Mosfet as a level shifter. I don't understand - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: November 12, 2021, 12:55:59 pm
content: Hi, today I watchedexplaining how to use a single mosfet as a logic level shifter basically.What I'll be referring to appears after around 1:50 minutes on the video. I'll try and capture what I find puzzling.The circuit is this.First of all I do understand that when both sides are high, regardless of who is the input and who is the output, the other side is high too, due to the pullups.The problem is with low (0v). Assuming left side (3.3v) as output and right side as input, Vgs=0: how the hell does the mosfet manage to pull down the right side?Actually the same goes for the other way around, the left side, which controls the gate, has it pulled up to 3.3v, and the source voltage is 3.3v too: Vgs=0, how the hell this gets forced to 0v?of course I understand that I'm missing something big on mosfets... what is it plz?Thanks

author: Psi
author_rank: Super Contributor
timestamp: November 12, 2021, 01:04:35 pm
content: My brain isn't working atm, it's 2AM here.but its probably something to do with the body diode conducting and the fact that a mosfet will conduct in both directions when on.If the 5V side goes low, then GND is lower than 3.3V so the body diode conducts and pulls the 3.3V side low?

author: bitwelder
author_rank: Super Contributor
timestamp: November 12, 2021, 01:05:41 pm
content: Didn't watch what the video says, but check the brief explanation of exactly this circuit here:http://www.penguintutor.com/electronics/mosfet-levelshift

author: dmills
author_rank: Super Contributor
timestamp: November 12, 2021, 01:08:03 pm
content: The mosfet is configured as a common gate amplifier, with the gate held at 3.3V all the time.When the right hand side goes low, the body diode conducts and starts to pull the left hand side down, as soon as the left hand side drops Vgs(on) volts below the gate voltage, the mosfet proper conducts and takes the voltage the rest of the way down.In the other direction, just pulling the source down makes the mosfet conduct (since the gate is above the source) and in this mode it is simply a cascode stage.HTH.

author: ledtester
author_rank: Super Contributor
timestamp: November 12, 2021, 01:12:49 pm
content: This explanation might help:(Go to the section "Testing your Logic Converter" in the page)https://circuitdigest.com/tutorial/bi-directional-logic-level-controller-using-mosfet#:~:text=Testing%20your%20Logic%20ConverterWhat it looks like to me is:When SW1 is pressed the MOSFET is turned on allowing current to flow through R2 and thus lowering the voltage at multimeter 2.When SW2 is pressed current flows through R1 and the body diode lowering the voltage at multimeter 1.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 12, 2021, 01:20:27 pm
content: The mosfet acts the same as in any other circuit.First you have the body diode that either blocks or conducts.The other part is that when there is a voltage between gate and source, then there is a low impedance path between drain and source.The mosfet does not connect anything to GND here, because none of the mosfet terminals is connected to GND. The only thing it can do is create a low impedance connection between the left and the right side.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 12, 2021, 03:21:05 pm
content: A most accurate description. Perhaps turning the MOSFET -45 ashttp://www.penguintutor.com/electronics/mosfet-levelshiftshows helps clarify the mode of operation

author: Moriambar
author_rank: Unknown
timestamp: November 12, 2021, 03:36:30 pm
content: Thanks!I get it now

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2021, 04:50:32 pm
content: The circuit will also work with a BJT. I recently did a simulation of it. C1 just speeds it up and is optional.Level shift bi-direc BJT.asc