Lab 4 - EE 420L
Authored
by: Roman Gabriele Ocampo
Email: ocampor5@unlv.nevada.edu
Date: March 3, 2014
Op-amps II
Prelab:
- Watch the op-amps II video available on the EE420L Lab 4 page
- Read the lab write up provided on the lab website
Lab
Description and Goals:
- Estimate, using values from the datasheet, the bandwidths for
non-inverting op-amp topologies having gains of 1, 5, and 10.
Experimentally verify these estimates.
- Repeat the steps above using the inverting op-amp topology with gains of -1, -5, and -10.
- Design two circuits that measure the slew-rate of the LM324. Once
circuit must use a pulse input while the other must use a sinewave
input.
Non-inverting op-amp topologies
The
gain bandwidth product describes the relationship between an op-amp's
gain and bandwidth by the formula |Av|*BW=GBP. Bandwidths for different
gains can then be estimated using the formula BW = GBP / |Av|.
The gain bandwidth product of the LM324 is given in its datasheet. Therefore, we know GBP = 1.3MHz for the LM324.
Using the formula above, estimates for the BW can be made:
| Gain |Av| | Bandwidth BW |
| 1 | 1.3MHz |
| 5 | 260kHz |
| 10 | 130kHz |
For
the experiment, the following non-inverting amplifier was created. R1
was set to 10k and the values for R2 were changed to achieve the
correct value for the gain (from the formula Av = 1+ [R2/R1]). Four
oscilloscope readings were performed. The first at 1kHz and then at
another frequency below bandwidth to illustrate normal
operation. The frequency was then modified until the experimental -3dB
point was reached (70.7% of bandwidth operation), and then finally
modified until unity gain frequency (gain of 1).
Gain of 1 (R2=0, calculated BW=1.3MHz):
The first two images were taken at 1kHz and 500kHz. We should expect to
have an output of about 70.7mV at the -3dB frequency, which was
achieved at 790kHz in the third image. This is lower than our
calculated BW.
Gain of 5 (R2=40k, calculated BW=260kHz):
The first two images were taken at 1kHz and 100kHz. We should expect to
have an output of about 353.5mV at the -3dB frequency, which was
achieved at 160kHz in the third image. This is lower than our
calculated BW. The gain is unity at 550kHz.
Gain of 10 (R2=90k, calculated BW=130kHz):
The images was taken at 1kHz. We should expect to
have an output of about 707mV at the -3dB frequency, which was
achieved at 83kHz in the third image. This is lower than our
calculated BW. The gain is unity at 550kHz.
Inverting op-amp topologies
The
gain |Av| in the GBP formula from above is the non-inverting gain. The
BW for the inverting op-amp topologies, therefore, cannot be calculated
using the gain for the inverting op-amp, but rather by using the gain
for the given resistor values as if it were in the non-inverting
configuration. The formula for estimating BW then becomes BW = GBP /
(|Av| +1).
| Gain |Av| | Bandwidth BW |
| 1 | 650kHz |
| 5 | 217kHz |
| 10 | 118kHz |
For the experiment, the following non-inverting amplifier was created.
R1 was set to 10k and the values for R2 were changed to achieve the
correct value for the gain (from the formula Av = -[R2/R1]).
Four oscilloscope readings were performed. The first at 1kHz and then
at another frequency below bandwidth to illustrate normal
operation. The frequency was then modified until the experimental -3dB
point was reached (70.7% of bandwidth operation), and then finally
modified until unity gain frequency (gain of 1).
Gain of -1 (R2=10k, calculated BW=650kHz):
The
first two images were taken at 1kHz and 200kHz. We should expect to
have an output of about 70.7mV at the -3dB frequency, which was
achieved at 680kHz in the third image. This is higher than our
calculated BW.
Gain of -5 (R2=50k, calculated BW=217kHz):
The first two images were taken at 1kHz and 100kHz. We should expect to
have an output of about 353.5mV at the -3dB frequency, which was
achieved at 143kHz in the third image. This is lower than our
calculated BW. The gain is unity at 500kHz.
Gain of -10 (R2=100k, calculated BW=118kHz):
The first two images were taken at 1kHz and 20kHz. We should expect to
have an output of about 707mV at the -3dB frequency, which was
achieved at 71kHz in the third image. This is lower than our
calculated BW. The gain is unity at 520kHz.
Slew Rate Measuring
From the datasheet, the slew rate is given as 0.4V/us.
To
experimentally measure slew rate, a non-inverting configuration with a
gain of 10 was used. However, the configuration isn't very important,
because the slew rate can easily be measured by providing an input to
the op-amp with a steep enough voltage change to cause the output of
the op-amp to be slew rate limited.
For the first experiment, a pulse signal was inputed into the op-amp. The following waveform is the output:
The
output cannot immediately change to the pulse's amplitude, so it
approaches the steady amplitude at its slew rate. The change in voltage
over time can be calculated using the data gathered from points a and
b. (220mV-(-780mV))/(4.56us-1.04us) = 1.00V/3.52us = 0.28V/us.
For
the second experiment, a sinewave signal is required. As such, I
increased the amplitude of the sine signal until the change in
amplitude of the output remained constant through amplitude increases.
The
output cannot change as quickly as the sinewave, so it changes at its
slew rate. The change in voltage over time can be calculated using the
data gathered from points a and b.
(1.16V-(-1.58V))/(12.6us-1.76us) = 3.24V/10.84us = 0.299V/us.
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