AIM: EXPERIMENT TO SHOW THE USE OF AN OPERATIONAL AMPLIFIER AS A
NON-INVERTING AMPLIFIER AND A SUMMING AMPLIFIER.
OBJECTIVES
 To observe and determine the gain and phase shift of an amplifier.
 To implement and analyze non-Inverting and summing amplifier circuits
 To determine the ability of the summing amplifier to provide an output voltage equal to
the sum of voltage present at the input.
APPARATUS
 Functional generator.
 Dual trace oscilloscope.
 Operational amplifier.
 Digital multimeter.
 Jumpers.
THEORY
Operational Amplifier (Op Amp) is a three terminal electronic device which has two inputs of
high impedance. The first input is called inverting(-), and the other terminal is called noninverting input(+). The third terminal serves as output terminal.
Figure 1. Op-Amp Symbol
THE NON-INVERTING CONFIGURATION
Fig 2: Illustrates the non-inverting configuration of an Op amp.
Figure shows a non-inverting amplifier. The output Voltage Vo is of the same polarity as the
input voltage VI. The input signal is applied directly to the non-inverting (+ve) input terminal of
the amplifier and the feedback resistance are connected between the output terminal, the (-ve)
input terminal and ground. The Minimum Gain of the Non-Inverting Amplifier is 1.
Closed loop gain of a non-inverting configuration.
Assuming that the op amp is ideal with an infinite gain, a virtual short circuit exists between its
two input terminals. Hence the difference VId = (v2-v1) input signal is
𝑉𝑜
VId= 𝐴
for A=infinity
Thus the voltage at the inverting input terminal will be equal to that at the non-inverting input
terminal, which is the applied voltage vI. The current through R1 can then be determined as
and because of the infinite input impedance of the op amp, this current will flow through
R2. Now the output voltage can be determined from
𝑉𝑖
Vo=Vi+(𝑅1)R2
From which the Gain G can be determined as
𝑉𝑜
𝑉𝑖
𝑅2
=1+𝑅1
Where A=
𝑉𝑜
𝑉𝑖
The closed loop Gain A does not contain any negative, this means that the input signal of the
circuit gets amplified without changing its polarity at the output.
It is seen that from the expression of voltage gain above, the gain A will be unity when R2=0 or
R1=infinity.
That is when ;
R2=0,
R2
A=(1 + 𝑅1)
0
A=(1 + 𝑅1)
A=1
When R1=infinity,
R2
A=(1 + 𝑅1)
A=(1 +
R2
∞
)
A=1
Therefore when the feedback path is short circuited ,or when the external resistance is opened,
the gain will be 1.
THE SUMMING AMPLIFIER
A summing amplifier is an op amp circuit that combines numbers of inputs to give a single
output .A summing amplifier can be modified from an inverting circuit by simply connecting
several inputs in parallel to the inverting terminal.
It is an important application of the inverting configuration and it is shown in Fig.3 with a
resistance Rf in the negative-feedback path and a number of input signals v1, v2, . . . , vn each
applied to a corresponding resistor R1, R2, . . . , Rn, which are then all connected to the inverting
terminal of the op amp as shown below. Where n is the number of input terminals connected in
parallel.
Fig 3: Illustrates an inverting Op amp acting as a summer.
The non- inverting terminal is grounded meaning potential at the terminal is zero.
As an op amp is ideal there exists a virtual ground appearing at its negative input terminal and
from Ohm’s law the currents i1, i2, . . . , in are given by
𝑉1
i1=𝑅1
𝑉2
I2=𝑅2
𝑉𝑛
In=𝑅𝑛
All these currents sum together to produce the current i which is given as
i=i1+i2+…….+in
This current is then forced to flow through Rf since no current flows into the input terminals of
an ideal op amp. The output voltage vO is then determined by another application of Ohm’s law
to be
Vo=0-iRf=-iRf
Therefore
𝑅𝑓
𝑅𝑓
𝑅𝑓
Vo=-(𝑅1 𝑉1 + 𝑅2 𝑉2 + ⋯ + 𝑅𝑛 𝑉𝑛)
That is, the output voltage is a weighted sum of the input signals v1, v2, . . . , vn. This circuit is
therefore called a weighted summer. It should be noted that each summing coefficient may be
independently adjusted by adjusting the corresponding “feed-in” resistor (R1 to Rn).
PROCEDURE
The figure was connected as shown by connecting jumpers J3, J15, J18 to provide
current flow.
