EE 420L Engineering Electronics II Lab -
Lab 4
Op-amps II, gain-bandwidth product and slewing
Spring 2019
Authored by Shadden Abdalla
Email: abdals1@unlv.nevada.edu
February 26, 2019
Prelab work:
Watch the
second op amps two video, review the notes and simulate the circuits.
This lab utilizes the LM324 op-amp with VCC+=5V
and VCC-=0V.
The following lab report contains:
1.
Estimation using the datasheet, the bandwidths
of noninverting op-amp topologies with gains of:
a.
1
b.
5
c.
10
2.
Experimental verification of these estimates
with a common-mode voltage of 2.5V.
a.
Report will have schematics of the topologies
with photos and results
b.
Associated comments with reasons for any
differences between estimates and experimental results.
3.
Repetition of the above steps using gains of
-1. -5, and -10.
4.
The design of two circuits that measure the
slew-rate of LM324
a.
One circuit with pulse input
b.
One circuit with sinewave input
c.
Support for the design decisions
d. Comments on
any differences between measurements and datasheet specifications
1.
Estimation using the datasheet, the bandwidths
of noninverting op-amp topologies with gains of:
a.
1
b.
5
c.
10
The gain bandwidth product of the LM324 is
1.3MHz based on the datasheet.
Using the
above equation for finding bandwidth and the value of gain bandwidth product
that we already have we can calculate the bandwidth for the non-inverting op
amps.
Calculated Bandwidth:
|
Gain |
Bandwidth
(BW) |
|
1 |
1.3MHz |
|
5 |
260KHz |
|
10 |
130KHz |
NON-
INVERTING AMPLIFIER
Below is the
schematic I used for all my non-inverting op-amp experiments. R1 is set to be
10k in all these experiments. R2 is changed in order to change the gain.
Order of operations:
In order to
find the correct simulation, we need to find the -3dB frequency which
corresponds to the input signal I used, 100mV.
To find the
-3dB:
-3dB= 
= about 70mVpp. To find the frequency, we
changed it until we had a waveform that measured 70mVpp.
To find the gain:
The
gain of a non-inverting amplifier is found using the equation:
Gain =
1 + R2/R1
The resistor values I used for R2 for a
non-inverting amplifier are:
|
|
Gain of 1 |
Gain of 5 |
Gain of 10 |
|
R2 Value |
0 ohms |
40k ohms |
90k ohms |
EXPERIMENTAL
RESULTS
Gain of 1:
Gain = 1 + R2/R1 = 1 + 0 = gain of 1.
You can see in the LTSpice sim that the roll
off frequency is about 1.2MHz.
The experimental result of my non-inverting op amp
with a gain of 1 matches the bandwidth given in the data sheet and the LTSpice
simulation and is measured at 1.3Mhz at 68mV peak to peak, seen below in the
photo. This result matches both the LTSpice simulation and the calculated value
seen in the highlighted portion of the calculation chart.
Gain of 5:
Gain = 1 + 40k/10k = 1 + 4 = gain of 5.
Based on the LTSpice simulation below, you can
see that the frequency should be a bit less than 200kHz, where the calculated
value we found was about 260kHz.You can find the value in the spice sim by
finding the -3dB frequency in the simulation. I did not put a cursor on it for
a more concise lab, however, it is easy to find the value by looking at the -3dB
frequency on the graph.
My experimental result was
different than both the calculated result and the LTSpice simulation result, at
16.547kHz at 112mV. It was very difficult to get this simulation to go to 70mVpp
and this is the the lowest it would get with a gain of 5.
You can see based on the
highlighted line in the calculations table that the value is very off of the
calculation.
Gain of 10
Gain = 1 + 90k/10k = 1 + 9 = gain of 10
LTSPICE
Simulations show that the frequency should be at about 90kHz.
The experimental results gave a similar value,
at 124.16kHz. The frequency found was like both the calculated value and the
LTSpice value.
Results of
Non-Inverting Topology:
|
GAIN |
CALCULATED |
SPICE |
EXPERIMENTAL |
|
1 |
1.3MHz |
1.2MHz |
1.3MHz |
|
5 |
260kHz |
200kHz |
16kHz |
|
10 |
130kHz |
90kHz |
124kHz |
|
|
|
|
|
Conclusion and analysis of these results: It is apparent that
the experimental results are not always very close to the experimental or LTSpice
results. It is important to note that the values in LTSpice and in calculation
neglect practical factors and those factors are not accounted for. It is common
for the experimental results to differ from the calculated ones because of many
outside factors that can affect the performance of the circuit and the output
values. The reasons could vary from bad equipment, wires, different chips,
capacitance that was not accounted for, and inaccurate input signals. Many
things can affect variations, but the important part is that these are the
genuine values from the experiments and have not been altered in order to match
the calculated values. It is important when performing experiments to preserve
the experimental integrity of the values and find the actual value outputted.
