I need to implement a simple logic inverter..
iv found here a simple one using n-channel and p-channel mosfets, but is there an easier way?

 

CD4049, CD4069, CD40106 for voltage up to 15V, 74HC14 for voltages up to 5V

 

i need a small chip.. i made a hardware error on a project, i can fix with one mosfet, but im trying to find extremely small (passive?) solution.. so no ics

 

One NFET and one pullup resistor on its output (Drain) is all you need.

 

so.. my picture, withthe pchannel replaced by a resistor?
and is that a weak or strong pull up?

 

I'm not going to another site to see your picture.

You need a NFET;
The input is the Gate (and is driven from your original logic output).
The Drain is the output and needs a pullup resistor to Vdd.
The source is ground (Vss).

The pullup resistor depends on the speed you need it to work at, if it works at higher frequencies the resistor should be smaller. Generally 470ohm or 1k will work for most apps.

 

ah yeh ok (note to self post pictures not links) i tell other people to, dunno why i didnt think of it..
yeh i get the circuit now, just wanted to make sure it was how i thought..
ill post the image up for any googlers or future readers.

 

2N2222 NPN transistor, 1KΩ resistor in series with the base and 10KΩ pull-up load resistor at the collector.

 

Not all inverters are created equal. The application may dictate the design. What will it be used for?

 

just to invert the logic for a mosfet switch =] sadly i cant change the polarity of the trigger, it is active high, and my circuits switch is active low.

 

just to invert the logic for a mosfet switch =] sadly i cant change the polarity of the trigger, it is active high, and my circuits switch is active low.

I hope you are going to put it in front of the driver.
What is the MOSFET driver? Is it an IC, or is it discrete? Give us a part number, or a schematic (preferred).
How fast does it have to switch?
What is the minimum pulse width?

 

I hope you are going to put it in front of the driver.
What is the MOSFET driver? Is it an IC, or is it discrete? Give us a part number, or a schematic (preferred).
How fast does it have to switch?
What is the minimum pulse width?

sorry "in front of the driver"?
no sorry when i said mosfet switch.. its a p-channel mosfet, acting as a switch, its a bodge to correct a prototype circuit, but the pin it would be driven by is active high auto controlled by the pic32 im using.
it doesnt have to switch fast, its just an on/off style switch.. power on, power off.. i just needed to invert the polarity to make sure it worked correctly.

 

sorry "in front of the driver"?
no sorry when i said mosfet switch.. its a p-channel mosfet, acting as a switch, its a bodge to correct a prototype circuit, but the pin it would be driven by is active high auto controlled by the pic32 im using.
it doesnt have to switch fast, its just an on/off style switch.. power on, power off.. i just needed to invert the polarity to make sure it worked correctly.

Why can't you change the polarity coming out of the PIC32? Don't you have control of the software?
I hope you are using a logic level MOSFET. Otherwise, it won't work.

 

its a USB library function controlling the bus voltage
and sorry probably a terminology thing.. "logic level mosfet" i.e. triggerable by logic levels?

 

its a USB library function controlling the bus voltage
and sorry probably a terminology thing.. "logic level mosfet" i.e. triggerable by logic levels?

Yes. Rds(on) needs to be specified at or below 5V Vgs. If it isn't the MOSFET may turn on weakly, and the on resistance may be too high

 

4.5V phew! id only looked at the threshold voltage..

 

4.5V phew! id only looked at the threshold voltage..

The threshold voltage spec for a Mosfet is when it is almost turned off. Don't you want it to be turned on very well?

 

The threshold voltage spec for a Mosfet is when it is almost turned off. Don't you want it to be turned on very well?

I think he means that the device is spec'ed for Rds(on) at Vgs=-4.5V, i.e., it's a logic level device.

 

He said he looked at the "off" threshold voltage.

An IRL540 has a threshold voltage of 1V to 2V when it is almost turned off with a current of only 0.25mA. Its is spec'd to be turned on very well with a max on-resistance of only 0.058 ohms when its gate voltage is 5.0V.

 

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I like the MOSFET idea, I would have gone with the classic common emitter, two resistors and a BJT.