Download The Differential Mode Op-Amp

The Differential Mode Op-Amp
What is the Differential Mode ?
• The op-amp can be connected up in
various ways or modes.
• What it does depends on how it is
connected up.
• When connected up in the differential
mode, it finds the difference between the
two input voltages and multiplies it by the
gain.
The Differential Mode Op-Amp
The input voltages
are fed in through the
input resistors ( R1
and R2 ).
Rf
R1
-
The feedback
resistor ( Rf ) is
used to control the
gain of the op-amp.
+Vs
R2
+
V1
V2
R3
-Vs
Vo
0V
R3 is always the same size as Rf.
Revision – The Gain of an Amplifier
You may remember from Standard Grade that
the gain of an amplifier is the ratio of the output
and input voltages.
𝑉𝑜𝑢𝑡
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑔𝑎𝑖𝑛 =
𝑉𝑖𝑛
The gain can also be expressed in terms of the
resistors used to connect up the op-amp.
𝑅𝑓
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑔𝑎𝑖𝑛 =
𝑅1
The Differential Mode Gain Equation
𝑉𝑜𝑢𝑡 =
𝑅𝑓
𝑅1
(𝑉2 −𝑉1 )
Example 1
Calculate the output voltage from the op-amp
below.
150kΩ
20kΩ
-
+Vs
20kΩ
+
0.3 V
150kΩ
0.5V
0V
-Vs
Vo
Example 1 - Solution
𝑉𝑜𝑢𝑡
𝑉𝑜𝑢𝑡
𝑉𝑜𝑢𝑡
𝑅𝑓
=
(𝑉2 −𝑉1 )
𝑅1
150𝑘
=
x 0.5 − 0.3
20𝑘
= 1.5V
Example 2
What size of feedback resistor would be required
to produce an output voltage of – 6 volts ?
Rf
20kΩ
-
+Vs
20kΩ
+
0.6 V
R3
0.3V
0V
-Vs
-6V
Example 2 - Solution
𝑅𝑓
𝑉𝑜𝑢𝑡 =
(𝑉2 −𝑉1 )
𝑅1
𝑅𝑓
𝑉𝑜𝑢𝑡
⟹
=
𝑉2 − 𝑉1
𝑅1
−6
⟹ 𝑅𝑓 =
x20k
0.3 − 0.6
⟹ 𝑅𝑓 = 400kΩ