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Old 26th August 2003, 12:52 PM   #1
kyrgeo is offline kyrgeo  Greece
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Default A different input stage

Virtually all differential input stages have a common emitter differential pair. This configuration suffers from the Miller capacitance multiplication effect, and has therefore low bandwidth.
Cascoding with a common base stage reduces the Miller effect, and increases bandwidth. This is straightforward and has been done often.

I was wondering why the common collector (emitter follower) "cascoded" by a common base stage is not used. The emitter follower inherently does not suffer from the Miller effect, but does not provide voltage gain. Therefore we use a common base stage, where the Miller effect is also absent, to provide the voltage gain. The emitter follower should be more linear than the common emitter stage...

Why has the CC-CB configuration not caught on, like the CE-CB cascode?
Has anybody any experience with it?
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Old 27th August 2003, 03:53 AM   #2
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This type of input stage was first used in the LM101 in 1966 or so.
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Old 27th August 2003, 04:49 AM   #3
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Hello -

I don't really see how one could make a differential amplifier with your proposed input stage.

Charles Hansen
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Old 27th August 2003, 04:57 AM   #4
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Quote:
Originally posted by john curl
This type of input stage was first used in the LM101 in 1966 or so.
Hello John,

I found the schematic for the LM101 on National's web site at:

http://cache.national.com/ds/LM/LM101A.pdf

I looked at it for around 3 minutes, but my head exploded before I could understand how it could possibly work. Any insights would be appreciated!

Thanks,
Charles Hansen
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Old 27th August 2003, 05:22 AM   #5
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Default Re: A different input stage

Quote:
Originally posted by kyrgeo
Why has the CC-CB configuration not caught on, like the CE-CB cascode?
Dunno. Been wondering the same about the Xfrmr/CC configuration.

se
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Old 27th August 2003, 05:39 AM   #6
andy_c is offline andy_c  United States
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Quote:
Originally posted by Charles Hansen

I found the schematic for the LM101 on National's web site at:

http://cache.national.com/ds/LM/LM101A.pdf

I looked at it for around 3 minutes, but my head exploded before I could understand how it could possibly work. Any insights would be appreciated!
Hi Charles,

There's a PDF file at this site:

http://www.stanford.edu/class/ee214/.../ho18opamp.pdf

which discusses detailed design information of the 101 among others. It's a good read.

Andy C
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Old 27th August 2003, 05:40 AM   #7
jam is offline jam  United States
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Default Warning! Off Topic Post.

Charles,

I thought you might find this a source of amusement.............

http://www.lunaticlounge.com/fringefiles/explode.html

Regards,
Jam
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Old 27th August 2003, 06:41 AM   #8
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Quote:
Originally posted by andy_c
There's a PDF file at this site:

http://www.stanford.edu/class/ee214/.../ho18opamp.pdf

which discusses detailed design information of the 101 among others. It's a good read.
Hello Andy,

Wow! Thanks for the great link. I haven't had time to fully digest it (my head hasn't fully re-integrated after the recent explosion), but it's really, really good! Did you take this class, or did you just stumble across the file?

I still have a question. In the file you referenced, the author states, "connect the PNPs (Q3/Q4) in a differential common-base configuration".

I didn't know you could do that! Does anyone have any references for that circuit? It seems like the beta for the two transistors would have to match *exactly* for this to work. Even then, it seems a bit sketchy to me.

Thanks,
Charles Hansen
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Old 27th August 2003, 06:58 AM   #9
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Quote:
Originally posted by Charles Hansen
I didn't know you could do that! Does anyone have any references for that circuit? It seems like the beta for the two transistors would have to match *exactly* for this to work. Even then, it seems a bit sketchy to me.
How 'bout if you look at the transistors as transconductance devices rather than current amplifiers as John once advised?

se
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Old 27th August 2003, 09:04 AM   #10
bocka is offline bocka  Germany
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With discretes it's very difficult to get a well balanced differential stage. transistors must have the same . So you have to use duals for NPN and PNP and control the Ibias for the differential pair. It's only good/simple solution for chips
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