1. SUMMING AND DIFFERENCE
AMPLIFIER

2. Summing Amplifier
 The summing amplifier is a versatile device for
combining signals. It can either add signals directly, or
scale them to fit some predetermined combination
rule.
 Summing several signals with equal gains is done in
an audio mixer.
 A summing amplifier with different resistors on the
inputs gives a weighted sum. This can be used to
convert a binary number to a voltage in a digital to
analog converter.
 A summing amplifier can be used to apply a DC offset
voltage along with an AC signal voltage. This is done
in a LED modulation circuit to keep the LED in its
linear operating range.

3. The Summing Amplifier
 Scale factors for the 2
inputs can be
independently adjusted by
the proper choice of R2 and
R1.
 Any number of inputs can
be connected to a summing
junction through extra
resistors.
 This circuit can be used as
a simple digital-to-analog
converter. This will be
illustrated in more detail,
1
1
v
1
i
R

2
2
v
2
i
R

3
ov
3
i
R

Since the negative amplifier
input is at virtual ground,
Since i-=0, i3= i1 + i2,

vo
R
3
R
1
v
1

R
3
R
2
v
2

4. Difference Amplifier
 Then differential amplifiers amplify the difference
between two voltages making this type of operational
amplifier circuit a Subtractor unlike a summing amplifier
which adds or sums together the input voltages. This
type of operational amplifier circuit is commonly known
as a Differential Amplifier configuration and is show in
the next slide.

5. The Difference Amplifier
 This circuit is also called a
differential amplifier, since it
amplifies the difference
between the input signals.
 Rin2 is series combination of
R1 and R2 because i+ is zero.
 For v2=0, Rin1= R1, as the
circuit reduces to an inverting
amplifier.
 For general case, i1 is a
function of both v1 and v2.
1
v
1
2-v
1
21)-v
1
v(
1
2-v
21
i-v
22
i-vov
R
R
R
RR
R
R
RR



















v 
R
2
R
1
R
2
v
2
Also,
Since v-= v+ )
2
v
1
(v
1
2v 
R
R
o
For R2= R1
)
2
v
1
(vv o

6. Difference Amplifier: Example
 Problem: Determine vo
 Given Data: R1= 10kW, R2 =100kW, v1=5 V, v2=3 V
 Assumptions: Ideal op amp. Hence, v-= v+ and i-= i+= 0.
 Analysis: Using dc values,

A
dm

R
2
R
1

100kW
10kW
10
Vo A
dm
V
1
V
2





10(53)
Vo20.0 V
Here Adm is called the “differential mode voltage gain” of the difference amplifier.

7. THANK YOU FOR WATCHING