# Homework Help: Opamp ckt with temp sensor

1. Mar 5, 2012

### likephysics

1. The problem statement, all variables and given/known data
I am trying to figure out how this ckt works. But no idea, where to even begin.
see attachment tp.png

2. Relevant equations

3. The attempt at a solution
Q1 is diode connected. The Vbe temperature dependence is amplified to indicate the temperature. -2mV/ deg C
I simplified the ckt to the one in pdf attachment.
The -15v is added to inverting terminal and -13.85v is added to non-inverting terminal.
Assuming Vbe is 0.64v at room temp, -13.85 and -0.64v add up to -14.5v
The difference between invering and non-inverting terminals is 0.5v.
Can't go any further.

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2. Mar 6, 2012

### Staff: Mentor

Sweet! That circuit is from near the end of National Semiconductor's App Note AN-31, an Opamp Circuit Collection. Classic AN.

Well for one thing, with negative feedback, the voltage between the + and - inputs is most definitely not 0.5V.

You might have a look at National Semiconductor's AN-56 (which is in the same book as AN-31 if you have it, their "Linear Applications Handbook"). Figure 6 in AN-56 is an Electronic Thermometer opamp circuit, very similar to the one you posted. AN-56 has an explanation of how the circuit works, so that should help you figure out this AN-31 circuit.

Let us know what you figure out!

3. Mar 7, 2012

### likephysics

Okay. That helped.
I simplified the ckt to the following.
A 1.2v reference for constant current biasing of temp sensor.
A -1.2v reference for zeroing.
I am not sure how the zeroing works. To zero a diode with +1.2v bias, you need another diode or a resistor with its resistance equal to diode resistance with a -1.2v bias. correct?
I don't know why a 5k pot is used.
I've attached the AN56 ckt and my simplified ckt pdf.

#### Attached Files:

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• ###### opamp temp reduced2.pdf
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4. Mar 11, 2012

Anyone?