EE 420L Engineering Electronics II - Lab 3

 

Authored by David Flores

Email: flored6@unlv.nevada.edu

Due: February 20, 2019

 

Lab Description

In this lab we worked with op-amps specifically the LM324 chip. We learned how to read the data sheets to get important information, as well as all the components that affect an op-amp. The following lab is done at Vcc+ = 5V and Vcc- = 0V.

 

 

Pre-lab

 

Lab Instructions

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.

  
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.

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

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Experiment 0: Reviewing Data Sheet

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?

The maximum common-mode voltage is VCC-1.5V which is 3.5V and the minimum is 0V

Support your answer with an entry from the electrical characteristics table in the datasheet.

As shown here on the data sheet for 5V VCC+ and 0V VCC- the maximum Vcm would be Vcc - 1.5V = 3.5V. The minimum Vcm would be 0V. But this OP-AMP can have a VCC+ up to 30V. The max Vcm at VCC+ 30V is 30 - 1.5V = 28.5V at (25C)

 

What is a good estimate for the op-amp's open-loop gain? 

According to the plot below (Open Loop Frequency Response) at 10Hz frequency we have a gain of about 100dB = 20log(x) which is equal to 100k. This is supported characteristic table which states that the gain is about 100V/mV which is 100k at (25C). We can see in the plot below (Large Signal Voltage Gain) that the Gain does not change to much in reasonable temperatures.

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?

According to the Electrical Characteristics table a good estimate for the offset voltage would be 2mV. The worst-case design would be 9mV. These values are based on a VCC+ = 5V and at 25C.

 

 

Experiment 1: Building OP-AMP Circuit 1

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?

The common-mode voltage is defined as the average of the two inputs of the op-amp in this case it would be 2.5V this value is calculated with a voltage divider; which has an input voltage of 5V and two 10k resistors. The capacitors are open in DC operations.

Vcm = Vcc * (10k/20k) = 2.5V

 

What is the ideal closed-loop gain?

The ideal closed-loop gain for an inverted op-amp would be –(RF/RI) which in this case would be equal to -1. The output waveform would just be Vin but out of phase by 180

Oscilloscope measurement with -1 gain

 

What is the output swing and what is it centered around?

The output swing is 100mV in the picture above we can see that it is about 121mV.

 

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.

If not centered around 2.5V (VCM) either below or above the waveform might clip because it could run into +Vcc or -Vcc sooner than expected which would cause the waveform to clip. We can see in the picture below that the waveform clips because input is more than 2.5V. Since the waveform is centered at 2.5V and the amplitude is more than 2.5V it clips at 5V since that is the maximum allowable range of voltage. This happens because VCC+ is equal to 5V.

Oscilloscope waveform clipped                                                       Ltspice simulation not centered at 2.5V

                

 

 

What is the maximum allowable input signal amplitude? Why?

The maximum allowable input signal amplitude is 2.5V. The waveform has an amplitude of 2.5V and it is centered around 2.5V if the input exceeds this value it will surpass the VCC which is 5V.

 

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

The maximum allowable input signal if the gain is increased to 10 is 250mV. If the gain is increased to 10, (Vout/Vin) = 10 so the new allowable input value would have to be 0.1 * 2.5 =250mV. This would take care of the increase in gain.

Example of how clipping occurs at 250mV with a gain of 10

 

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?

 

These capacitors are known as decoupling capacitors. They help with the amount of noise that is seen in the waveform. They do not affect the voltage divider because they act as an open in DC. The values that are used are not important 0.1uF, 1000pF, 1uF these can all 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 affected 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?

Our Vcm would change depending on the size of the resistors. Even if the current is very small the resistors could be large which would cause a notable voltage drop. This voltage will add to Vcm and could clip the output since the allowable max input is 2.5V

The input offset current is the current offset of the two input terminals. Here are both the offset current (below) and the input bias current (above) the max values are shown in the column before the nA. Average value is shown before that. Values are all with VCC+ = 5V, VCC- = 0, and at room temperature.

30MEG resistors showing voltage drop

 

The amplitude of the waveform gets smaller because the little bit of input bias current causes a notable voltage drop with the big 30MEG resistors.

 

 

 

Experiment 2: Building Circuit 2 with 4 Different OP-AMPS

 

 

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

In an ideal op-amp the two input terminals would be equal so VM would equal VCM so there would be no current flowing through the R1 resistor. The closed loop gain for an inverting op-amp is –(Rf/RI) which is -20. The offset could then be:

 

 

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.

 

For this experiment we used 4 different op-amps to see the difference in offset. We used the same LM324 chip but 4 different chips and we still got a different offset for each of the different measurements. Here we can see as mentioned in class that even if we are using the same LM324 chips we can still have different offsets.

 

Here we wanted to measure small offset voltages so we increased the gain by increasing  RF to 100k. The four different OP-AMPS are shown below.

Offset of 1mV                                                                                Offset of 2mV

         

Offset of 1.2mV                                                                             Offset of 1.6mV

         

 

 Here we can see that even though we are using the same LM324 chip but 4 different chips we still get a variation in the offset. Like we learned in class the only way we could get away with having the same offset would be to use chips that were manufactured the same by the same people maybe even the same date.

 

Conclusion: This lab was very helpful for my understanding about OP-AMPS it helped reaffirm the information that I learned in Circuits 1. As well as taught me new information like voltage offset and how it can affect the circuit. We also learned how useful a data sheet can be to find a lot of different information that we can use to better understand the chips being used. I relearned the closed loop gain and how to derive it. As well as how the open loop gain affects the output with the two input node voltages.

 

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