Biomedical Engineering Reference
In-Depth Information
−
inverting
A
output
+
non−inverting
Fig. 7.1.
A differential amplifier. The device has two inputs,
V
+
and
V
−
, and an
ouput
V
out
=
A
(
V
+
−
V
−
), where
A
is the gain of the amplifier
1. The voltages satisfy
V
+
∼
V
−
.
2. The current into the device is negligible.
It is not complex to show that in this operating regime, the device illustrated
in Fig. 7.2a is a summing amplifier: since one of the inputs (
V
+
) is grounded,
V
−
∼
0. Therefore, the currents through the resistances are
I
j
=
V
j
/R
j
.
Since the device takes virtually no current, the sum of all those currents
passes through
R
F
. Therefore,
V
R
1
V
R
2
V
R
3
V
out
R
F
+
+
=
−
.
(7.20)
By choosing the resistances, the output voltage can be chosen to be (modulo
an inversion) the sum of the input voltages.
a)
R
F
R
1
V
1
R
2
−
V
2
A
V
R
3
out
+
V
3
C
b)
R
−
V
in
A
V
out
+
Fig. 7.2.
(
a
) A summing amplifier. This arrangement of resistances and an op-amp
allows us to obtain an output voltage that is the sum of a number of voltages in
the circuit. (
b
) An integrator. By connecting a capacitor and a resistance as shown
in the figure to an op-amp, we are able to integrate the signal
V
in
