Lab 9: Op Amp Limitations
U.C. Davis Physics 116A
INTRODUCTION
Now replace R2 with a 10kΩ resistor and
then with a 100kΩ resistor and measure AV and
fB for each. (You don't need to draw a complete
graph for these; just find fB .) Compare all three
of these gain-bandwidth products. Is AV f B
approximately constant? For your lab report,
compare all three AV f B 's, and calculate what
AV would be for fB = 300, 000 Hz for this op
amp.
Previously, we have used op amps as if they
were ideal. This week, we will measure some of
their real-life limitations.
1. SLEW R A T E
The slew rate is the maximum dvout dt that
the op amp is capable of. This idea is pictured in
figure 1.
vout
3. I NPUT O FFSET V O L T A G E
A real-life op amp behaves as if it has a
small battery between its + and - inputs. This is
called the "input offset voltage", Vio, and here we
will try to measure it.
Use the circuit in figure 2. Resistors R+
and R− balance the effects of the bias current
(which we'll measure in the next section) by
balancing the input resistances to ground. For
your report, measure the output voltage and
compute Vio = Vout / AV . (You can use either the
calculated gain for this circuit or you can measure
it.)
dvout
= this slope
dt
t
Figure 1: The slew rate of an op amp.
Build
a
noninverting
amplifier
using
V
in
(not used)
R1 = R2 = 1 kΩ , the unity gain configuration.
For your lab report, measure the slew rate for
your op amp. To do this, use a square wave
input and measure the slope of the leading edge
of the output waveform, as in figure 1.
+
_
R
+
100 Ω
2. G AIN -B ANDWIDTH P RODUCT
V
out
100k Ω
R_
100 Ω
Op amps can only amplify at their full
calculated gain up to a certain maximum
frequency, fB , the "break frequency".
This
maximum frequency is dependent on the gain of
the circuit the op amp is in, but the "gainbandwidth product" AV f B is roughly constant
for any particular op amp IC.
Construct a noninverting op amp initially
with R1 = 1kΩ and R2 = 33kΩ . Measure and
plot AV vs. f for f = 100Hz through
f = 100kHz .
From your graph, find the
frequency where AV drops to 0.707AV,low freq..
This is the break frequency fB . Calculate the
gain-bandwidth product AV f B . For your report,
include the graph of AV vs. f, the break
frequency, and the gain-bandwidth product.
Figure 2: Circuit to measure input offset bias.
4. Z EROING V I O
Vio is typically around 20 mV. For some
applications, like the ones we used last week,
this is a negligibly small voltage and we can
simply ignore it. For other applications, like for
measuring a 2 mV signal, it can be disastrous.
So, the manufacturer has provided a way to
deliberately add a small voltage to Vio so that the
total input offset voltage is 0. This process is
called "zeroing" or "nulling" the op amp.
20
-15V
-15V
10 k Ω pot
V
pin 1
in
pin 5
(not used)
Figure 3: Add this to figure 2 to zero Vio.
Use the same circuit as above with the
addition shown in figure 3.
Adjust the
potentiometer until the output voltage is zero.
For your lab report, note how close to zero you
can get the total Vio to be. You now have an op
amp with Vio ≈ 0 which is suitable for
amplifying very small voltages. Keep this
circuit for the sections that follow.
R+
10k Ω
R4 = 10kΩ
Figure 4: Circuit to measure input offset current.
The ideal op amp requires zero current into
its inputs. A real-world op amp, however,
requires a very small current, I bias, to drive it.
Here, we will measure this current.
Continue to use the zeroed op amp circuit
from above and replace R+ with a 10k resistor.
To calculate I bias, note that Vin = − I bias R+ and
that the rest of the circuit is a noninverting
amplifier with Vout = AV Vin so
−Vin −Vout
=
.
R+
AV R+
(We are neglecting the voltage drop produced by
R− since R− << R+ so V− << V+ .) For your
report, measure V out and calculate your I bias.
6. I NPUT O FFSET C U R R E N T , I O S
The currents required by the two inputs are
never exactly equal in the real world. Here, we
measure the difference between the two input bias
currents, which is called the input offset current,
I o s.
Modify your previous circuit by adding R4 as
shown in figure 4. R4 should have the value
R+ − R− = 10kΩ − 100Ω = 9 .9 kΩ but we can
approximate this with 10kΩ. Now, if the two
I bias currents are equal, V+ − V− = 0 and Vout = 0 .
If one or the other is greater, Vout ≠ 0 and
Io s =
V
100k Ω
R_
100 Ω
5. B IAS C U R R E N T , I B I A S
I bias =
+
_
−Vout
.
R+ AV
Include your I o s in your report.
21
out