MOSFET voltage follower

[mayor]

Hello,

I've googled a lot to find an answer, so I am thinking I'm missing something very obvious here.

In a MOSFET voltage follower circuit like, for example:



I am wondering how one can reach saturation in a case like this? Vgs is going to be essentially the threshold voltage, right? Such that Vgs = Vt, thus Vgs-Vt = 0 ?



As I understand it, Id would be very, very low, whatever Vgs is ?

             

[danadak]

Vgatetoground = Vgs + Id x Rsource.

Or Vgs = Vgatetoground - Id x Rsource.

Google "bias mosfet common source", lots of hits.


Regards, Dana.

[Probb]

You may be confusing "voltage follower" with "source follower" in this case. Voltage follower circuits tend to incorporate op-amps and act as buffers.
The circuit you've shown is dependent on a few parameters, specifically the pull-down resistance at the gate, and the resistance at your source.
What's your application?

[Audioguru]

Most Mosfets specify the threshold current at only 0.25mA when the Mosfet is almost turned off and is barely turned on. The Vgs at threshold for most Mosfets is 2V to 4V.

[mayor]

You may be confusing "voltage follower" with "source follower" in this case. Voltage follower circuits tend to incorporate op-amps and act as buffers.
The circuit you've shown is dependent on a few parameters, specifically the pull-down resistance at the gate, and the resistance at your source.
What's your application?

Hi, thanks for getting back.

My application is to control voltage, and use the mosfet to provide more current. The gate of the mosfet is driven by an opamp, which outputs the desired voltage, which I aim to have across DS.

[madires]

Please see
- http://www.ti.com.cn/cn/lit/an/snoa600b/snoa600b.pdf
- http://cds.linear.com/docs/en/application-note/an18f.pdf

[mayor]

Hi,

this is closer to what I am inquiring about. Still puzzled on how the mosfet can operate in saturation mode with the above?



9.4 Voltage followers (also called Emitter or Source follower or Common collector or drain amplifiers)
The Emitter or Source follower is often called a common Collector or Drain amplifier because the collector or drain is common to both the input and the output. This amplifier configuration, figure 9.4, has its output taken from the emitter/source resistor and is useful as an impedance matching device since its input impedance is much higher than its output impedance. The voltage follower is also termed a “buffer” for this reason.



Figure 9.4:Basic n-type Voltage follower or common collector/drain circuit (neglecting biasing details)

The gain of the voltage follower is always less than one since rEand RLor rS and RL form a voltage divider. The input to output offset is set by the VBE drop of about 0.65 volts below the base for the BJT and VGS below the gate for the MOS. This configuration’s function is not voltage gain but current or power gain and impedance matching. The input impedance is much higher than its output impedance so that a signal source does not have to supply as much power to the input. This can be seen from the fact that the base current is on the order of 100 times (?) less than the emitter current. The low output impedance of the emitter follower matches a low impedance load and buffers the signal source from that low impedance.

[grouchobyte]

Hello,

I've googled a lot to find an answer, so I am thinking I'm missing something very obvious here.

In a MOSFET voltage follower circuit like, for example:



I am wondering how one can reach saturation in a case like this? Vgs is going to be essentially the threshold voltage, right? Such that Vgs = Vt, thus Vgs-Vt = 0 ?



As I understand it, Id would be very, very low, whatever Vgs is ?

           

@mayor

Not sure of your application, unless your question is purely academic. If so, disregard my answer.

Your circuit is only linear over a 1.5 volt range and is potentially thermally unstable.
Whack an opamp in there with some feedback (I would sketch something up but I am one handed after a shoulder fracture and limited to my ipad). Anyone else care to help out here?

This way you can have linear control over a several volt range especially down below 2 volts and it would not be dependant on the mosfet Vgs vs Id characteristics or temperature.

@grouchobyte

[Delta]

Use a BJT! :-)

[danadak]

A Spice overview, attached.

Regards, Dana.