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 Post subject: Unity Gain Buffer
PostPosted: 31 Dec 2019, 21:06 
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Joined: 07 Jan 2018, 07:13
Posts: 50
Hello,

I’m looking to build a simple cathode follower buffer stage for unity gain with a 12AU7. My question is how do you calculate the plate resistor value?

I’ve searched for the subject but only found examples pertaining to stages other than cathode followers. Some cathode followers don’t even seem to have plate resistors. Other than that I can’t find any detailed description for calculating its value.

Thank you


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 Post subject: Re: Unity Gain Buffer
PostPosted: 01 Jan 2020, 12:04 
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Joined: 06 Apr 2009, 10:08
Posts: 1707
Location: US Pacific Northwest
Look at the buffer on this amplifier stage to see an example: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Note that the design of a simple cathode follower is based on a normal common cathode amplifier stage adjusted for 100% feedback. Attached are a couple of pages from one of my design notebooks on cathode follower derivation.
Attachment:
Cathode Follower Formulas - Page 1s.jpg

Attachment:
Cathode Follower Formulas - Page 2s.jpg

I hope this helps.


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It's all about the Glass!
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 Post subject: Re: Unity Gain Buffer
PostPosted: 01 Jan 2020, 20:28 
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Joined: 04 Jun 2008, 20:59
Posts: 4284
Location: Arizona, USA
There is no plate/anode resistor as one is not needed. BTW the gain will always be less than unity. With a 12AU7 it is about 0.90.

Good listening
Bruce

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