Page 258 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

238 MICROSENSORS

Figure 8.10 (a) Basic transistor circuit for a PTAT sensor and (b) photograph of a commercial
integrated silicon temperature 1C. From Wolffenbuttel (1996)

Transistors are the most attractive elements for measuring temperature either in a
discrete device or in a part of a standard 1C. For example, Figure 8.10(a) shows a simple
PTAT circuit that uses two identical p-n-p transistors to divide the current equally into
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two n-p-n transistors with different emitter areas. The voltage dropped across the resistor
R is simply the difference in base-emitter voltages for the n-p-n transistors; therefore,
the current I o flowing out is

Figure 8.10(b) shows a commercially available integrated temperature IC based on the
PTAT circuit. In the temperature IC (RS 590kH), the output current I o has been set by
laser-trimming of the resistor to 298.2 ± 2.5 uA for a temperature of 298.2 K, and the
temperature sensitivity S T is 1.0 uA/°C over the range of —55 to +150°C.
There are a number of variations in this type of PTAT circuit, such as using a set
of eight identical n-p-n transistors of equal emitter area and adding a reference offset
voltage to have an output closer to zero at room temperature. However, this type of
temperature sensor is simple to make in a standard IC process and has a good sensitivity
and low dependence upon process variation because of the ratiometric principle employed.
Therefore, it is an attractive option in many cases.

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Commonly referred to as a current mirror.