Are MOSFET's voltage, not current, devices?

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.

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.

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.)

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