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neonlamp

Direct gain measurement for bipolar transistors

Status: Proposal
1661
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October 4, 2013
This circuit uses an analogue divider to calculate Ic/Ib so that the gain of a transistor can be measured at a specific collector current, unlike many simple measurement circuits which pass a fixed current into the base and use the collector current as an indication of the gain. To perform the analogue calculation, an op-amp circuit is used with a bipolar transistor feedback to create a log circuit, then the log(ib) is subtracted from log(ic) and anti-logged in an exponential circuit. The output is in 1 volt per gain of 100 and can be set to measure gains to 1000. Using inverse transistor pairs, the log circuits operate on positive and negative currents equally, thus the majority of the circuit works with npn and pnp transistors (who said you could not have logs of negative numbers?- electronic engineers find a way!) with only a few stages needing to be switched to change polarity. It is not yet built but only simulated.
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