
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
13th February 2003, 03:53 PM  #1 
diyAudio Member
Join Date: Nov 2002
Location: Norway

Output impedance
Could somebody help me with this problem, I'm just wondering what the output impedance is for the circuit beneath.
I have tried to find the formula for the output impedance, but I don't know if it's correct. the formula I found is: Zout= Rk  (rp+Ra+u*Rk) or (Rk*(rp+Ra+u*Rk)) / (Rk+rp+Ra+u*Rk) 
13th February 2003, 04:53 PM  #2 
diyAudio Moderator Emeritus
Join Date: Jan 2003
Location: Near London. UK

Zout = Rk // [(Ra+ra)/(u+1)]
To put it in words, looking up into the anode, all resistances are divided by a factor of (u+1), and that little lot is in parallel with your cathode bias resistor. The equation you quoted would imply that loading a cathode follower with a constant current sink insted of a cathode resistor would significantly increase its output resistance.
__________________
The loudspeaker: The only commercial HiFi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference... 
13th February 2003, 05:41 PM  #3 
diyAudio Member
Join Date: Nov 2002
Location: Norway

thanks
Do you have some more hints about how I can derive this formula?? I have tried a couple of times but haven't got the right answer yet 
13th February 2003, 06:04 PM  #4 
diyAudio Senior Member
Join Date: Aug 2002
Location: Belgium

Zo AT THE CATHODE.
Hi,
The easiest way is: Output impedance (cathode): Rk' = (Rl+ra)/(mu+1) Rout = Rk'Rk Where: Rp = the plate resistor Rl = the load resistance, or the input resistance of the next stage Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl. If there is no Rl, Ra = Rp. ra = the internal plate resistance of the tube mu = the mu of the tube Keep in mind this for a bypassed Rk. Cheers,
__________________
Frank 
13th February 2003, 06:13 PM  #6 
diyAudio Senior Member
Join Date: Aug 2002
Location: Belgium

Bypassed Rk?
Hi,
Meaning : cathode resistor bypassed with a capacitor. Should have clarified that. For the unbypassed common cathode circuit you can use this: Rout = [(Ra + ra)/(mu + 1) * Rk] / [(Ra + ra)/(mu + 1) + Rk] Where: Rk = the cathode resistor Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl. If there is no Rl, Ra = Rp. ra = the internal plate resistance of the tube mu = the mu of the tube Cheers,
__________________
Frank 
13th February 2003, 06:35 PM  #7 
diyAudio Moderator

Pardon my ignorant question, but since the output is being taken from the cathode, won't bypassing the cathode result in... silence?
__________________
“Just as soon as we get our opposable thumbs on an interesting concept, the rest of our chimp brains hijack it for a drunken joyride down the interstate.” 
13th February 2003, 06:51 PM  #8 
diyAudio Senior Member
Join Date: Aug 2002
Location: Belgium

I SEE.
Hi,
Sorry if I got anyone confused here. Output impedance (cathode): Rk' = (Rl+ra)/(mu+1) Rout = Rk'Rk Actually this is the output impedance calculated for a common cathode stage,signal is of course taken from the plate,cathode R is bypassed. Output impedance (plate): Rout = (ra * Rp)/(ra + Rp) Hope this sets it straight. Cheers,
__________________
Frank 
13th February 2003, 07:45 PM  #9 
diyAudio Member

The complicated way, of course, would be to graph operation, determine grid drive for output, then add output voltage to input voltage as NFB, and calculating Zo based on Rp, Rl and amount of NFB applied.
No doubt those equations simplify all this into a single operation... Tim "but I like the scenic route!" 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
3C24 in a SE output stage  output transformer impedance recommendation !!!  aldovan  Tubes / Valves  15  6th September 2008 10:18 AM 
Output impedance  overdrajv  Tubes / Valves  8  5th March 2008 05:54 PM 
Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms  kelticwizard  Everything Else  11  25th March 2007 05:17 AM 
Output Impedance?  ble0t  Parts  4  19th January 2006 05:45 PM 
Low output impedance pre  miguel2  Tubes / Valves  13  23rd July 2003 09:51 AM 
New To Site?  Need Help? 