Are MOSFET's voltage, not current, devices?

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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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

Guys: Thanks.

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.