The external transistor, Q₁, conducts the majority of the full-scale output current. Power dissipation in this transistor can approach 0.8W with high loop voltage (40V) and 20mA of output current. The XTR11x is designed to use an external transistor to avoid on-chip, thermal-induced errors. Heat produced by Q₁ still causes ambient temperature changes that can affect the XTR11x. To minimize these effects, locate Q₁ away from sensitive analog circuitry, including the XTR11x. Mount Q₁ so that heat is conducted to the outside of the transducer housing and away from the XTR11x.

The XTR11x is designed to use virtually any NPN transistor with sufficient voltage, current, and power rating. Case style and thermal mounting considerations often influence the choice for any given application. Several possible choices are listed in Figure 8-1. A MOSFET transistor does not improve the accuracy of the XTR11x and is not recommended. Although the XTR11x can be used without an additional external transistor, this configuration is not always practical at higher loop voltages and currents because of self-heating concerns.