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View Full Version : Cathode followers -- what are they?

tht00
05-23-2006, 08:08 PM
Alright, I'll have the first 'real' thread in here. ;)

I've seen these things in schematics before, and I know a little about them. They can be used in tube or transistor designs, and I've seen them a lot before tone stacks.

So... from what I understand, they don't provide any voltage amplification that a normal 'gain stage' would have, but they rather have the ability to source more current than a typical stage? Do I have the right idea?

Here's a few images I found from http://www.platenspeler.com/background/tubes/uk_tubesinfo_1.html

'Normal' gain stage:
http://www.platenspeler.com/background/tubes/figure_1.gif

Cathode follower:
http://www.platenspeler.com/background/tubes/figure_4.gif
What5647
05-23-2006, 08:13 PM
yeah, you have the right idea. in transistor terms it's a common collector amp. CC amps have a input resistance, high current gain and an approximate voltage gain of 1.
tht00
05-23-2006, 08:28 PM
yeah, you have the right idea. in transistor terms it's a common collector amp. CC amps have a input resistance, high current gain and an approximate voltage gain of 1.

Ok -- kinda like a voltage follower using an op amp.

So, then, what would be the most obvious uses of these, and is there a guideline of when one should use them and when you shouldn't? I've seen guitar circuits that have cathode followers before a tone stack, and I've seen some that don't. What's the determining factor?
What5647
05-23-2006, 08:43 PM
Ok -- kinda like a voltage follower using an op amp.

So, then, what would be the most obvious uses of these, and is there a guideline of when one should use them and when you shouldn't? I've seen guitar circuits that have cathode followers before a tone stack, and I've seen some that don't. What's the determining factor?
to tell you the truth, i don't really know what the most common use is, my textbooks tend to dwell more on learning how to analysing circuits than presenting common applications, so i guess whenever you need an amp that fits those parameters.
oh, i just remembered another thing about them: there's no phase inversion.