Biomedical Engineering Reference
In-Depth Information
FIGURE 5-9
Op
amp circuit for a
noninverting
amplifier.
ve input to the op amp is a virtual
earth, the input impedance of the inverting amplifier is equal to
R
1
, which is usually less
than 100 k
One further consideration is that because the
−
or so (depending on the actual input impedance of the op amp and the bias
current required). This can result in this amplifier circuit loading the input, with unforeseen
consequences (Kuphaldt, 2003).
5.3.2.3 Noninverting Amplifier
The relationship between the input and output voltage can easily be derived from
Figure 5-9. Consider that the input impedance of the op amp is extremely high; there-
fore, the current flowing into node A must equal the current flowing out.
V
in
R
1
=
V
out
−
V
in
R
2
(5.13)
from which it is easy to determine that
V
in
R
1
+
R
2
R
1
V
out
=
(5.14)
For the component values shown in Figure 5-9, the gain is
A
V
=
11
,
making the output
1.1 V for an input of 100 mV.
If the resistance of
R
2
is reduced to zero, the circuit reduces to that in Figure 5-7, and
the gain reduces to unity.
In this configuration, the input impedance of the amplifier is equal to the input
impedance of the op amp, which is extremely high, and hence no loading to any pre-
vious stages will occur.
5.3.2.4 Differential Amplifier
An op amp with no feedback already provides differential gain, amplifying the voltage
difference between the two inputs. However, its gain cannot be controlled, and it is gen-
erally too high to be of any use except as a comparator. In the previous examples, the
application of negative feedback to op amps has resulting in the practical loss of one of
the inputs, with the resulting amplifier good only for amplifying a single voltage signal
input. However, an op amp circuit maintaining both voltage inputs with a controlled gain
set by external resistors can be constructed as shown in Figure 5-10.
If all the resistor values are equal, this amplifier will have a differential voltage gain
of 1. The analysis of this circuit is similar to that of an inverting amplifier, except that the
noninverting input (
+
) of the op amp is at a voltage equal to a fraction of
V
2
rather than
being connected directly to ground. In this configuration,
V
2
functions as the noninverting
