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[AMPS] Re: Formula for GG Input Impedance

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Subject: [AMPS] Re: Formula for GG Input Impedance
From: (Jon Ogden)
Date: Wed, 25 Mar 98 14:24:44 -0600
>Since you have disputed the formula for input impedance for
>a GG class AB2 amplifier:
>Input impedance equals input drive power divided by peak cathode
>current squared. (I wrote it out in words...several have been
>confused by my attempts at writing equations on the refelector)
>perhaps you have been taught a "new math" in school and would like
>to share the correct formula with us. The above formula first
>appeared on page 18 of the January 1974 issue of Ham Radio magazine.
>It has never been disputed or changed. Please share with us the correct
>numbers, which you should have done when you trashed my post!


I didn't trash your post.  There was a post from someone, I don't know if 
it was you or not who put out a formula for impedance where the end unit 
was VOLTS and not OHMS.  The formula I quoted in my message was copied 
EXACTLY from the message with the incorrect units.  I was not the only 
person who pointed this out.

What you are saying above is correct.  Z=P/(I^2).  It follows easily from 
Ohms Law.

However, someone did try to say that as you decrease power, impedance 
stays the same becuase current also decreases.  If the current decreases 
linearly at the same rate that the power does, this is not true.  The 
impedance will NOT stay the same.  For example:  If at 100 Watts of drive 
the grid current is 200 mA, then the  Z=100/(.2^2)=100/.04=2500 Ohms.  If 
at 50 watts of drive, the current is 100 mA, then Z=50/(.1^2)=50/(.01) = 
5000 Ohms.  These numbers are purely hypothetical,but prove the point 
that in order for the impedance to stay 2500 Ohms at 50 watts, the 
current would then need to be: 2500=50/(I^2) => I^2=50/2500=.02 => 
I=141.42 mA.

Phil, in one of your messages you said some correct stuff, I didn't trash 
that.  I almost copied it into my message and said it was incorrect but 
when I re-read it, I realized that what you said was correct and 
therefore I wasn't going to trash it.

Please point out to me the correct information you gave that I so 
wantonly trashed.  I notice you didn't quote any of my message to support 
your claim.  If I am in error, show me and I will apologize to the world. 
 As for now, though, I stand by the message I sent and the information 
contained therein.




Jon Ogden

"A life lived in fear is a life half lived."

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