Department of Electrical Engineering, IIT Bombay
EE-319 Experiment No. 6
Non Idealities in Op-Amps
General notes:
For experiments 1 through 5 we assumed ideal operational amplifiers while designing and testing
circuits. However real life operational amplifiers deviate from this. In this experiment, you will
measure some of the non-ideal parameters of LM741.
References:
Ramakant A. Gayakwad, Op-Amps and Linear Integrated Circuits, 4th Edition, chapter 4.
Sedra & Smith, Microelecronics Circuits, Fifth edition, chapter 2.
Experiments:
1. Input off-set voltage: We expect the output voltage of an op-amp to be zero when both
differential input terminals are connected to ground. In reality, the output voltage would be
nonzero. In fact it would be at either negative or positive saturation if the open loop gain is
sufficiently large. This is a consequence of the unavoidable mismatches between transistors
that forms the input differential stage and is called the input off-set voltage. The op-amp output
can be set to zero by applying an appropriate voltage to one of the input terminals (typically the
non-inverting terminal). The offset voltage can be modeled as a voltage source connected to the
non-inverting terminal of an ideal op-amp (offset free op-amp).
a. Wire up LM741 so that both the input terminals are shorted and connected to ground.
Observe the output.
b. See the datasheet of LM741 (http://www.national.com/ds/LM/LM741.pdf) and note
down the range of input off-set voltage.
c. The input off-set voltage is typically very small. It can be measured easily if it can be
amplified. Design a test circuit using the model described above and an appropriate
amplifier configuration (choose between inverting/non-inverting configurations).
d. Compare the measured value with the value given in the datasheet. Also compare the
data you obtained with that obtained by 5 other groups. Comments?
e. Wire up the circuit as in 1a. Connect a 10 kΩ potentiometer to the off-set null terminals.
Connect the wiper of the potentiometer to the negative power supply and adjust the
wiper to obtain zero volts at the output terminal.
f. Some of the op-amps (example: quad op-amp LM324 used for some of the previous
experiments) do not have off-set null terminals. Design an off-set null circuit for one
op-amp from LM324. Submit the design as part of your journal. Datasheet of LM324:
http://www.national.com/ds/LM/LM124.pdf
2. Input bias current: A small current is required at the inputs to the op-amps. In op-amps built
with bipolar transistors, this is essentially the base currents of the differential pair. MOSFET
input stages would have protection diodes and consequently reverse saturation current of the
diodes flows into the terminals. Even though small, the input offset current would flow through
the components connected to the input terminals of the op-amp. The resulting voltage could be
amplified resulting in a significant output voltage. Input bias current is defined as the average
of the currents that flows into the two input terminals of the op-amp. Input off-set current is
defined as the difference between the two input currents.
a. Wire up the circuit as in 1e and obtain zero at the output.
b. Bias current flowing into the noninverting terminal can be determined by connecting the
op-amp in voltage follower configuration and connecting an appropriate resistor from
the noninverting terminal to the ground. The bias current into the noninverting terminal
would cause a voltage drop across the resistor which can be measured at the output of
the op-amp.
c. Refer to the 741 datasheet and note typical values of bias currents.
d. Design the test circuit discussed in 2b, wire it up around the circuit obtained in 2a, and
determine the bias current into the noninverting terminal.
e. Bias current flowing into inverting terminal can be determined by connecting the
noninverting terminal to ground and an appropriate resistor between the inverting
terminal and the output. The bias current into the inverting terminal would cause a
voltage drop across the resistor which can be measured at the output of the op-amp.
f. Design the test circuit discussed in 2e, wire it up around the circuit obtained in 2a, and
determine the bias current into the inverting terminal.
g. Calculate the input bias current and the input off-set current. Compare the values with
the LM741 datasheet and data obtained by 5 other groups. Comments?
h. What are the restrictions imposed by input bias current on the choice of resistance
values that can be used in inverting and noninverting amplifiers?