FET saturation and Power amplifier saturation

m_oylulan · 2007-08-13T21:43:36+00:00

Can anyone please help me understand the relationship between a FET'ssaturation region and the saturationpoint of a power amplifier? From what I have read, the FET has threeregions of operation: ohmic, saturation and cut-off. The descriptionof the cut-off region seems the most straightforward: Vgs Vthreshold, Ids ~ (Vgs - Vthreshold)*Vds. Ids is dependent onVds, the Power amplifer is like a voltage controlled resistor.I don't understand the purpose of the ohmic region from a poweramplification perspective, but it is the saturation region that reallyhas me confused. In saturation: Vgs > Vthreshold, Ids ~ (Vgs - Vthreshold)^2. Ids does not depend onVds, but increases as Vgs increases.As I understand it, this is called the 'saturation region' of the FETbecause for any particular value of Vgs, Ids is constant for allvalues of Vds > Vgs - Vthreshold. So Ids is saturated.What I can't figure out is, at what point does a power amplifierbiased in the FET saturation region become saturated i.e. when thepower amplifier reaches its maximum output power and begins tocompress? If you plot Ids against Vgs you get like a risingexponential. My thinking is that the part of the exponential where theslope is zero corresponds to the power amplifier saturation. Is thislogical?Any suggestions welcome.Thanksmees

Aug 13, 2007 26 Replies

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Michael A. Terrell 19 years ago

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Joerg 19 years ago

Thanks! I also like Cajun and Gospel so this should be cool. Will try out this afternoon since the PC in the office doesn't have speakers. The one in the lab sounds like a fox after three packs of Marlboros but I can hook something up to it. There is an old amp somewhere in the garage.

I don't like any sports on radio or TV. With the exception of Brazilian soccer commentators. They really rock.

Regards, Joerg http://www.analogconsultants.com

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Michael A. Terrell 19 years ago

Do you need a decent set of computer speakers? I have several sets, so you could have a set for the shopping costs.

Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida

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Joerg 19 years ago

Thanks, Michael, but I've got a few big ones (this amp would blow PC speakers). Plus some smaller ones from a DVD player from a yard sale. It's surround sound which we don't need, meaning lots of little speakers that are quite good. Yeah, we have a DVD player now. Number of DVDs viewed in the three months we had it: One movie, one short DVD from church. So I'm not sure we really need a DVD player ....

Regards, Joerg http://www.analogconsultants.com

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m_oylulan 19 years ago

FET'ssaturationregion and thesaturationpointof a power amplifier? From what I have read, the FET has three

Thanks all for your responses, the summary of which seems to be power amplifier saturation and FET saturation are not necessarily related. The same word is just used to apply to different siutations, and for power amplifiers saturation happens when the output voltage/current peaks at the power supply limit.

Thanks again. Mees

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Joerg 19 years ago

FET'ssaturationregion and thesaturationpointof a power amplifier? From what I have read, the FET has three

Not necessarily, but usually. The younger amplifier stages begin to drive the FETs more and more into saturation when the amp has run out of steam, when the rail voltages have been reached.

Regards, Joerg http://www.analogconsultants.com

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