FET saturation and Power amplifier saturation

Think of a power amplifier as a voltage regulator. It puts out a voltage half way between V+ and V-, in other words zero volts on a symmetrical supply, when there is no input. When you strike a cord on a guitar it attempts to steer the output proportionately to the input. The scale factor depends on where you have set the volume.

In this mode the FETs are neither off nor saturated.

Saturation sets in when you have the volume at 10 and play "Stairway to Heaven" with full gusto and the neighbors hollering at you :-D

Seriously, when the gate is driven more above Vth but there is no more voltage headromm left to increase current then the current into the load remains flat, which causes clipping. The FET will reside in saturation but it doesn't help. On the other cycle it's the opposite FET going to the same extreme.

Then there are class D amplifiers where the FETs are switched between fully off and saturated but that's a whole 'nother ballgame.

--
Regards, Joerg

http://www.analogconsultants.com

You are confused by lingo, and possibly characteristics.

In the FET world, since at low Vds, the characteristic looks ohmic, and the resistance varies with gate voltage, it was named the "linear region". Likewise, as Vds rises, Ids "saturates" to some value, thereby naming that region as well.

This is in contrast to BJTs, which because they act more like diodes, the voltage-saturation region, where Vce is small, was named the saturation region. The linear region is where linear signal amplification takes place, which is out in the middle of flatsville (where Ice varies little for a change in Vce).

And nowadays, MOSFETs are widely used as "saturated" switches, which horrendously means the voltage is saturated = as low as it can get.

Confused enough yet?

Now, to actually address your question. MOSFETs, BJTs, tubes, burnt-galvanized-steel-negative-resistance-junctions, whatever -- all operate in the linear region, where a change in input causes a larger change in the output. The output can be current or voltage, it only matters that the circuit around it is capable of turning that into a useful output.

On the curve of input vs. output, there is always SOMETHING which limits the maximum output. Usually, it's that a transistor is bumped up right against the power-supply rail and therefore simply cannot supply a higher voltage. It can be said that the amplifier is saturated (cannot go any closer) to that rail. This is analogy to switching circuits, which being designed for switching, are usually better at it. Rail-to-rail amplifiers, typically built with MOSFETs, are capable of switching-grade coverage, which seems to be what your question was about.

Tim

-- Deep Fryer: A very philosophical monk. Website @

** First you need to get the meaning of the term " saturation " right - beware, it changes depending on the context.

" Saturation: a condition in which a quantity no longer responds to some external influence " is the definition that applies to electronic devices and circuits best.

Note, the term is not locked specifically to current, but an expected response to an input.

With a single MOSFET , the input is gate voltage and the response is the resulting drain current.

However, with most amplifiers, the input may be voltage or current and the output is typically voltage.

So, amplifiers are said to " saturate " when some output VOLTAGE limit is reached.

....... Phil

FET's set up with no feedback clip different than amplifiers with feedback.

D from BC

Saturation is the opposite - like a shorted (closed) switch. Device is turned on completely and its resistance is as low as it ever gets - irrespective of power supply or signal fluctuations for all practical purposes.

BJT also exhibit the same sort of thing - but enough current in the base and the collector emitter voltage is as low as it can get.

Clipping in an amp is usually the result of running out of power supply - not necessarily because the mosfet is saturated, in a well designed amp it will still have some headroom left or it wouldn't be linear in the region it is used in.

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Yep, and when you place a weak FET amp such as a CD4049UBE hanging on a lowish VCC up front you can almost make the whole thing sound like a tube amp ;-)

--
Regards, Joerg

http://www.analogconsultants.com

So what is clipping but a...nonlinearity?

If you mean keeping certain parts of the circuit in their linear range while others crash, well no, that still doesn't count as what I think you said, because something went out of bounds. I mean, you could intentionally add clamps to limit voltage and current swing and call it a feature, but it's still nonlinear. The best thing you can do is design and build your circuit to handle such things gracefully. I'm listening to an amp right now that comes out of saturation within microseconds, with a perfectly smooth recovery -- no transient top or bottom.

Tim

-- Deep Fryer: A very philosophical monk. Website @

The RCA ones sound better.

The amplifier "sticking" issue (as Tim alludes to) is a big deal. Everything has a breaking point, but failing gracefully is a selling feature. One of the best ways of seeing this in real life is to look at the "residual" output of a distortion analyzer. This also shows the onset of clipping. I'm not sure if anyone ever did a write up on using a distortion analyzer in this manner, but it is common knowledge. By comparing input versus output, you can determine if the source of distortion is at the zero crossings, slewing related, soft clipping, hard clipping, and sticking.

As long as it has a really mean sound when you strum the twang box real hard ;-)

--
Regards, Joerg

http://www.analogconsultants.com

Yeah well . . . have to define "amplifier" and "signal processor," if that's the case. Guitar amp is a special function - part of the instrument.

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The best thing you can do is to design your system so that it remains linear up to the "threshold of pain." Anything less is a compromise.

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Le Tue, 14 Aug 2007 19:16:24 -0700, MooseFET a écrit:

Why didn't RCA made a 3 inverters version in octal socket? That would've been a real killer...

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Thanks,
Fred.

Le Wed, 15 Aug 2007 07:45:53 -0400, default a écrit:

To me, with some 'modern music' anything above a mW is above the threshold of pain.

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Thanks,
Fred.

I have to agree with you there. The cannons in the 1812 Overture take a lot of amplifier and big speakers . . . Ride of the Valkyrie, Bolero, Totentaz, etc..

When all is said and done, speakers are the weak link followed by listening rooms. Headphones.

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In 2006 and 2007, will there be any songs that will become classics recognized for decades?

D from BC

Even a blind squirrel gets a nut or two.

I think the pop music industry has taken all the inventiveness, spirit and soul out of music. Too much emphasis on marketing to the detriment of the music.

If the Internet can keep the open free-for-all spirit that it currently enjoys - it is just possible that music will see a resurgence that could put the renaissance to shame.

But I don't expect that will happen. Too much money Too much greed Too many corporations and politicians

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I think it can happen but not on the radio. They seem to mostly play what the big labels want them to, over and over. Or, ahem, what the labels pay for. However, the web allows solo artists to thrive. Most of them seem to still follow the old routes though, by local distribution of their products. I know a guy who writes his own songs, lyrics, then sings and plays the guitar. A real one-man show.

--
Regards, Joerg

http://www.analogconsultants.com

Sure. In the 'Country Music' and 'Bluegrass' genres.

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Michael A. Terrell
Central Florida