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BJT and FET modes of operation

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Advanced Member level 6
Jan 20, 2011
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So a while back I got into a heated conversation with another engineer when talking about FET amplifiers. Basically came down to what are the correct names for the operating regions. He was adamant that in switching amplifiers and power supplies, they operate in saturation when conducting, while I said they operate in the linear region when conducting. He thought I was nuts because "It's not a linear amplifier! How can you say it's operating in a linear region?"

I'm pretty sure I'm right, and his misunderstanding comes from conflating FETs with BJTs. With BJTs, he would have been correct (saturation is when their Vce is at a minimum and they look like a low impedance voltage source), but with FETs, it's the complete opposite (saturation is when they operate as current sources with a high output impedance).

I just wanted to get an idea of which ways everyone else was taught and what terminology you use normally. I see this happening a lot (but normally it doesn't turn into a dispute).

yes. there is a confusion in 'saturation' and 'linear'

i have my own version.
i differentiate the region in device separately and the circuit separately.

so , my way is:
when the device is in saturation the circuit (with which the device is constructed) is in linear operation.

and when the device is in linear region , the circuit is said to be in saturation.

Hi mtwieg, you are right - there is some confusion in the definition of saturation.
For example, here is an excerpt from "Microelectronic Circuit Design" (R.C. Jaeger):

"If both junctions of the BJT are forward-biased, the transistor is operating in the saturation region and appears as a closed switch."...."It is important to note that the saturation region of the bipolar transistors does not correspond to the saturation region of the FET. This unfortunate use of terms is historical in nature and something we just have to accept".

My comment:...knowing the terms seems to be important.

Well . . . . . . . . . .
BJT amplifier in active or linear region . . . . . . . switching in saturation and cut off

MOSFET amplifier in saturation . . . . . . . . . . . .switching in triode and cut off

in BJT the region of saturation VCE aprox 0 corresponds to triode region VDS aprox 0

in BJT the linear or active, thats between saturation and cutoff, corresponds to the saturation region.

please tell meif I could expalin well.

I like to add to the above good points... that the word 'saturation' may refer to voltage or current.

So when the collector current Ic of a BJT transistor is said saturated it means that the transistor is biased so that Vce ( > 1V for example) won't affect Ic (approximately).

And when the Vce is said saturated it means that the transistor is biased so that its current Ib ( > Ic/20 for example) won't affect Vce (approximately).

In the first case, the transistor works as a current source at C having a current gain (Ic/Ib >> 1).
In the second case, the transistor works as a variable resistor between C and E (in function of Ib).


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