Lab 3 - EE 420L 

Authored by Marco Muņiz,

Email: munizm1@unlv.nevada.edu

02/13/2019

  

  

Lab description :

  

This lab will utilize the LM324 op-amp (LM324.pdf).

Review the data sheet for this op-amp.

For the following questions and experiments assume VCC+ = +5V and VCC- = 0V.

  

  

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Part 1: LM324 Questions

  

  

                              file:///C:/Users/mmuni/Pictures/Lab3/node_volt.JPG

  

From the given data sheet information, we can see that the common mode voltage range is from 0V minimum to [VCC - 1.5V] maximum. In our case, VCC is 5V so the maximum would be [5-1.5]=3.5V. It is important to note that these values are for an ambient temp. of 25 Degrees Celsius. 

  

  

   

From the Open Loop plot on the left, we can see that the open-loop gain at 1kHZ is 60dB, or a gain of 1000. (dB=20log(x)). From the plot on the right, we can see that the gain does not have much variation with changes in temp. or supply voltage, thus we can assume an gain of 1000 @ 1kHz will be close to continous. 

  

                                      file:///C:/Users/mmuni/Pictures/Lab3/openloop_freq_resp.JPG    file:///C:/Users/mmuni/Pictures/Lab3/Largesignal_V_gain.JPG

                                      file:///C:/Users/mmuni/Pictures/Lab3/volt_gain.JPG

  

  

  

                           file:///C:/Users/mmuni/Pictures/Lab3/offset_voltage.JPG

  

From the datesheet, we can see that the offset voltage should be approximately 2mV at room temp. (25 Degrees Celcius).

  

  

  

From the same data sheet snip in the previous question, we can see that the maximum offset voltage would be approximately 9mV. I would assume this value would be the worst case. 

  

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Part 2: Build, and test, the following circuit.

  

  

                                         file:///C:/Users/mmuni/Pictures/Lab3/op_amp_circuit.JPG 

                                                                                    (Given Op-Amp circuit)

  

The Common-mode Voltage is the voltage that is at the input points of an op-amp. For our given circuit, the common-mode voltage is 2.5V which we are providing through the use of a voltage divider of two 10K resistors. The capacitors are acting as opens for the DC so we they are not influencing the voltage division but will help with stabalizing from any changes.
 
The ideal closed-loop gain is the gain the op-amp supplies while it is inverting. In the case of this op-amp, that would be [-Rf/Ri] = -1. The negative gain means that the output will have a gain of 1 but with a 180 degree phase shift.
 
 
The output swing of this circuit is the 100mV ac signal witch swings above and below the 2.5V DC offset voltage. So with the voltage centered at 2.5V, we will see an increase of 100mV, as well as a decrease of 100mV.  We can see this illustrated in the images below, particularly in the spice simulation where we can see the swing going up to 2.6V and down to 2.4V, centered around 2.5V.

                                                       file:///C:/Users/mmuni/Pictures/Lab3/Oscill_reading.JPG
                                                                   (Oscilloscope illustration of the output swing)

                                            file:///C:/Users/mmuni/Pictures/Lab3/spice_sim.JPG

                                                                           (Spice Simulation illustrating output swing)

  

For this circuit, the maximum allowable input signal amplitude would be 2.5V because our output for op-amps is limited by your supply rails. (+VCC and -VCC). In our case, we have a +VCC of 5V and a -VCC of 0V so, with our signal centered at 2.5V and a gain of 1, we are limited to this value. If we were to go above this value, we would begin to clip our output signal as seen below.
   
                                                    file:///C:/Users/mmuni/Pictures/Lab3/Oscill_reading2.JPG
                                                                                      (Clipped output)
 

With a gain of 10, our output would be the equivilant at 1/10th of our input. Thus, the maximum allowable input signal would be 1/10th of 2.5V or 250mV. 

   

The capacitors found in the VCC to VCM voltage divider are used as decoupling capacitors which aid in keeping the divider voltage to stay at a consistent value with no variation, as well as to help keep any effects from noise to a minimum.
 

Based on the information given on the data sheet, we are shown that this op-amp has an input bias current of roughly 20nA. Because of this current, if we were to use resistors in the Giga or higher range, we we would some some substantial change in voltage (based on V= I*R). If we were to use resistors in this range, our signals would see some changes in centering and could run into various issues such as clipping. 

  

                                        file:///C:/Users/mmuni/Pictures/Lab3/offset_current.JPG
                                                                                                (Input Offset Current Sheet)
 
 The input offset current (Iio) is the difference between the input bias currents that are on the terminals of the op-amp. (Iio= Iib+ - Iib-).
For this op-amp, we can expect bias currents of around 20nA and an offset current of around 2nA.
 
 
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Part 3: Measuring of 4 different op-amp offset voltages
 
 
                                                             file:///C:/Users/mmuni/Pictures/Lab3/offset_circuit.JPG
                                                                           (Offset Voltage measure ring circuit)
 

For this circuit shown, we have VCM as the input of both terminals on the op-amp. We must remember that the op-amp will try to keep Vm equal to the input voltage of VCM. However, the op-amp will not be able to keep Vm exactly equal to VCM, the voltage potential at Vm will be VCM + (some offset voltage). This offset voltage will also be amplified by the gain, in this case Rf/Ri = 20 and the
Offset Voltage = VCM-Vout.
 
                                                                                                     LM348

                                                            file:///C:/Users/mmuni/Pictures/Lab3/lm348.JPG
                                                                              Offset Voltage = |3.22-2.68|/20 = 27mV
   
                                                                                                      LM339
 
                                               file:///C:/Users/mmuni/Pictures/Lab3/lm339.JPG
                                                                          Offset Voltage = |3.22-2.68|/20 = 10mV
 
                                                                                                       TL081  
 
                                                             file:///C:/Users/mmuni/Pictures/Lab3/tl081.JPG
                                                                                Offset Voltage = |1.62-2.40|/20 = 39mV
 
                                                                                                       UA741  
   
                                                             file:///C:/Users/mmuni/Pictures/Lab3/ua741.JPG
                                                                                  Offset Voltage = |2.62-2.56|/20 = 3mV
 
 
Conclusion:
 
This lab provided a good amount of practice with real Op-Amps to reinforce the theoretical knowledge we learned in lecture. Furthermore, we were able to see how differently op-amps can act and vary in real world application. We also learned some new things, such as measuring offset voltages.
 
   
 
 
Back up:
 
../Pictures/Lab3/back_up.JPG

 

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