Why MOSFET must be biased in saturation region for amplification?

[liudehua]

Why MOSFET must be biased in saturation region for amplification instead of triode region?

i have realised that hand analysis is much easier by biasing the mosfet in the saturation, but now equipped with simulation tools, triode region might give us some improvements on the functions of amplifiers while not imposing much challenge in the process of analysis.

P.S. i know that biasing in the saturation region gives higher voltage gain Av for a common source amplifier, but i doubt it's the same for other amplifier applications.

please give me your opinion on why we use saturation region?:smile:

 

[jiripolivka]

I do not know about simulation but any amplifier device including a MOSFET must be biased to amplify. If you rather use the MOSFET to switch on/off, saturation setting can be useful for a good efficiency.
Low-level signal amplifier is usually biased in the "linear" response region; high-level amplification calls for saturation setting but one should expect some distortion. Instead of sinusoidal output it is rather rectangular pulse series, with harmonics.

 

[LvW]

..... triode region might give us some improvements on the functions of amplifiers while not imposing much challenge in the process of analysis.

Why do you think so? Can you justify this claim?

In the saturation region the device acts as a signal current source (of course: non-ideal).
And that is the property which each transistor-based linear voltage amplifier should have:
The input signal voltage is transferred to an output signal current (Id) that produces a voltage across an external resistor Rd.
That`s how voltage amplification works.

 

[liudehua]

Why do you think so? Can you justify this claim?

In the saturation region the device acts as a signal current source (of course: non-ideal).
And that is the property which each transistor-based linear voltage amplifier should have:
The input signal voltage is transferred to an output signal current (Id) that produces a voltage across an external resistor Rd.
That`s how voltage amplification works.

u r correct!:razz: thank you

- - - Updated - - -

Why do you think so? Can you justify this claim?

In the saturation region the device acts as a signal current source (of course: non-ideal).
And that is the property which each transistor-based linear voltage amplifier should have:
The input signal voltage is transferred to an output signal current (Id) that produces a voltage across an external resistor Rd.
That`s how voltage amplification works.

u r correct!:razz: thank you

 

[Marvel_tronix]

Why MOSFET must be biased in saturation region for amplification instead of triode region?

i have realised that hand analysis is much easier by biasing the mosfet in the saturation, but now equipped with simulation tools, triode region might give us some improvements on the functions of amplifiers while not imposing much challenge in the process of analysis.

P.S. i know that biasing in the saturation region gives higher voltage gain Av for a common source amplifier, but i doubt it's the same for other amplifier applications.

please give me your opinion on why we use saturation region?:smile:

Best way to understand the need for saturation region can be understood using voltage-transfer characteristics (VTC).
Fig. 1 consists of basic structure of CS amplifier while Fig2 shows its VTC.
We want LINEAR amplification of a signal i.e. the amplified signal should be linearly related to input signal so that we get distortion free output. U can see in Fig2 that this linear region(corresponding to amplifier characteristics) is from pt. A to pt. B where NMOS is in saturation region. If u bias NMOS in triode or linear region, as u want;-), u would get a highly distorted output.
Ahead, as above, similar to other members explanation, MOS can be understood to be acting like a current source(with some parallel resistance to it) in saturation region causing its i-v curve slope to be almost constant. While in triode region it acts as a voltage-controlled resistor where the slope is not constant & changes with the applied input voltage. Actually, from this behaviour the VTC arises which is pretty clear.
Hope u will get clear now.

 

[liudehua]

Best way to understand the need for saturation region can be understood using voltage-transfer characteristics (VTC).View attachment 94424View attachment 94425
Fig. 1 consists of basic structure of CS amplifier while Fig2 shows its VTC.
We want LINEAR amplification of a signal i.e. the amplified signal should be linearly related to input signal so that we get distortion free output. U can see in Fig2 that this linear region(corresponding to amplifier characteristics) is from pt. A to pt. B where NMOS is in saturation region. If u bias NMOS in triode or linear region, as u want;-), u would get a highly distorted output.
Ahead, as above, similar to other members explanation, MOS can be understood to be acting like a current source(with some parallel resistance to it) in saturation region causing its i-v curve slope to be almost constant. While in triode region it acts as a voltage-controlled resistor where the slope is not constant & changes with the applied input voltage. Actually, from this behaviour the VTC arises which is pretty clear.
Hope u will get clear now.

thank you. this is the most comprehensive answer i could find online so far.:grin:

 

[dick_freebird]

Voltage gain is gm*rout, rout is the parallel sum of the
gain device, its load device and the attached impedance
(driven load or successor stage), and rout is lousy in
linear region. If your driven load is high impedance then
the gain stage needs to be high impedance (saturation)
presuming you want your amplifier to, you know, amplify.