Lab

Lab X - ECE 421L

Authored by Worku, Yetneberk

E-mail: workuy@unlv.nevada.edu

Today's date 03/01/14

Pre-Lab work

A. watch the video op amps I, review op amps I.pdf (associated notes), and simulate the ciruits

B. read data sheet of LM324 amplifier (LM324.txt)

Lab description:

>> Build up the first and second op-amp.

1. for the forllowing questions and experiments assume VCC+ = +5V and VCC- = 0V.

A. 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 voltage?

:- The Minimum VCM, and The Maximum VCM at 25 degree range min 0 to Vcc -1.5V

B. what is a good estimate for the op-amp's open-loop gain?
:- open-loop gain is100dB

C. what is a good estimate for the offset voltage? And for worst case design what value would you use?
:- the good estimation of input offset voltage is 5mV, and the worst case is 9mV.

2. Build, and test the following circuit.note that a precise value for the 5k resistors is n't important. you can use 4.7k or 5.1k resistors.

A. what is the common-mode voltage, VCM? Does VCM change? why or why not?

:- VCM is 2.5V. VCM does n't change because 10k voltage divider and op amp 1, however the following schematic is showing value does n't chage too.

also, the hand calculation prove it, see the following hand calculations.

>> Lab experiment result showing below

B. what is the ideal closed-loop gain?

:- closed loop-gain is A = (Vout/Vin) = (-R2/R1)

C. what is the ouput swing and what is it centered around?What happens if the input is n't centered around VCM, that is 2.5V? provide a detailed discussion illustrating that you understand what is going on.

:- swing from 2.39V to 2.61V. Centered around is 2.5V. when the input voltage increasing, the output voltage is decreasing. the following schematic is showing what happening.

D. what is the maximum allowable input signal amplitude? why?
:- the maxumum allowable input signal amplitude is 1V. because maxmimum VCM is 3.5V, then 3.5V-2.5V =1V
E. what is the maximum allowable input signal if the magnitude of the gain is increased to 10?
:- input amplitude will divided by 10
F. what is the point of the 0.01uF capacitors from VCC and VCM to ground? are these values critical or could 0.1uF, 1,000pF, 1uF, etc capacitors be used?
:- the point of the capacitors used to reduced nose, also the capacitors does not effect DC voltage at all.
G. the data sheet show that this op-amp has an input bias current that flows out of the op-amp's inputs of typically 20nA.This current flows out of oth 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 largeer. Explain what is going on. what is the input offset curent? what des this term describe?
:- the electrical characteristics table in the datasheet show input offset current and input bias current.

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

A. 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 hoing on.

:- there is no current flow throw R1 and RF because op-amps is ideal with zero offset. therefore, the following hand calculaton and schematic is shown.

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

:- the following schematic is shown that

Summary:-

Indeed, we learn how to operate ideal op-amp and real op-amp.

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