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tim
2/22/2002 1:56 AM
(perhaps a dumb q) what is "common
cathode"?
This (fine) GlassWare article seems to describe something I'm more accustomed to calling
"differential":
http://www.glass-ware.com/tubecircuits/Common_Cathode_Amplifiers.html Seems fair to call it
"common cathode", two triode sections sharing a common cathode resistor and voltage.
Yet, this (equally fine) Aiken article refers to a single triode section:
http://www.aikenamps.com/CommonCathode.htm I don't see any commonalities between
cathodes (as there is only one cathode).
Both articles seem immensely helpful... but seem to be addressing somewhat different circuit
methodologies under the same banner. I'm confused!
Any thoughts? Thanks :)
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Todd N.
([email protected])
2/22/2002 3:36 AM
Re: (perhaps a dumb q) what is "common cathode"?
Hey, tim. I can see why you are confused. From what I've learned, this circuit shouldn't be called
"common-cathode". Going from what I've learned, "common-cathode" should mean the cathode is
basically attached to ground [simplified answer]. When someone says an element of a triode is
"common", they're basically saying that element is held steady, with no signal variation [a bit of an
oversimplification, others here could explain better].
In a common-cathode topology, you're trying to hold the cathode at a steady voltage, while the
grid voltage is used to control the current from the cathode to the plate. In common-grid (or
gounded-grid) mode, the grid is held steady, and voltage variations at the cathode, in relation to
the grid, control the current to the plate.
The amplifier in the GlassWare article is really a cathode-follower input (used to buffer the input)
coupled to a grounded-grid amplifier. The 1st half of the triode lowers the impedance of the signal
as it's taken from its cathode. That signal is then applied to the cathode of the 2nd triode, which
then varies in relation to the grounded-grid, controlling current through the plate, and causing
amplification. This circuit is a good way to get around the problem of high-frequency roll-off due to
the input capacitance of a tube, and doesn't invert the phase of the signal.
I wish I could explain further, but I'll end up tripping over my words further :D. Like I said, other
here could explain better. I hope I haven't confused you more. Take care!
Todd
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M Cook
([email protected])
2/22/2002 4:27 AM
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=169803
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Re: (perhaps a dumb q) what is "common cathode"?
Tim,
I could be way off here but I have always understood the term "common cathode" to mean the
common (ie. regular, normal, most common) type "cathode biased voltage amplifier"
I think that the "common" in this sense does not refer to the cathode being in common with
anything.
Someone please clear this up if I'm wrong!
Michael :)
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Liam
([email protected])
2/22/2002 11:04 AM
Re: (perhaps a dumb q) what is "common cathode"?
I think that the "common" in this sense does not refer to the cathode being in common
with anything.
Someone please clear this up if I'm wrong!
'fraid you are ;). "Common cathode" means that it sees no signal variation (not directly, anyway),
as in the other reply. It doesn't have to be grounded. If it's raised from ground with a resistor you
get local negative feedback. Common cathode amplifiers are used in standard voltage amplifiers.
The grid is used for signal input and the signal is taken from the anode.
The other type of circuit is the cathode follower, also known as the common anode. In this one the
anode sees no signal (it's grounded by B+ for AC purposes). The signal is taken from the cathode,
and hence the term 'cathode follower'. It provides no voltage amplification and is used for buffer
and drive circuits.
Now I'm not sure if you get "common grid" valve stages. I'd assume you do. I think the transistor
equivalent (common base) is used for current amps.
Aiken is the place to look at this. Just ignore the math and try to see the principles.
Liam
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markh
([email protected])
2/22/2002 10:47 AM
Re: (perhaps a dumb q) what is "common cathode"?
All three-element amplifying devices (tubes and transistors) have three modes, using one element
as input, one as output, and the third as "common" to both the input and output (the third being
neither input nor output). The easiest way to see this is if the common element is grounded, but
this is not always possible given practical circuit constraints such a biasing, etc.
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In the case of the tube/valve, the three are:
1) common cathode
- grid is input, plate is output
- highest power gain (voltage/current combo)
- output is 180 deg. out of phase WRT input
2) common plate
- grid is input, cathode is output
- highest current gain, voltage gain is "almost 1"
- output is in phase WRT input
- Also called the "cathode follower"
3) common grid
- cathode is input, plate is output
- highest voltage gain, best HF performance
- output is in phase wRT input
In most guitar amps, most of the stages (including the power tubes) are common cathode. The
common exceptions are the cathode follower driving the typical plexi Marshall tone stack, and the
"right half" of the phase inverter, which sometimes is common grid (if the grid is AC grounded via
a .1 cap like it is in most Marshall and Fender designs)
HTH,
--markh
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