Current Mode vs Voltage Mode

I've seen a few designs that operate in 'Current Mode', and I can only assume this is as opposed to Voltage Mode. Am I correct in saying that current mode means the voltage is kept constant while the current varies according to the source signal?

So what is the real practical difference? What are the advantages of either method? What are the major differences between a current mode schematic and a voltage mode design? Is the decision to use current mode or voltage mode a personal preference sort of thing, or is it a direct consequence of some other design decision? Isn't it kind of strange to use a constant-current sources and sinks to drive a current mode amp?

Any help understanding this concept would be most appreciated, I'm looking at an amp design now where the input is a current mode amp, but the output is not (assumed to be voltage mode), and I'm having trouble understanding the ramifications of this. Thanks everyone.
- Jonathan
 
"Current mode" can probably mean a number of things. But my guess is that:

1) The input signal is a current
2) The output signal is a current
3) Current feedback is used

Or perhaps a combination. If the amplifier requires an input current, then you can expect to see a rather low input impedance. Hey, maybe an ohm or less. You can convert from your voltage source signal to such an amplifier using a series resistor.

An amplifier may have current sources/sinks for an output stage, yet still produce an output voltage into a load with reasonable accuracy by applying feedback. The feedback will lower the effective (closed loop) output impedance.

There are advantages to either voltage or current mode amps - depends on the application or performance goals. If a load is capacitive, then a current mode type can offer a more stable (less prone to oscillation) closed loop system.

jh
 
Hello G-Daddy,
I think the tems that you mean are,
Constant current mode, and
Constant voltage mode.
Usual audio amps are CV mode - ie for a given input voltage level, the output will track this and supply current into the load according to the load impedence (V/R=I).
For CC mode the amplifier will vary its output voltage in order to force a given current into the load, regardless of load impedence.
Drawback of CC mode is no damping at resonance of speaker drivers.

Regards Eric.
 
Sorry, my terminology was lifted from Kevin Gilmore's article at headwise, here http://headwize.com/projects/showproj.php?file=gilmore2_prj.htm . In all other regards, Mr. Gilmore seems to do a great job designing and explaining his amplifier design. He refers to his first 2 stages as running in 'current mode'. Perhaps examination of his amplifier design can suggest better, more descriptive terms. As always, thanks for everyone's time and attention.
- Jonathan
 
So...
How many angels can dance on the head of a pin?

Worry not your heads about spurious nomenclature intended to obscure the fact that Ohm's Law prevails. In general, when someone speaks in terms of 'current mode' they're saying that they're operating into a low impedance. A speaker is a good example of a device that operates in 'current mode.'
'Voltage mode' means that they're operating into a high impedance, i.e. that something like a 100k input impedance will draw very little current at any voltage that we in audio would think of as normal. (Lightning strikes are another matter entirely...)
At each and every moment, IR=E <i>will</i> prevail and the current and voltage will balance according to the impedance into which they operate. When people start throwing terms like that around, just think of it as advertizing hype; meaningless, empty phrases intended to impress the unwary and up the word count for those who are getting paid by the word.

Grey
 
How many angels design circuits?

The difference between the right word and almost the right word is the difference between the lightning and the lightning bug. Neslon is absolutely right about using more meaningful and standard terms. Voltage mode and current mode are pretty meanless terns to anyone versed in actual circuit design. Stick to meaningful terms like voltage feedback, current feedback, transconductance, ect. There is enough B.S. marketing techno-babal out there as it. Please excuse my "Rant Mode" post and will will wait for any positive or negative feedback from my peers.

H.H.
 
Voltage output and voltage feedback are two different things. You can have a current output with voltage feedback and voltage output with current feedback, etc.

Audio amplifiers are voltage sources, that is, they amplify a voltage to produce a like voltage with gain to drive the speaker (sorry to state the obvious). A constant voltage source would be a power supply. And so would the constant current source.

The typical transistor audio amp uses voltage feedback. Most pentode amps (Williamson, Dynaco) use current feedback. Both are perfectly fine.

jh
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Of course Voltage Output and Current Output are simply
extremes of the same thing.

Similarly Voltage Feedback and Current Feedback are
extreme examples of the same thing.

In both case, the key is the impedance of the circuit.

With Voltage Feedback, we are usually referring to feedback
to a high impedance element, such as the Gate/Base of
a differential pair. With Current Feedback, we are usually
feeding back to the Source/Emitter of a gain device with a
low impedance network.