Lab 3 - EE 420L
Pre-lab work
Question 1: 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?
In the datasheet for the LM324 we can see that at room temperature the minimum VCM is 0 and the maximum is Vcc-1.5 = 5-1.5 = 3.5V.
Question 2: What is a good estimate for the op-amp's open-loop gain?
In the datasheet we can see a minimum gain of 50V/mV and a typical gain of 100V/mV. A good estimate for the open-loop gain would be 100V/mV.
In the below plot we can see that the voltage gain changes based on power supply voltage. For us (Vcc = +5V) our gain will be around 100dB
Question 3: What is a good estimate for the offset voltage? For worst case design what value would you use?
At room temperature the typical offset voltage is 2mV with a maximum offset of 5mV. For worst case scenarios we should design for the maximum temperatures. The maximum offset voltage is 9mV for high temperatures.
Experiment 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 5.1k resistors.
What is the common-mode voltage, VCM? Does VCM change? Why or why not?
In our circuit VCM is set to 2.5V. This is because VCM is the output of a voltage divider with VCC set to 5V.
VCM will not change because the two 10nF capacitors will route AC current to ground.
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 VCM, that is, 2.5V?
The output swing is 100mV centered around 2.5V.
If the input isn't centered around VCM the output will be offset in the opposite direction from the input.
What is the maximum allowable input signal amplitude? Why?
The maximum allowable input signal amplitude is 2.5V because of the 5V Vcc. If the signal goes above 2.5V the output signal will begin to clip.
In this image the input is at 2.6V and the output signal (blue) begins to clip.
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, the maximum allowable input signal will be reduced by 10. This maximum allowable signal will be 250mV. If you go above 250mV your output signal
will begin to clip due to the limitations of the Vcc+ = 5V and Vcc- = 0V.
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. capacitor sbe used?
The capacitors route the AC current to ground to keep VCM as stable as possible. The size doesn't really matter for this experiment.
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. 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?
If R1 and R2 are much larger, the 20 nA current flowing into the divider circuit will cause VCM to increase which may cause the circuit to work incorrectly.
For example
If we choose R1 and R2 to be 100MEG we will see an increase in our VCM by 1V. Our VCM would be 3.5V instead of 2.5V which is the maximum value we can use as our VCM.
The input offset current is the difference between the bias currents flowing out of the op-amp terminals.
