Measurement of the current transfer coefficient with a multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Many radio amateurs use relatively cheap DT830D digital multimeters or similar ones that have a mode for measuring the static current transfer coefficient of the h21E base. The instructions for them say that this parameter can only be measured for silicon transistors. Meanwhile, with simple mathematical calculations, the multimeter can also be used to test low-power germanium transistors.
Take a look at the simplified multimeter circuit (see figure) operating in the h21E measurement mode. Naturally, only one tested transistor should be included in the multimeter sockets. A voltage of 2,8 V is supplied from the ICL106 microcircuit stabilizer (domestic analogue - KR572PV5, see "Radio", 1998, No. 8, p. 62-65), the PA1 milliammeter is a digital current meter with a limit of 20 mA. The base current of the transistor under test is Ib \u2,8d (1-Ube / R, where Ube is the base-emitter voltage of the transistor, R is the resistance of the resistor in the base circuit (R2 or RXNUMX).
When measuring h21E of silicon transistors, Ube is approximately 0,6 V, and the base current is quite accurately 10 μA. As a result, the multimeter readings correspond to the true value of the transfer coefficient.
For germanium transistors, Ube is approximately 0,2 V, and the base current is approximately 12 μA - hence the overestimation of the measurement result by 20%. In addition, the readings increase due to the through current of the transistor. Therefore, as in the "good old days", when only germanium semiconductor devices existed, you should first measure the through current of the transistor. To do this, insert the collector and emitter leads into the "K" and "E" sockets, respectively, and, without connecting the base lead, record (or remember) the reading of the device.
After inserting the base output, the multimeter reading is again recorded, the previous reading is subtracted from it and the result is divided by 1,2 - this will be the true base current transfer coefficient.
It should be added that the multimeter can measure the reverse current of diodes and other semiconductor devices, including them in the appropriate polarity in the "K" and "E" sockets. The result on the display will be displayed in microamperes.
Author: P.Aleshin, Moscow
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