Cathode Follower Buffer Input Impedance

xcortes · 30th November 2005, 04:50 PM

An someone tell me what determines the input impedance of a cathode follower circuit?

Thanks,

xavier

xcortes · 30th November 2005, 04:56 PM

The circuit I'm considering is the enclosed one.

It's taken from:

http://www.siteswithstyle.com/VoltSecond/Foreplay_Unity_gain/Unity_gain_Foreplay.html

The circuit shows two circuits. One is after a volume control and thus this part tells the input impedance. The other is directly after the input and is tagged "tape out". This is the one I don't know what the input impedance is.

Thanks,

xavier

cerrem · 1st December 2005, 06:36 AM

A tube follower has very high input impedance..
So the input impedance, mid band, is the grid to ground resistance.... In the case of the schematic you show...the input impedance is 100K...since you have two 200K resistors in parallel, then they are bypassed to ground via 100uFcap....
The complete input impedance, would include the low frequency roll-off from this 100uF bypass cap....also the tube capacitance would affect high frequenncy roll-off... The plate to grid capacitance in parallel with the plate to cathode capacitance...
The grid to cathode capacitance is not significant in a follower since there is not much voltage differential across those nodes..

Some more info may be needed since the value of the pot the wiper goes to should be included..
Chris

xcortes · 1st December 2005, 02:36 PM

Quote:

Some more info may be needed since the value of the pot the wiper goes to should be included

I will not include the pot, wiper and selector switch but intend to connect the circuit directly after my phono pre as a buffer to make it capable of driving a CD Recorder with 10K ohm input impedance. The phono stage "is designed to operate into line stages with 50K or higher input impedance" so what I need to know is if this will work.

thanks

cerrem · 1st December 2005, 05:59 PM

I may have looked at the schematic from the wrong perspective...
If you are looking into the circuit from the perspective of the wiper terminal, then you will have 200K input inpedance at mid-band...

Chris

xcortes · 1st December 2005, 06:10 PM

thanks a lot!

30th November 2005, 04:50 PM	#1
xcortes diyAudio Member Join Date: Feb 2004 Location: Mexico City	Cathode Follower Buffer Input Impedance An someone tell me what determines the input impedance of a cathode follower circuit? Thanks, xavier

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1st December 2005, 06:36 AM	#3
cerrem diyAudio Member Join Date: Jun 2005 Location: San Diego, CA	A tube follower has very high input impedance.. So the input impedance, mid band, is the grid to ground resistance.... In the case of the schematic you show...the input impedance is 100K...since you have two 200K resistors in parallel, then they are bypassed to ground via 100uFcap.... The complete input impedance, would include the low frequency roll-off from this 100uF bypass cap....also the tube capacitance would affect high frequenncy roll-off... The plate to grid capacitance in parallel with the plate to cathode capacitance... The grid to cathode capacitance is not significant in a follower since there is not much voltage differential across those nodes.. Some more info may be needed since the value of the pot the wiper goes to should be included.. Chris

1st December 2005, 05:59 PM	#5
cerrem diyAudio Member Join Date: Jun 2005 Location: San Diego, CA	I may have looked at the schematic from the wrong perspective... If you are looking into the circuit from the perspective of the wiper terminal, then you will have 200K input inpedance at mid-band... Chris

1st December 2005, 06:10 PM	#6
xcortes diyAudio Member Join Date: Feb 2004 Location: Mexico City	thanks a lot!

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