Mosfets, source & drain, p and n channel, where to place load, I'm always mixed up...

dl324

Joined Mar 30, 2015
12,871
The first is how it's usually done.

The second can be used, but the voltage required to turn the MOSFET on depends on the voltage drop on the load. The preferred way doesn't have that problem.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
448
The first is a common-source circuit, the second is a source follower.
The first applies full supply voltage to the load, the second applies the supply voltage minus the gate-source turn-on voltage.
So a "source-follower" might have a problem turning on at low supply voltages?

I realized the following isn't a schematic, but it's what I am working with today...
If I understand, in my following pictogram, my 2n7000 is a "source-follower" and the
bs250 is a "common-source"?



1610774094181.png
 

shortbus

Joined Sep 30, 2009
8,568
Probably going to be told I'm wrong, but... The easiest way for me to remember is to look at the diode given in most drawings of the mosfet, P or N. If you look at how the intrinsic diode gets biased it will tell you the correct way to use the mosfet.
 

crutschow

Joined Mar 14, 2008
27,188
So a "source-follower" might have a problem turning on at low supply voltages?
Yes.
But the common-source also needs to have Vgs well above its threshold voltage (typically a Vgs of 10V) to fully turn on as a switch.
If you look at how the intrinsic diode gets biased it will tell you the correct way to use the mosfet.
To make sure that the intrinsic diode is always reversed biased (except in certain applications, such blocking reverse voltage to a circuit or when used as an active fly-back diode in a switch-mode power supply).
 

jpanhalt

Joined Jan 18, 2008
11,088
Horowitz and Hill describe both transistors and mosfets from the standpoint of sourcing or sinking current in their usual configuration. PNP transistors and P-channel mosfets source current; NPN transistors and N-channel mosfets sink. That doesn't mean N-channel mosfets and NPN transistors can't be used to source.

1610816247528.png

Of course, as shortbus said, the diode is a sure sign how to connect mosfets. For BJT's, the arrow always points toward ground.
 

dl324

Joined Mar 30, 2015
12,871
So a "source-follower" might have a problem turning on at low supply voltages?
With low supply voltages, you can use logic level MOSFETs that have lower threshold voltages.

I used an A03401 in a current limiting circuit and I limited the gate voltage swing to a couple volts. With that voltage range, typical on resistance at 1A is ~60 milliohms.
clipimage.jpg
I limited the voltage swing because I also wanted to be able to use an NTA4151P that had a maximum Vgs of 6V. The difference in on resistance for the A03401 at Vgs=10V wasn't significant for my circuit.
 
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