My values are off; however, they are real values. Computer simulations and
calculations are used to gather a ballpark value; however, practical circuitry
does not always follow calculations.
INVERTING AMPLIFIER
Order of
operations:
In order to find the correct simulation, we
need to find the -3dB frequency which corresponds to the input signal I used,
100mV.
To find the -3dB:
-3dB= 
= about
70mVpp. To find the frequency, we changed it until we had a waveform that
measured 70mVpp.
First, we need to find the Bandwidth of the
inverting topology.
Gain of
inverting topology= -RF/RI.
Gain of 1:
= 650khz
Gain of 5:
= 216khz
Gain of 10:
= 118khz
|
Gain |
Inverting Bandwidth
(calculated) |
|
-1 |
650kHz |
|
-5 |
216kHz |
|
-10 |
118khz |
EXPERIMENTAL
RESULTS:
Gain of -1: the bandwidth calculated
was 650kHz. In LTSpice it is about 700kHz.
My experimental results matched the calculated values and the
lowest it would go down to was about 85mVpp. The value I found experimentally
was 650kHz.The LTSpice value and the experimental value were very close and the
experimental value actually matched the calculated one exactly.
Gain of -5: The bandwidth I
calculated was 216kHz. In LTSpice,
the value is lower at about 140kHz.
The experimental results were extremely different because I did not
reach 76mV until I had inputted a 1.026MHz frequency. This could be because of
external factors that we did not take in account such as capacitance and
different equipment that may cause skewed results. I did not change any values
to force my output to be accurate based on calculations in order to preserve
accuracy and experimental integrity, even though the value was very different.
Gain of -10: The value I calculated for the gain of ten was
118kHz. In LTSpice it was lower, at about 75kHz.
The experimental result was extremely different than what I
had calculated at about 538kHz. This could be because of external factors that we did not take in
account such as capacitance and different equipment that may cause skewed
results. I did not change any values to force my output to be accurate based on
calculations in order to preserve accuracy and experimental integrity, even
though the value was very different.
FINAL
INVERTING OP-AMP RESULTS
|
Gain |
Inverting Bandwidth
(calculated) |
LTSPICE |
Experimental |
|
-1 |
650kHz |
700kHz |
650kHz |
|
-5 |
216kHz |
140khz |
1.02MHz |
|
-10 |
118khz |
75kHz |
538kHz |
Conclusion and analysis of these results: It is apparent that
the experimental results are not always very close to the experimental or
LTSpice results. It is important to note that the values in LTSpice and in
calculation neglect practical factors and those factors are not accounted for.
It is common for the experimental results to differ from the calculated ones
because of many outside factors that can affect the performance of the circuit
and the output values. The reasons could vary from bad equipment, wires,
different chips, capacitance that was not accounted for, and inaccurate input
signals. Many things can affect variations, but the important part is that
these are the genuine values from the experiments and have not been altered in
order to match the calculated values. It is important when performing
experiments to preserve the experimental integrity of the values and find the
actual value outputted. My values are off; however, they are real values.
Computer simulations and calculations are used to gather a ballpark value;
however, practical circuitry does not always follow calculations. Engineering
consists of using calculated values to get a good estimate of the type of
values we should input, in order to see what the practical results were.
SLEW RATE
The circuit I used to measure slew rate was the unity follower:
noninverting with a gain of 1. Below is the circuit on the breadboard.
The ideal slew rate found in the datasheet is 0.4V/us.
I continued to change the frequency until the output slewed, in order
to measure the slew rate. I took two measurements, the time delta and the
voltage delta in order to find the slope and the slew rate.
Sine wave:
127kHz input frequency to the left, the time measurement with a delta
of 4us in the middle and the voltage measurement with a delta of 1.02V on the
right.
We can measure the slew rate by finding the slope, the voltage delta
over the time delta.
Slew rate of sine wave: 
Square pulse:
1kHz input frequency to the left, time measurement of 4.2us in the middle and
voltage delta of 1.10V on the right.
Slew rate of square wave: 
Slew rate conclusion: The slew rates that I found were 0.255V/us for a sine wave and
0.261V/us for a square wave where the ideal one was 0.4V/us. The value is not
extremely close to the ideal one in the datasheet, however it is close enough
to be considered an accurate approximation. As stated before, it is very
unlikely to produce the same values experimentally that exist through
calculations because of outside factors that we did not take in account for the
calculations or in the datasheet. These factors include variances in equipment
quality, capacitance and overall experimental factors that add variance to the
results. The results are very close considering that we did not use the same
parameters in the data sheet.