Page 173 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

166 Chapter Eight

8.2.4 LED temperature coefficient
When low tempco photodiodes are used, the real problem is usually the LED
that is used as the source (Fig. 8.1). These have variable, and often unspecified
tempcos of the order of -0.1 to 1 percent/°C, when driven by a constant current.

8.3 Temperature Compensation
If the intrinsic tempco of the optoelectronic components is still too high, tem-
perature compensation may be attempted. Figure 8.2 shows how thermistors
can be used with a photodiode or source LED to improve stability. The best
choice of values, ability to interchange devices without recalibration, and
variety of packages is available with negative temperature coefficient (NTC)
thermistors. At room temperature the resistance of common devices varies by
about -3.5 percent/°C, large enough to compensate any LED or photodiode.
The temperature sensitivity of thermistors results from the temperature
dependence of majority carrier concentrations (see Sze 1969). This leads to a
resistance R for semiconductors that varies as:

R = R e [ B(1 T-1 T o )] (8.1)
o
where R o is the resistance at the reference temperature T o (usually 25°C =
298K), and B is a characteristic constant (units K). The temperature coefficient
at any temperature T is then given by:

a = 1 dR = -B ∞C -1 (8.2)
R dT T 2

Temperatures are in K. The constants R o and B are published in the device’s
data sheets. For example, the common GM102 thermistor has a nominal re-
sistance of 1k at 25°C and a B value of 2910K, from which we can calculate a
temperature coefficient a of about -3.5 percent/°C at room temperature (Fig.
8.3a).

Improved
linearization: R R
R L
NTC Th Th
+ A
-
Thermal +
coupling PD -
LED Thermal
Cuvette NTC
coupling
Figure 8.2 Effects of the high temperature coefficients of LED output
and photodiode responsivity can be reduced using thermistors closely
coupled to them. Ladder networks can improve linearity.

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