Vasile Surducan wrote...

>I wasn't follow this subject but if delta means diference I have some
>doubts about the interchangeability of the sensors. Maybe a ratio.

This "delta Vbe" technique is a well established and documented
technique.  Linear Technology, Inc. application note AN45, by Jim
Williams,  discusses this method and shows an example design on page 7,
=46igure 11.=20

Circuits of this type produce a voltage output which is proportional to
the log of the ratio of the two currents times the absolute temperature.

The output shows very little dependency on transistor characteristics or
device type; Williams obtained less than 0.4 degrees C spread over 25
randomly selected 2N3904s and 2N2222s from various manufacturers.

>There are five basic methodes of measuring temperature with a =
transistor:
>1. Ib =3D constant, measuring Vbe, only be junction is used
>2. Vbe =3D constant, measuring Ib, only be junction is used
>3. Ib =3D constant, measuring Ic
>4. Ib =3D constant, measuring Vce
>5. Ib =3D constant, measuring both Vce and Vbe

>But all methodes are too complicated if we think to the LM35 family.

I think that last statement oversimplifies just a bit: there are many
ways to measure temperature, and each method has its strengths,
weaknesses, and a set of applications where its use is appropriate.

The "delta Vbe" method of sensing temperature has two application areas
where it is especially attractive:

1.  When the temperature sensing element MUST be a semiconductor
    junction, such as measuring the die temperature of a CPU chip
    for purposes of cooling control.  The delta-Vbe technique allows
    highly accurate die temperature measurement without any need for
    calibration.

2.  When there is a need for EXTREMELY low sensor cost (but NOT total
    circuit cost), together with a requirement that sensors be
    interchangeable without re-calibration.  For example, I can bond
    a small, cheap transistor to an assembly to monitor its
    temperature during testing or some other process (e.g., potting)
    and then discard it when I'm done, at little expense.

If you'd like to go back and follow the original thread to see how we
got to this discussion, look in the archives for "[PIC]: current
generator with a PIC" by Ashly Dearden on 27 December; that thread will
provide some context.

Dave Dilatush

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