EE 420L Engineering Electronics II Lab - Lab 3
Op-amps I, basic topologies, finite gain, and offset

Pre-lab work

• Watch the video op_amps_I, review op_amps_I.pdf (associated notes), and simulate the circuits in op_amps_I.zip.
• Read the write-up seen below before coming to lab.

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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.
• 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.

• What is the common-mode voltage, VCM? Does VCM change? Why or why not?
• What is the ideal closed-loop gain?
• 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.
• What is the maximum allowable input signal amplitude? Why?
• What is the maximum allowable input signal if the magnitude of the gain is increased to 10? Why?
• 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?
• 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.
• What is the input offset current? What does this term describe?

Explain how the following circuit can be used to measure the op-amp's offset voltage.

• 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.
  

Ensure that your html lab report includes your name, the date, and your email address at the beginning of the report (the top of the webpage).
When finished backup your work.