Becasue thats a Course project and i tried alot to design it but im having some problems when simulating it, it gives wrong results.Why not just use an LM35?
Thanks a lot for your help but could u please help me with the calculations.That circuit should work.
The diode-connected transistor in the feedback loop has about a -2mV/°C change in voltage drop. (Diode-connected transistors tend to have a more ideal diode behavior as compared to a standard junction diode).
That op amp configuration generates a constant current through the transistor, as determined by the value of R1, so that the voltage change due to temperature does not affect its current, which would cause an error in the voltage.
So you select the values of R2 and R4 for an output op amp gain that gives the desired 10mV/°C change in Vo for the -2mV/°C change at the output of the left op amp.
You then select the value of R3 to cancel the offset voltage of the transistor of about 0.7v (typically to give 0V out at 0°C for an easy to interpret readout of 0 to 1V for 0 to 100°C).
These resistor values will need to be tweaked for the actual temperature coefficient and offset of the transistor you use.
[For the LT spice simulation I did, the 2N3906 coefficient (I used that PNP as recommended in a Linear Technology design paper) was ≈-1.86mV/°C and the offset at 0°C was ≈686mV at a bias current of 500μA as determined by R1].
Below are the results of my simulation.
I'll leave the resistor value calculations up to you.
View attachment 165718
This circuit is a course project which the professor gave it to us , I'm not sure where he got it fromhi 5,
If you posted a link from where you sourced the circuit, it most likely has descriptive text, component types and calculation maths, that we could guide you through.
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Did you understand what I said about the circuit?Thanks a lot for your help but could u please help me with the calculations.
I know how op amps work, its just that when i try to analyze the circuit i cant make those temperature relations and im not sure what assumption on resistors i should make. and again thanks a lot for your time.Did you understand what I said about the circuit?
If not, what questions do you have about the calculations?
Do you understand how op amps work?
We need to know your level of understanding.
Great.I know how op amps work
What "temperature relations"?its just that when i try to analyze the circuit i cant make those temperature relations
Hi again , Thx alot for your time , and yea actually i dont know what the first op amp with that transistor feedback do, all i tried to do until now is trying to analyze the circuit but i always get a negative value for R4.Great.
If you do, then you should be able to tell me what the first op amp is doing.
What is the circuit configuration of that op amp (open or closed loop, inverting, non-inverting)?
What "temperature relations"?
That tells me little about where you are having a problem.
Is it with how a transistor voltage changes with temperature, or how the op amp is used to generate the output voltage?
Or something else?
We are not going to spoon-feed you a solution.
You need to be more specific about where you are having a problem and show us what you have attempted.
Okay.i dont know what the first op amp with that transistor feedback do
hi 5,
Post what you calc the voltage output is from the 1st OPA,.?
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If you have a diagram with some component values, please post it.
ok This time i will post my solution and orcad simulation.Okay.
So, ignoring R3 and the second op amp, if you apply a negative voltage to R1, what does the op amp do if the transistor in the negative feedback loop were just a resistor?
What is the current through that resistor?
Then substitute the transistor for the resistor and what is the result?
(Remember that an op amp with negative feedback, will do whatever it can at its output to keep the voltage between the two inputs at essentially 0V.)
by Jake Hertz
by Jake Hertz
by Aaron Carman