes infinite input impedance
and open loop gain, and zero output
=∞, Z = 0, and K =∞.
impendance, Z in out

triangle is always used as a symbol of amplification in electrical circuits. The symbol of a
basic op-amp shows the following five terminal, connections,

−
+
1. Power supply (bipolar) terminals (V , V , i.e., ±15 VDC, ±12 VDC, ±9 VDC,
±6 VDC).
2. Inverting (−) and noninverting (+) input terminals, each referenced to ground, with
−
+
voltages v and v .
3. Output terminal referenced to ground, where output voltage is designated as V .
o
This is an open loop op-amp. Most uses of op-amps involve adding external components
to it between its terminals to implement the desired functionalities. However, key to under-
standing the applications of op-amps (open loop or closed loop) is its open loop properties.
Here are the idealized assumptions of an open loop op-amp. Notice that in practice, actual
parameters are very close to the idealized assumptions that the performance difference
between the two is negligible in most cases.
Idealized assumptions on op-amps are as follows (Figure 5.24):

1. Input impedance of the op-amp is infinite. In reality it is a very large number compared
to source impedance.
2. Output impedance of an op-amp is zero. In reality it is a small number compared to
load impedance.
3. Open loop gain of the op-amp (K ) is infinite. In reality, it is a very large number,
OL
6
5
such as 10 to 10 .
4. Bandwidth of dynamic response is assumed to be infinite, but in reality it is a finite
large number.
The implications of these assumptions are as follows, which are very useful in deriving the
input–output relationship of any op-amp configuration.

1. Because input impedance is infinite, current flow into the op-amp from either input
−
+
terminal is zero, i = i = 0.
+
2. In addition, the differential voltage between the two input terminals is zero, v = v −
−
+
or E = v − v = 0.
d