Are MOSFET's voltage, not current, devices?

Actually, MOSFETs have a very high output impedance, typically in the megohms for a modern power device. Bipolars do too, although slightly less as reduced by the Early Effect. The output impedance of pentodes is lower, typically in the tens of k-ohms for a power device, and the output impedance of the triode is lower yet, with output impedances typically around one k-ohm for a power device. What transistors do have is a much higher current density capability than tubes. Tubes generally handle higher voltages better than transistors, and thus the need for an output transformer (or ZOTL). One must not confuse high current with low impedance when it comes to active devices as these concepts are very different. All ss amps that deliver low output impedance do so with high feedback. This can be done without loop feedback in the form of local feedback. Emitter follower or source follower configurations are common local feedback schemes that deliver low output impedance, and these are classified as feedback circuits. Only triode output stages can produce acceptable levels of damping without feedback (through a transformer or zotl). This self-damping characteristic of triodes, I believe, is the reason that triodes produce the least agressive sound of any of the amplifiying devices, all else being equal.

Guys: Thanks.

Jeffreybehr,yes,Herman did magnificently.I was of course referring to 'simply' being the key word in reference to non-tech lingo,so appreciated by laymen/rookies in this field such as I.Keep the ball rolling,good stuff,Bob

Hmm...I think Herman 'nailed it' when he wrote "Well, that should be enough information, mis-information (sic), and half truths to thoroughly confuse you."

Let's see if I can add anything useful with my just-enough-to-be-dangerous technical knowledge. (Just the other day I misstated on Audio Asylum the conversion of picoFarads to microFarads. There went my credibility! :-) )

First, generally, and compared with the impedances of typical loudspeakers, transistors are low-impedance devices and tubes are high-impedance devices. That's one reason the vast majority of tubed amps need output transformers and the vast majority of solidstate (SS) amps don't. Because the output impedance of a SS amp is MUCH lower than the impedance of a loudspeaker*, SS amps often are called constant-Voltage devices, since they can maintain Voltage as they flow more current as the load impedance decreases. Tubed amps can’t do that. At rated output, tubed amps can’t increase current flow (or at least current flow increases very little), so their Voltage output sags as load impedance decreases. Hence, their label as ‘constant-current’ devices.

Cford is correct in his contrasting of MOSFET v. other transistors. As a MOSFET increases in temperature, its internal impedance increases, thereby resisting further increases in current flow. As a germanium-based transistor (GBT) heats, its internal resistance decreases, thereby allowing even more current flow, thereby heating more, thereby passing more current...ad infinitum. (That's called 'thermal runaway'.) I believe MOSFETS are inherently higher-impedance devices compared with GBTs and therefor are not able to flow as much maximum current as GBTs.

However, compared with tubed amps, MOSFETS are low-impedance devices, just not as low as GBTs, and therefor a MOSFET-based amp will still increase current as the load impedance decreases. Therefor they’re still ‘constant-Voltage’ devices, just as GBT-based amps are.

* That characteristic is measured by 'damping factor', which is the ratio of the typical 8-Ohm loudspeaker to the output impedance of the amp.
.

Cs,simply put,you nailed it,thanks,Bob

Well, that should be enough information, mis-information, and half truths to thoroughly confuse you.

What GS5556 said is right. The job of the output device is to control the amount of current through the speaker just like a water valve controls the amount water through a hose. That's why tubes are called valves by the Brits.

I'm not sure how Hack's comments pertain to the question so I'll leave that alone. Gs5556's comments may not have been a thorough description of transistor action but I didn't see them as descriptive of a diode.

What SdCampbell said is rather unconventional as it really makes no sense to call something a voltage device or a current device.

What Cford says is correct, the key word being control.

In audio these devices are set up to control the amount of current that is flowing through their loads. They are in turn controlled by an input signal.

All types of FETs are controlled by an input voltage on the gate and this voltage draws little if any current.
All tubes with the input signal on the grid are controlled by a voltage but may draw significant current on positive peaks in some situations.
All bipolar transistors are controlled by an input current.

FETs and tubes are often said to be similar because they are both controlled by an input voltage and they produce the same types of distortion.

I don't know what amps you were looking at but the low output curent is not a charecteristic of all FETs. Some can handle in excess of 100 amps (that's a lot.)

Mosfets are voltage controlled devices, that is the more voltage on the gate the more fully they turn on and once turned on, they act like a resistor. The only current required to control them is to charge up any gate capacitance.
Bipolar transistors are current controlled devices, that is the more current flowing into the base, the more current can flow thru them. The only voltage required to turn them on is to overcome the voltage drop from base to emitter. Once fully turned on they have a fairly constant voltage drop, unlike a mosfet whos voltage drop = current x on resistance.

In a nutshell, transistors of all types are current devices, whereas tubes are voltage devices. In most of the "better" solid-state amps, the input stage uses FET's, the gain stage has MOSFET's, and the output stage has bi-polar transistors. This arrangement is usually chosen because it capitalizes on the best characteristics of each type of transistor.

Gs,

I think you may be a bit confused, you described a diode.

From a quick google:

"Acronym for metal-oxide semiconductor field-effect transistor, a common type of transistor in which charge carriers, such as electrons, flow along channels. The width of the channel, which determines how well the device conducts, is controlled by an electrode called the gate, separated from channel by a thin layer of oxide insulation. The insulation keeps current from flowing between the gate and channel.

MOSFETs are useful for high-speed switching applications and also on integrated circuits in computers."

MOSFETS do switch faster than traditional NPN or PNP transistors, however they also produce measurable (and more importantly audible) noise. So there's a tradeoff between transisents and noise, both approaches have known design techniques to deal with the shortcomings. For what it's worth most solid state amplifiers that I prefer are discreet amps, MOSFETS seem a bit edgy to my ears.

Reagrds, Jeff ('phile and semiconductor dude for the last 20+ years)

Transistors and tubes do not create voltage or current. They let current pass through them. The voltage that moves the current comes from the power supply. The current output is determined by the power supply voltage and resistor values around the transistors.

Transistors/tubes act like water valves - the valves neither create the water nor the water pressure behind them. They simply let water pass that's already there - just like transistors which pass current that's already there (simply put).