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Old 20th October 2002, 08:39 PM   #1
Koy is offline Koy  Czech Republic
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Default BJT Aleph input

After finishing the Aleph I started experiments with it (my recommendation - Aleph is easy to understand, lot of fun). I replaced the MOSFETs in input differential pait to BJT. But now I am not able to reach zero ( or almost zero) DC offset when using current mirror. Offset ( about 0.6 to 2V) moves with bias current. Why? Has it something to do with lower input resistance of BJTs? Or different "opening" voltage of BJTs? Why is not the pair with current mirror able to set 0V offset? Any literature to learn about?
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Old 21st October 2002, 08:27 PM   #2
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Try loading the BJT's with resistors instead of
current mirrors and see if the problem goes away.

Also, BJT's with high current gain would be the best
for low offset.
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Old 22nd October 2002, 04:52 AM   #3
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What is a BJT? A Boxy Unijunction Transistor? I missed this one along the technolocical way someplace.....
Mark
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Old 22nd October 2002, 05:09 AM   #4
hifiZen is offline hifiZen  Canada
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Bipolar Junction Transistor... aka your standard garden-variety transistor, available in either NPN or PNP flavours.
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Old 22nd October 2002, 06:30 AM   #5
Koy is offline Koy  Czech Republic
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NP:With only resistor - yes. But I think mirror offers more linearity. thats why I asked. Maybe low gain of input BJTs?
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Old 23rd October 2002, 08:39 PM   #6
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It is my experience that current mirrors offer more gain,
not necessarily more linearity.
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Old 24th October 2002, 12:54 AM   #7
eLarson is offline eLarson  United States
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Quote:
Originally posted by Nelson Pass
It is my experience that current mirrors offer more gain,
not necessarily more linearity.
What about a current "sink" at the source (emitter) pins of a common-emitter or diff-pair? Does that help linearity (akin somehow to emitter degeneration with a resistor, but with a much higher dynamic impedance)?

Erik
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