Lab

Lab 3 - EE 420L

Authored by Iain Drews, drewsi2@unlv.nevada.edu

2/19/2015

Lab description

This lab focused on the implementation and limits of OP-AMPs

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?

Minimum: -0.3
Maximum: Vp: VCC-1.5 but can do +32V without damage
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?

AOL: 100

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?

VCM= DC:2.5V AC: 100mV as it has an AC component the Voltage would be changeing

What is the ideal closed-loop gain?

Gain of 1 or Unity Gain *see above image

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

Output= DC:2.5V AC: 100mV *see above image

What happens if the input isn't centered around around VCM, that is, 2.5 V?

Output= 100mv ideally

Provide a detailed discussion illustrating that you understand what is going on.

Since VCM is still fixed at 2.5V that puts Vm at 2.5V which would mean there is no DC difference between Vin and Vm so the only current to go through would be the AC current.

What is the maximum allowable input signal amplitude? Why?

Maximum Amplitude is (VCC-GND)/2 so 2.5V since the output is limited by the Vs+ and Vs- nodes of an op amp

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

Maximum Amplitude is (VCC-GND)/(2*10) so .25V again this is because of the limits set by Vs+ and Vs-

What is the point of the 0.01 uF capacitors from VCC and VCM to ground?

To absorbe noise and give a more steady VCM

Are these values critical or could 0.1 uF, 1,000 pF, 1 uF, etc. capacitors be used?

Values are not Critical, lower value capacitors might allow more noise however.

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.

while normally 20nA is not that much current when pushed through a 30Meg Resistor it causes a voltage drop of 600mV

What is the input offset current? What does this term describe?

input offset current is the difference between the input bias currents in the + and - node of an op amp

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.
since VCM is going into both the positive node and the input of the op amp the only way a current can happen is if it there is an offest voltage. with this particular topology the circuit will amplify the offset voltage by 20x and show in the output

Measure the offset voltage of 4 different op-amps and compare them.

Voffset=44.5mv/20=2.225mv	Voffset=26.9mv/20=1.345mv

Voffset=33.1mv/20=1.655mv	Voffset=36.3mv/20=1.815mv

Extra Scope Reads

26DB gain at 5Hz

note the smaller input signal to allow a larger gain

Attempted 26DB gain at 100kHz

note that as we get closer to the GBP it gets harder to achieve much gain

Attempted 60DB gain at 100Hz

note the cutoff point at the top and bottom of Vout this is because Vout was going above the Vs+ and below Vs-

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