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Homework Help: Opamp ckt with temp sensor

  1. Mar 5, 2012 #1
    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
    Gain is about 50.
    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.

    Attached Files:

  2. jcsd
  3. Mar 6, 2012 #2


    User Avatar

    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. :smile:

    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! :smile:
  4. Mar 7, 2012 #3
    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:

  5. Mar 11, 2012 #4
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