Page 178 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 171

temperature, or even below it for photodetector noise reduction. Convenient
control and driver modules for Peltier elements are available, driven by the
market for controlled telecommunication laser sources, or you can design your
own. When stabilizing at reduced temperatures condensation of water onto
the source/detector must be avoided by direct bonding or sealing in of a dry
atmosphere.

8.4.2 Self-regulating transistors
If heating and temperature stabilization are to be applied locally to a small pack-
aged optoelectronic device such as a photodiode or LED, the physical sizes of
conventional sensors and heaters can become inconvenient. Combining these
two functions is a useful trick. An elegant technique using a transistor both as
sensor element and heater has been published by Woodward (1998). Figure 8.8
is developed from that article. A package of 4 CMOS gates is used to measure
the base-emitter voltage of the power transistor, and then heat it by applying
short current pulses. By dispensing with a transistor load resistor and relying
on its current gain to limit dissipation, the circuit size is minimized and all
heating energy goes into the transistor. By using a high current transistor for
Q1 in a small package (e.g., Zetex ZTX451 1A in the E-Line package) or a
surface-mount device, very compact and low-cost point-of-use temperature con-
trols can be made.

+12 V

REF195 C3 C2 R8
6 2 0.1μ 10μ
C5 C4 Q3 3
0.1μ 1μ 4
Tant 15
R1 R2 R4 LED1
1M AOT 1k0 1 2 NC
Q1 C6 R9 C7 Q 4
0.1μ
TIP41A 470μ 2 10R 7 9 16 R6 10
Q4 100k
Q2 C1
-
c 6 5 3 0.1μ
b 3
4
e 4 13
+
LMC7101 R5 11 14
R3 10k R7 10k Q 4
AOT 8 6,7
12 GND
Figure 8.8 A transistor can be used both as a heater and a sensor of its own temperature, making for a
very compact arrangement. Circuit after an elegant design by Woodward (1998). Reproduction by per-
mission of the author.

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