External pass transistor, Q₁, conducts the majority of the signal-dependent 4mA-to-20mA loop current. Using an external transistor isolates the majority of the power dissipation from the precision input and reference circuitry of the XTR106, maintaining excellent accuracy.

The external transistor is inside a feedback loop; therefore, the characteristics are not critical. Requirements are: V_CEO = 45V min, β = 40 min and P_D = 800mW. Power dissipation requirements can be lower if the loop power supply voltage is less than 36V. Some possible choices for Q₁ are listed in Figure 7-1. The XTR106 can be operated without an external pass transistor. Accuracy, however, is somewhat degraded as a result of the internal power dissipation and subsequent self-heating. Operation without Q₁ is not recommended for extended temperature ranges. A resistor (R = 3.3kΩ) connected between the I_RET pin and the E (emitter) pin is advised for operation less than 0°C without Q₁ to maintain the full 20mA full-scale output, especially with V+ near 7.5V.

(1) For operation without external transistor, connect a 3.3kΩ resistor between pin 6 and pin 8. See text for discussion of performance.

Figure 7-2 Operation Without an External Transistor