Terminal 2 was connected to the ground and then connected to the functional generator
with sine wave of 1kHz, 1Vpp
(i)
The resistance values are given below
R14 = 1KΩ
R7 = 9.5KΩ
R11 = 1KΩ
R15 = 100KΩ
The gain of the amplifier ( calculated gain)based on the
above circuit is given by
G =(1 +
R14
𝑅7
)
= 1.11V/V
(ii)
The oscilloscope channel 1 is connected across the terminal 2(Vin, input) and
channel 2 at terminal 3(Vo output)
(iii)
The gain, Go(measured gain) is given by
G= Vo/Vin
Vo = 1.34V
Vin = 1.20V
G = 1.12V/V
(iv)
The phase shift observed is given by Ø = 0◦
(v)
R14 is replaced by R11 and R15 at one time and the jumper J29 and connected
to J26 and then to J30 respectively and the following above procedures are
followed.
The results for R11 and R15 were obtained as follows;
For R11;
Calculated gain, G = 1.11V/V
Vo = 2.44V
Vin = 1.20V
Measured Gain, G = 2.03V/V
Phase shift observed = O◦
For R15;
Calculated gain, G = 11.53V/V
Vo = 200mV
Vin = 0.144V
Measured gain = 1.39V/V
Phase shift = O◦
PART 2 (SUMMING AMPLIFIER/WEIGHTED SUMMER)
PROCEDURE
(i)
Jumpers J2, J8, J9, J28, J6, J15, and J21 were inserted to produce
the circuit shown below
(ii)
(iii)
(iv)
(v)
Using the functional generator, a sine wave of 1kHz, 2Vpp and
zero average value to both terminals 1 and 2 is applied.
The multimeter is converted into an ohmmeter and resistances
are measured and recorded as follows;
R1 = 9.85KΩ
R7 = 9.5KΩ
R13 = 10KΩ
R11 = 10KΩ
R15 = 100KΩ
The theoretical output voltage value is obtained by;
Voltage output, Vo = -R13( 𝑽𝟏/ 𝑹𝟏 + 𝑽𝟐/ 𝑹𝟕 )
Vi=2.04V
Vo=-10K(2.04/9.85K+2.04/9.5K)=-4.218V
Practical Vo obtained=4.16V
The jumper J11 is connected such that resistors R13 and R11 are
parallel.
R11 and R13 when connected in parallel yields 5kΩ
Theoretical value of output, Vo = -5k ( 𝟐.𝟎𝟒/ 𝟏𝟎𝒌 + 𝟐.𝟎𝟒 /𝟏𝟎𝒌 )
=-2.04V
(vi)
The measured output voltage, Vo = 2.08V
(vii) The peak current through R13, Ip = 2.08/10K
= 0.208mA
(viii) The jumper J26 is removed and the output was determined as
Vo = 4.08V
(ix)
The output functional generator is removed and a common DC
voltage of 5V was applied at both terminals 1 and 2 and the
output, Vo was measured.
Vo = 0.9mV
OBSEVATIONS
NON-INVERTING AMPLIFIER
 The gain for non-inverting amplifier was found from:
G=Vo/Vin=1+R2/R1
Vo is for output voltage, Vin is for input voltage and R2=R14, R1= R7
Therefore the value of G obtained was G=1.11V
 There was no shift between input and output voltage as it was practically proven .from
the formula;
Vo=Vin(R1+R2)/R1
Since Vo and Vin have same signs( can be negative and negative or positive and positive), they will travel in phase.
SUMMING AMPLIFIER
 Summing amplifier output voltage, Vo is given by the formula;
𝑅𝑓
𝑅𝑓
Vo=-(𝑉1 ∗ 𝑅1 + 𝑉2 ∗ 𝑅2)
Where V1 and V2 are the input terminal voltages, R1 and R2 are the input resistances and Rf is
the feedback resistance and also R1 and R7 serve as input voltages and R13 is the output
voltage. When the input voltage is obtained Vpp, our output voltage equals Vo = 4Vpp.
However , if we Rf is replaced with equivalent resistance of 5kΩ, the measured value gets two
times less. Hence Vo = 2Vpp.
CONCLUSION
 From the experiment observed that for a non-inverting configuration, the output
signal was in phase with the input signal while for the inverting configuration, the
output was 1800out of phase with the inverting signal. It is also observed that the
gain of the Op amp was determined by the external components for example
resistors.
 We were able to work with amplifiers and prove the theoretical formulas for their
gains and output voltages and also experienced how to build circuits of non-inverting
and summing amplifiers.
 We got to understand how to analyze the output signals of the non-inverting and
weighted summer circuits with the help of oscilloscope and multimeter.
REFERENCES
EE_3101_Experiment_8.pdf
Lab_Report_5_Operational_Amplifier_Circu.pdf