Output impedance - diyAudio
 Output impedance
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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)
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 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 Hi-Fi 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, 06:08 PM #5 diyAudio Moderator     Join Date: Oct 2002 Location: Chicagoland Blog Entries: 2 Bypassed Rk? __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 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     Join Date: Oct 2002 Location: Chicagoland Blog Entries: 2 Pardon my ignorant question, but since the output is being taken from the cathode, won't bypassing the cathode result in... silence? __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 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     Join Date: Jan 2003 Location: Milwaukee, WI 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!"

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