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Old 10th Dec 2017, 3:43 am   #30
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Matched Transistors

I re-tested to circuit and also to see how close calculation & measurement might agree if one was interested in the hfe:

Replacing the bias network with a simple 270k resistor to keep the collector current close to 4.8mA, also increasing the 200R emitter bypass cap to 1000uF(the 10uF one I used had a high esr it turned out it was an NP type) also increasing the input coupling cap to 10uF so that it didn't factor in, the generator voltage is 110mV pp for 6vpp at the collector.

The 110mV pp signal is distributed as: The voltage across the series 1k2 is 60mV and the B-E voltage of the OC72 50mV pp.

The current in the series 1k2 to the base is therefore 50uApp, ignoring the current into the 270k, this is nearly all dynamic base current. (the dynamic or signal current in the 270k is 50mV/270k or 0.18uA and so doesn't count).

The dynamic collector current is 6vpp/1k2. or 5mA. This makes the measured hfe 5mA/ 50uA or 100 exactly.

From the DC perspective the base current via the 270k bias resistor is approx 44uA from the 12V supply, and the DC collector current 4.8mA, so that makes the HFE around 109, which agrees with the large signal test.

Since the voltage gain comparing the base and emitter dynamic voltage is 6Vpp/50mVpp or 120, and the collector resistor is a 1k2 then the measured value of re is 1k2/re = 120, making re = 10 ohms in practice. However, the theoretical value of re is 25/4.8 = 5.2 ohms.

Trying to asses the input impedance by using hfe/gm or hfe(re) by calculating re (or gm) from the collector current is not as accurate as measuring it, it would appear.

I've attached an image of the test circuit. Obviously this is not an ideal way to bias for an amplifier , but actually I have decided, for comparing two transistors it is better as it also helps compare the C-B leakage for germanium types.

Please ignore the error in post 28. And I mis labelled the 1000uF electro polarity in the diagram !
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Last edited by Argus25; 10th Dec 2017 at 3:59 am.
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