Lab 3 - EE 420L 

Authored by: Roman Gabriele Ocampo
Email: ocampor5@unlv.nevada.edu
Date: February 24, 2014
  

Op-amps I

Prelab:
Lab Description and Goals:
The goal of this lab to understand the function of an op-amp, as well as recognize basic topologies and realize imperfections such as finite gain and offset.

Review of the LM324.pdf

The maximum allowable common-mode voltage is Vcc - 1.5V. For our purposes, Vcc is set to 5V, so the maximum and minimum common-mode voltage range is -0.3V < Vcm < 3.5V.
 

 
From the diagram and datasheet entry below, a good estimation of the open-loop gain is 100dB or 100,000.
 


 
A good estimate for the input offset voltage is 5mV. For worst case design, you would use the value of 9mV.
 
 
Inverting Amplifier Circuit
 
The schematic and a picture of the built circuit is found below:
 

 
VCM is created through a voltage divider, therefore it is at a constant 2.5V. The ideal closed loop gain is given by -RF/RI = -1.
 
The function generator that I had access to in the lab could not be set to provide an AC signal smaller than 700mV, so the following waveforms have been captured using an AC signal of 1V.
 

 
With an AC input of 1V, the output swings between 1.5V and 3.5V, and it is centered around 2.5V. The centering of Vout and Vin are inversely related. No current flows into the op-amp, so the KCL equation at the Vm node is (Vin-VCM)/RI = (VCM-Vout)/RF. RI and RF are equal, so an increase in Vin causes a decrease in Vout, and vice versa.
 
The maximum allowable input signal amplitude is 1V. This is because, as illustrated above, the maximum VCM is 3.5V. So, for a Vin centered at 2.5V, an AC signal of 1V will cause VCM to hit its maximum. Any higher than that, and the input MOSFETs of the amplifier will move into triode, affecting the operation of the op-amp. If the gain of the amplifier is increased to 10, the maximum input amplitude will drop to 0.1V.
 

The waveforms above are the result of a 1V signal through an amplifier with a gain of 10.
 
The capacitors from VCC and VCM to ground are used to decouple the nodes, causing them to be free from noise effects. The values on the capacitors is not critical.
 
The input bias current is the (average) current that flows out of both the non-inverting and inverting inputs of the op-amp. For the op-amp used in this lab, the input bias current is 20nA. In an ideal op-amp however, no current flows into the input nodes. Therefore, input bias currents will cause deviation from the ideal output voltage when large RI and RF resistances are used, because the input bias current will load them. The input offset current is the difference between the currents in the two input terminals.
 
Measuring an Op-Amp's Offset Voltage
 

 
The circuit above is used to measure the op-amp's offset voltage. Both inputs are connected to VCM. Ideally, no current would flow through RI and RF, so Vout would be equal to VCM. However, Vout will devate from VCM if the op-amp amplifies its own input offset voltage. Therefore, this circuit can be used to measure an op-amp's offset voltage by taking the difference between Vout and VCM and dividing the difference by the closed loop gain. Using this technique, and LTspice, the offset voltage of 4 different op-amps can be calculated as part of the experiment.
 

 
From the above diagram, each op-amp's input offset voltage can be found by taking the value of Vout, subtracting 2.5V, and then dividing by 200.
 
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