BJT Aleph input

Koy · 20th October 2002, 08:39 PM

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?
Thanks!

Nelson Pass · 21st October 2002, 08:27 PM

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.

Mark A. Gulbrandsen · 22nd October 2002, 04:52 AM

What is a BJT? A Boxy Unijunction Transistor? I missed this one along the technolocical way someplace.....
Mark

hifiZen · 22nd October 2002, 05:09 AM

Bipolar Junction Transistor... aka your standard garden-variety transistor, available in either NPN or PNP flavours.

Koy · 22nd October 2002, 06:30 AM

NP:With only resistor - yes. But I think mirror offers more linearity. thats why I asked. Maybe low gain of input BJTs?

Nelson Pass · 23rd October 2002, 08:39 PM

It is my experience that current mirrors offer more gain,
not necessarily more linearity.

eLarson · 24th October 2002, 12:54 AM

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

20th October 2002, 08:39 PM	#1
Koy diyAudio Member Join Date: Jan 2001 Location: Zlin	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? Thanks!

22nd October 2002, 05:09 AM	#4
hifiZen diyAudio Member Join Date: May 2001 Location: Mountain View, CA	Bipolar Junction Transistor... aka your standard garden-variety transistor, available in either NPN or PNP flavours. __________________ - Chad.

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21st October 2002, 08:27 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	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.

22nd October 2002, 04:52 AM	#3
Mark A. Gulbrandsen diyAudio Member Join Date: Sep 2001 Location: Salt Lake City	What is a BJT? A Boxy Unijunction Transistor? I missed this one along the technolocical way someplace..... Mark

22nd October 2002, 06:30 AM	#5
Koy diyAudio Member Join Date: Jan 2001 Location: Zlin	NP:With only resistor - yes. But I think mirror offers more linearity. thats why I asked. Maybe low gain of input BJTs?

23rd October 2002, 08:39 PM	#6
Nelson Pass The one and only Join Date: Mar 2001	It is my experience that current mirrors offer more gain, not necessarily more linearity.