Lab

Lab 3 - ECE 420L

Authored by Kyle Butler, butlerk2@unlv.nevada.edu

2/20/2019

Pre-lab work:

Lab work:

Introduction questions:

This lab will utilize the LM324 op-amp, 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?

The max common mode voltage is VCC-1.5V, so with VCC=5V the max common mode voltrage is 3.5V.

With a supply voltage of 5V, the estimated gain is 115dB. Resulting in AOL=10^(115dB/20) = 526K

From the datasheet the typical offset is 2mV, while worst case design offset is 9mV.

Build and test the circuit:
Real world circuit Real world measurement

Circuit simulation Measurement simulation

The VCM is the voltage seen across the input terminals, which is 2.5V. Since the inverting input is at ground and the non-inverting terminal is 2.5V, the VCM must be 2.5V. This can be seen in both the real world measurement and the simulation.

Closed loop gain for inverting orientation is -(Rf/Rin), since both resistors are 5k the resulting closed loop gain is -1. This can be seen by the phase shift between Vin and Vout in the measurement simulation.

Output Swing Clipping

From the simulation measurements the output swing is 100mV, while the real world measurement is 111mV.
Additionally the output swing is centered around the DC offset or VCM of 2.5V, if it is not centered around 2.5V it is possible for the output to reach the maximum or minimum voltage of the rails. This causes clipping around 0V and 5V.

Considering the VCM of 2.5V, the max input signal amplitude is 2.5V. Any larger then 2.5V results in a clipping at the rails around 5V.

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

By increasing the gain by factor of 10 the max allowable input signal must reduce by a factor of 10. The max input signal with a gain of 10 is 250mV. The output will swing from 5V to 0V centered around VCM

Similarly to the reasoning of putting capacitors to ground on an Opamps supply voltage, it is to reduce the noise or rippling of the voltage.
The 0.01uF capacitors can be replace with 0.1uF-1000pF because it does not effect the output voltage and the input signal is a DC source, no filtering of ripple is neccessary.

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.

Since the input bias current is small (20nA) and the resistor values used for R1 and R2 are small we see a current in the range of 250uA, the input bias is negligible when compared to the curent flowing in the circuit.
However, if the resister values were much larger, say by a factor of 1000, the circuit flowing through the circuit will be in the nA range and the input bias current effect will be observed in this current range.
This change in current will result in a change in VCM, leading to the possiblilty of a clipping effect or at the very least a shift in the output voltage.

Op-amp offset voltage:

By using the following circuit layout we can measure the offset voltage as Vos=(VCM-Vout)/Gain. For the 4 different opamps the VCM will be 2.5V and the Gain will be 20.

The following Opamps used the same circuit layout as part 1, but with resistor values of 1k and 20k.
LM324
Vos = (2.53-2.49)/20 = 2mV
LM348
Vos = (2.51-3.17)/20 = -33mV
LT(forgot number, get from lab later)
Vos = (2.56-2.89)/20 = -16.5mV
LM393
Vos = (2.51-2.03)/20 = 24mV
The Opamps besides the LM324 had large offset voltages campared to the LM324. I beleive we should have selected to use a larger gain (200)) for these opamps and that our resistors values may have been effected from input bias current.