Biomedical Engineering Reference
In-Depth Information
The op amp circuit in Example Problem 9.21 provides an output proportional to the
difference of two input voltages. This op amp is often referred to as a differential amplifier.
EXAMPLE PROBLEM 9.21
Find the overall gain for the following circuit.
i
2
R
2
R
1
i
a
−
+
i
b
v
n
+
+
+
R
1
v
p
+
−
V
a
+
v
0
V
b
R
2
−
−
−
−
Solution
Assuming an ideal op amp, we note no current flows into the input terminals and that
v
n
¼
v
p
.
Apply KCL at the inverting input terminal gives
i
a
¼
i
2
or
v
n
V
a
R
1
þ
v
n
v
o
R
2
¼
0
and
R
1
þ
R
ð Þ
v
n
R
2
V
a
¼
R
1
v
o
The previous equation involves two unknowns, so we need another equation easily found by
applying the voltage divider at the noninverting input.
R
2
v
p
¼
2
v
b
¼
v
n
R
þ
R
1
Substituting this result for
v
n
into the KCL equation at the inverting terminal gives
R
2
V
b
R
2
V
a
¼
R
1
v
o
or
v
o
¼
R
2
R
1
V
b
V
a
ð
Þ
As shown, this op amp circuit, also known as the differential amplifier, subtracts the weighted
input signals. This amplifier is used for bipolar measurements involving ECG and EEG, since the
typical recording is obtained between two bipolar input terminals. Ideally, the measurement
