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2N2907 PNP transistor as temperature sensor

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alex.martinez:
Howdy!,

I have been recently into temperature sensing applications and I'm testing different methods to read this magnitude. Thermocouples and dedicated IC's are cool, but using a diode-connected BJT is even cooler (no pun intended). From what I have read in Analog Devices application notes, a current has to be injected into the diode-connected transistors. I achieve this by using a sketchy current source consisting of another 2N2907 and an opamp (I'm using an LM324).

In order to read the temperature I probe the Vbe through a voltage buffer (to isolate it) and then I try to subtract a DC offset of 500 mV for latter amplification. Point is, that I cannot get to amplify it correctly.

The current source is set to output 250 uA (probed, and it does maintain such level) and the Vbe stays at around 580 mV. Whenever I heat it there is a variation in temperature (drop in voltage due to a negative temperature coefficient). However, when it comes to probing and amplifying it, I'm getting some unknown issue to me. That is why I wanted to ask in the forum if there is any wrong assumption in the schematic, attached below.



Thanks in advance,
Alex.

EDIT1: Fixed opamps on schematic.

StillTrying:
The center op amp is connected in a nonsense way, input should be to its non inverting input.

alex.martinez:

--- Quote from: StillTrying on February 10, 2019, 07:51:00 pm ---The center op amp is connected in a nonsense way, input should be to its non inverting input.

--- End quote ---
Thank you, I assembled the schematic quickly on LTSpice, the opamp is wired correctly on the breadboard (I'm using an LM324N with 4 opamps).

StillTrying:
The feedback on the VOUT op amp is wrong! After that work out what the inputs of 580mV and 450mV should do.

alex.martinez:

--- Quote from: StillTrying on February 10, 2019, 08:09:33 pm ---The feedback on the VOUT op amp is wrong! After that work out what the inputs of 580mV and 450mV should do.

--- End quote ---

Yup...I messed up when orienting the opamps in the draft... :palm: Im an idiot. Just fixed it. with the resistor names in the re-uploaded schematics, VOUT = -Vbe*(R6/R5) + Vcc(Rb/(Ra+Rb))(1+R6/R5) = -4.54Vbe + 2.6

This amounts to -0.06 V at the output and I guess it's outside the rails of the opamp...Thanks.

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