it will be useful for engineering students in analog electronics subject. this ppt is showing basic of differential amplifier.

1. DIFFERENTIAL AMPLIFIER
Adani institute of infrastructure engineering
Presented by:-
Name:- raval arpit m.
Er.no:- 161310109050
Field:- electrical engineering
Sem.:- 3

2. What is differential amplifier?
 A differential amplifier is a type of electronic amplifier that
amplifies the difference between two input voltages . It is an
analog circuit with two inputs and one output in which the output
is ideally proportional to the difference between the two voltages.
(A differential amplifier)

3. Types of differential amplifier:-
There are three types of differential amplifiers:-
1) Differential amplifier using one op-
amp
2)Differential amplifier using two op-
amp
3)Differential amplifier using three op-
amp

4. Differential amplifier using one op-amp
 The difference amplifier and subtractor circuits are used to
obtain the subtraction of two input voltages
The circuit diagram of the differential amplifier using one op-
amp is given below.

5. Expression for output voltage:-
 We use the superposition theorem to obtain the expression for output
voltage.
Consider only v2 connected and assume v1 to be short circuited.
The output voltage due to only v2 be denoted by vo2.
so; vo2 = - Rf/R1*v2

6.  Now assume v2=0 that means v2 is short circuited to
ground and only v1 connected.
 it is a non-inverting amplifier with output voltage vo1.
The output voltage vo1 due to only v1 is given by, vo1= Avf * vA
But Avf = 1+RF/R1 AND VA = RF/R1+RF (V1)
so Vo = RF/R1(V1-V2)
Voltage gain is AD = VO/V1-V2 = -RF/R1

7.  Differential amplifier with two op-amps:-
 The gain and input resistance of the differential amplifier can be
increased if we use two op-amps.
A1 is non-inverting amplifier and A2 is differential amplifier.

8.  Voltage gain:-
 Step 1:- voltage gain of first stage:
 AV1 = 1+ R3/R2
 BUT R3 = R1 and R2 = RF
 so AV1 = (1+ R1/RF)
 Hence the output voltage of the first stage is ;
V3 = AV1 * V2
= (1+ R1/RF) V2

9. STEP 2:- output of second stage:
 The total output is given by ,
VO= VO due to V12 alone +VO due to V3 alone
= [1+ R1/RF] V1 – RF/ R1(V3)
 Substituting the expression for V3 we will get the final equation of
output voltage
VO = [1+ RF/R1] (V1 – V2)
 AND the equation of gain of the circuit is,
AD = 1+ RF/R1

10. Differential amplifier using three op-amps:-

11.  The difference amplifier discussed earlier has a low input impedance. Whereas
the input impedance of a high quality instrumentation amplifier should be very
high.
 This problem of the difference amplifier can be overcome by using a buffer
stage on the input side. Buffer is nothing but unity voltage follower circuit.
 The voltage gain of the buffer stage is 1. but it provides a very high input
impedance.
 The gain of the circuit is same as the gain of the difference amplifier.
 So we can write ; Va = V1. and Vb= V2
 The output of the differential amplifier is given by
Vo = R2/ R1( Vb – Va)
Vo = R2/ R1( V2 – V1)
 the gain of the differential amplifier is (R2/R1).