Lab 3 - ECE 421L 

Andrew Buchanan

Buchaa2@unlv.nevada.edu

2/20/2019

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.

• Knowing the non-inverting input, Vp, is at the same potential as the inverting input, Vm, (called the common-mode voltage, VCM) what are the maximum and minimum allowable common-mode voltages?
  • Support your answer with an entry from the electrical characteristics table in the datasheet.

• What is a good estimate for the op-amp's open-loop gain? 
  • Support your answer with a plot from the datasheet and an entry from the electrical characteristics table.

 Looking at the frequency response plot, the open-loop gain at 1kHz is about 60dB, or 1000. ( dB = 20log(x) ). From the other plot, we can see that the gain does not change much with power supply voltage, so we can estimate AOL = 1000 at 1kHz.

• What is a good estimate for the offset voltage? 
  • For worst case design what value would you use?
    

Build, and test, the following circuit. Note that a precise value for the 5k resistors isn't important. You can use 4.7k or a 5.1k resistors.
This is our circuit

This is our measurment

This is our spice simulation

• What is the common-mode voltage, VCM? Does VCM change? Why or why not?

The common mode woltage is the voltage sin at the non inverting input terminal. In our circuit it is 2.5V. It does not change because the cpacitance that are in parrealel with the voltage divider filter out any AC.

• What is the ideal closed-loop gain?

 closed loop gain of an inverting opamp is -Rf/Ri this means that if the resistances are the same for the Rf and the Ri that the closed loop gain is going to be -1 and this causes the output to be the same as the input with a phase offset of 90 degrees. this is shown in the simulation above.

• What is the output swing and what is it centered around?
  • What happens if the input isn't centered around around VCM, that is, 2.5 V? 
  • Provide a detailed discussion illustrating that you understand what is going on.

The output swing is  the 111mV signal which "swings" around the DC offset of 2.5V. similar to the simulation which has a swing of 100mV centered at 2.5V.  If we increase the amplitude beyond 2.5V we can see that the output hits the rails and cuts off

• What is the maximum allowable input signal amplitude? Why?

 The max input signal amplitude is 2.5V because the VCM is 2.5V and beyond this the circuit starts clipping

• What is the maximum allowable input signal if the magnitude of the gain is increased to 10? Why?

 If the gain is increased to 10 this would cause the output to swing 10 times larger and you would have to lower you amplitude by a factor of 10. So, the max amplitude before clipping would be 250mV

• What is the point of the 0.01 uF capacitors from VCC and VCM to ground? 
  • Are these values critical or could 0.1 uF, 1,000 pF, 1 uF, etc. capacitors be used?

 As i said above it helps keep the voltage from changing or getting any noise interference

• The data sheet shows that this op-amp has an input bias current that flows out of the op-amp's inputs of typically 20 nA.
  • This current flows out of both the non-inverting and inverting inputs through the resistors connected to these inputs.
  • Show how the operation of the circuit can be effected if, for example, R1 and R2, are much larger. Explain what is going on. 
  20nA is basically 0V if the resistor values are small, and the current is around 250uA. if the resitor values were high, 1k,
  

• Note that if the output voltage is precisely the same as VCM then the op-amp has no offset voltage (this is very possible).
  • To measure small offset voltages increase the gain by increasing RF to 100k or larger. Explain what is going on.

• Measure the offset voltage of 4 different op-amps and compare them.
  

 

The LM324 was the only op amp with a small ofset voltage if we would have selected higher gain we might have gotten better results.

Conclution:

In this lab we learned about ofset voltage of an op amp and bias current. I also learned how to read the data sheet to find the data that I need.

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