Lab 4 - EE 420L
Authored
by Marco Muņiz,
Email: munizm1@unlv.nevada.edu
02/24/19
Lab
description
Again, this lab will utilize the LM324 op-amp (LM324.pdf).
For the following questions and experiments assume VCC+ = +5V and VCC- = 0V.
- Estimate, using the data sheet, the bandwidths for non-inverting op-amp topologies having gains of 1, 5, and 10.
- Experimentally verify these estimates assuming a common-mode voltage of 2.5 V.
- Your
report should provide schematics of the topologies you are using for
experimental verification along with scope pictures/results.
- Associated comments should include reasons for any differences between your estimates and experimental results.
- Repeat these steps using the inverting op-amp topology having gains of -1, -5, and -10.
- Design
two circuits for measuring the slew-rate of the LM324. One circuit
should use a pulse input while the other should use a sinewave input.
- Provide comments to support your design decisions.
- Comment on any differences between your measurements and the data sheet’s specifications.
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Part 1:
- Estimate, using the data sheet, the bandwidths for non-inverting op-amp topologies having gains of 1, 5, and 10.
The
bandwidth of an Op-Amp is a range of frequencies where the closed loop
gain stays at the same value and acts constant. This can be seen easily
in the topologies for inverting Op-Amps, in which we see a constant
range then we begin to see a drop in the gain. This frequency is the
bandwidth. The Gain Bandwidth Product "GBP" would be the closed loop
gain times the bandwidth at that point. This value is considered a
constant. For example, the GBP for an inverting Op-Amp with gain of one
would be : (1*Bandwidth = GBP), where the GBP is equal to the frequency
of the Op-Amp at unity gain, also known as the unity gain frequency "fun".
From the data sheet of Op-Amp we are using, we get the open lood response. We can also see the GBP with a unity gain frequency "fun" is around 1Mhz.
(Open Loop Frequency Resp. Plot)
(Unity
Frequency for GBP)
In the case of the Non-Inverting Op-Amp, we can see calculations to estimate the bandwidth below.
(Hand Calcs for BW estimate for Non-Inverting Topology)
@ Gain =
1
@ Gain = 5
@ Gain = 10
- Experimentally verify these estimates assuming a common-mode voltage of 2.5 V.
Non-Inverting Op-Amp
(Gain of 1)
(Gain of 5)
(Gain of
10)
To
determine the BW (bandwidth) for this Op-Amp at gains of 1, 5, and 10,
we first set up the above circuits for a low input frequency so that we
can see a clear and accurate gain.
From this point, we will increase the frequency until we see the output
drop to 70.7% of its original value, or a 3dB drop from its original
value. The frequency were this occurs is the Bandwidth of the Op-Amp used for the current gain.
For
the Oscilloscope images below, we will be showing the gain for low
frequencys, as well as the gain for high frequencys point were we reach the bandwidth so that we can see
a clear illustration of the difference. However, the gain seen in the
oscilloscopes was fairly off from what we would expect from the
estimates dont previously. The images still show a 3db drop from the
low frequency gain to illustrate the bandwidth.
For the following images, the Blue channel is the Input and the Purple Channel is the Output wave.
@ Gain of 1
(Low Frequency)
(High Frequency)
@ Gain of 5
(Low Frequency)
(High Frequency)
@Gain of 10
(Low Frequency)
(High Frequency)
(Sorry! did not notice till after we had already left lab)
From
the table, we can see how different our experimental values diverged
from out estimated calculations with some cases almost twice the
bandwidth. A reason for this variation would be the +/- 10 %
Tolerance in our resistors which would make the gain more, or
less that what we believe it is. Additionally, we might be seeing this
large variation due to our low VCC voltage and unwanted capacitance
from the lab equiptment.
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Part 2:
- Repeat these steps using the inverting op-amp topology having gains of -1, -5, and -10.
Below,
we have the schematics used to measure the bandwidth for the Inverting
Op-Amp topology. As in the previous part, we will show two Oscilloscope
readings to show the an accurate gain at low frequencys, as well as for
high frequency.
Gain of -1
Gain of -5
Gain of -10
As
with the Non-Inverting topology, these oscilloscope readings show the
gain of the Op-Amp at low frequencys where we can clearly see the gain.
Also, we show the high frequency point where we reach 70.7% of our
output signal to illustrate the Bandwidth.
For the following images, the Blue channel is the Input and the Purple Channel is the Output wave.
@ Gain of -1
(Low Frequency)
(High Frequency)
@ Gain of -5
(Low Frequency)
(High Frequency)
@ Gain of -10
(Low Frequency)
(High Frequnecy)
From
the table for the Inverting Op-Amp, we begin to see much less variation
between our estimated bandwidth values and our experimental values. I
would also attribute the disparity in our bandwidths to inaccurate
resistors, a low VCC voltage or unwanted capacitance from the lab
equipment/cables.
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Part 3:
- Design two circuits for
measuring the slew-rate of the LM324. One circuit should use a pulse
input while the other should use a sinewave input.
For Op-Amps, the output voltage can only change at a specific rate. This rate is
called the "Slew Rate" of the Op-Amp. The Slew Rate for the Op-Amp used
in this lab is found on the provided data sheet plot seen below.
Additionally, we are shown in the data sheet how the slew rate is
measured in what is known as a "Voltage follower" setup seen below.
(Voltage Follower Response)
(Voltage
Follower Setup)
For the first experiment, we
set up the voltage follower circuit with the input as a Square Wave.
This set up will show us the Slew Rate directly since the output will
not have a fast edge, like the square wave input, but rather the
slanted rise of the output trying to change as quickly as it can.
(Square Input)
(Sine Input)
We
calculate the Slew Rate from by dividing the change in voltage(Line
Slope) by the change in time. Using the values from the oscilloscope
plots, we get roughly [ .5V/1.319us ] = 0.38V/us. This Value is acceptably close to the stated Slew rate, in the Data sheet plot of .4V/us, shown above.
For
the sine wave input, we see that the amplitude and slope increase in
unison. However, if we input a wave with a slope thats much greated
than the Op-Amp slew rate, our output begins to look erratic due to the
Op-Amp trying to catch up to the rate of change of the input signal but
not being able to.
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