The BC557 transistor is widely used as a silicon diode thermometer in which the collector-base voltage, at a constant bias current, is measured as the indicator of temperature. The normally applied bias current for this measurement is 10μA, however under some circumstances advantage may be gained by Massing at a higher current. Since its first use in thermometry the BC557 has undergone changes in its production technology and these have had some impact on its temperature characteristics. In this paper the sensitivity of calibration to bias current is discussed and data is presented on old and new technology devices which have been characterised at 10 and 50μA. At higher temperatures, a reduced spread in the V-T characteristics of the devices is observed with the higher current, giving an improved conformity to standardised calibration data.
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