"Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

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mediatechnology
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"Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by mediatechnology » Wed May 08, 2019 5:35 pm

"Low Noise Electronic Design," "Design with Feedback," Section 7-4, C.D. Motchenbacher, F.C. Fitchen, 1973.

The "op amp around the transistor" "current feedback" circuit shown by Demrow in 1968 https://proaudiodesignforum.com/forum/p ... =12&t=1052 and documented by Motchenbacher and Fitchen in 1973.
This is the single-ended half-circuit of the popular "Cohen" differential style preamp.


Image
"Low Noise Electronic Design," "Design with Feedback," Section 7-4, C.D. Motchenbacher, F.C. Fitchen, 1973. Figure 7-3.

Note the common base connection (figure 7-3a) with "current feedback."

"Low Noise Electronic Design," "Design with Feedback," Section 7-4, C.D. Motchenbacher, F.C. Fitchen, 1973. https://proaudiodesignforum.com/images/ ... on_7-4.pdf

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by JR. » Thu May 09, 2019 11:02 am

Page 124... 8-)

I've shared this story before... back in the late 70s I was living about one town away from Bridgeport where Fitchen was teaching at University of Bridgeport. His phone number was in the phone book so I called him. I wanted to pick his brain about a Leach low noise topology that was unusual (he didn't know either). After a nice chat he asked me to contribute to a chapter for his next book... I demurred, I was not an academic (college dropout), but I gather Motchenbacher was also a working design engineer, and Fitchen was the professorial half of the team.

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by mediatechnology » Thu May 09, 2019 12:35 pm

Thanks John for that story.
Yes, page 124.

I had posted that page here elsewhere but I thought it deserved its own thread and index.
Its one of the more practical books and I enjoyed leafing through it again.

You don't see the common base circuit often unless its a mic preamp re-purposed as a bus summing amp or "current input" moving coil phono preamp.

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by JR. » Sat May 11, 2019 3:04 pm

Yup we live in the time of "Bridging" input terminations that favor high Z.

MC is low Z source but even I tried to present a 10x termination impedance... but that was a very long time ago.

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by lsburden » Fri Oct 08, 2021 10:04 am

I tried simulating the inverting amplifier with no luck.
What am I missing?

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by mediatechnology » Fri Oct 08, 2021 10:47 am

Those look to be AC models without bias.

On figure (b) there should be a positive DC potential at the non-inverting op amp input. (Vbias)
(Vcc-Vbias)/Rc = Ic

For your SE model you also need to add a coupling cap in series with Re.
Ie is provided by the op amp.

The common base configuration of figure (a) has always been interesting to me.
It's what you get when you make a summing amp out of a CFB mic preamp like a THAT1510.

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by lsburden » Fri Oct 08, 2021 1:47 pm

ok.
I'm aware of the positive DC potential at the non-inverting input of the amplifier in figure b.
I was able to get something going with a negative DC potential at the non-inverting input of the amplifier in figure a.
This looks like a fun circuit to play with...

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by lsburden » Fri Oct 08, 2021 1:49 pm

I was also able to get something going by applying the positive DC voltage to transistor base...duh.

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by mediatechnology » Fri Oct 08, 2021 1:53 pm

In figure (a) I think you'll need a coupling cap in series with Rs.

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Re: "Low Noise Electronic Design," Motchenbacher and Fitchen, 1973

Post by lsburden » Fri Oct 08, 2021 1:57 pm

Makes sense.

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