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Old 13th September 2012, 12:53 PM   #1
Jehan is offline Jehan  Indonesia
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Default A Question About Positive Feedback

Just a simple question. How to apply positive feedback to grounded grid? Thanks.
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Old 13th September 2012, 01:13 PM   #2
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Since in grounded grid the output (anode) and input (cathode) are in-phase, any feedback from anode to cathode is pos feedback.

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Old 13th September 2012, 03:16 PM   #3
DF96 is offline DF96  England
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Perhaps the OP was referring to the cathode-coupled amp, or short tailed pair, which is sometimes (wrongly) called 'grounded grid'? For this, the feedback can go to the first grid as that is in phase with the output from the second anode.
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Old 13th September 2012, 04:52 PM   #4
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To me grounded grid amplifier is like this:
Click the image to open in full size.

But I have been noticing that also this "good old" name has today other meaning.
I am wondering why. Did'nt the inventor of "new grounded grid amplifier" know that this name was already in use ?
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Old 13th September 2012, 05:17 PM   #5
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Quote:
Originally Posted by artosalo View Post
To me grounded grid amplifier is like this:


But I have been noticing that also this "good old" name has today other meaning.
I am wondering why. Did'nt the inventor of "new grounded grid amplifier" know that this name was already in use ?
I resist the new imho incorrect usage vehemently, and agree with you and DF96, imho the originator of the misuse of this term did it for reasons of self promotion.

Not sure why someone would want to apply positive as opposed to negative feedback unless one also wanted to boost the OLG or wanted to make an oscillator of course.. :P
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Old 13th September 2012, 05:31 PM   #6
DF96 is offline DF96  England
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The funny thing is, most of the 'new grounded grid' circuits don't even ground the second grid but use it as a negative feedback point! Maybe they should call it 'ungrounded grid'? I still prefer STP as a name, but 'cathode-coupled amp' seems to be what the textbooks call it.
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Old 13th September 2012, 05:42 PM   #7
palmas is offline palmas  Portugal
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I think the grounded grid pre was named like that because to protect the tubes it had a metalic grid connected to the chssis (ground)! There cannot be another explanation, as the circuit was never a grounded grid and never will be.
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Old 13th September 2012, 06:10 PM   #8
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Putting positive feedback around a (real) grounded grid stage could increase the input impedance for the tube below it (cascode driving case), which could be quite useful for a triode driver's linearity. But one will more likely end up with a VHF oscillator unless extreme counter-measures are taken. (ferrite beads, ground plane for grid 1, plate cap for the grounded grid stage, cage, feedthru cap in ground plane)
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Last edited by smoking-amp; 13th September 2012 at 06:21 PM.
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Old 14th September 2012, 09:28 AM   #9
Jehan is offline Jehan  Indonesia
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Thanks for the replies. So, the short tailed pair with positive feedback looks like this?
Attached Images
File Type: jpg Schematic 3.jpg (156.1 KB, 63 views)
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Old 14th September 2012, 10:07 AM   #10
DF96 is offline DF96  England
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Yes. I'm curious: what is your application? Except for oscillators and bootstraps, positive feedback is usually avoided.
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