Creating CMOS inverter

Discussion in 'General Electronics Chat' started by surfline, Jan 28, 2010.

  1. surfline

    Thread Starter Active Member

    Aug 12, 2009
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    Hi,

    I want to build a CMOS inverter circuit (1 pmos on top, 1 nmos on bottom) on a breadboard. Is this possible to do with an nmos and a pmos you would buy from mouser or digikey? Or do you have to buy a special IC to use it? Can you find an NMOS and a PMOS that have the same parameters that would allow it to work? Further, I want a decent amount of current to flow through these, so could I make this inverter from typical power mosfets ?
     
  2. lambedan

    New Member

    Oct 21, 2009
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    What kind of voltage and current do you plan on working with?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is a quick sim of the idea. Note the "shoot-through" current (IDm2) as a function of Vdd. You would have to design a "stabilized bias" network similar to what is used in a class AB audio amplifier to control the quiescent current for supply voltages above ~4V.
     
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  4. surfline

    Thread Starter Active Member

    Aug 12, 2009
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    5 volt pulsed input at the gates, and the inverter wouldn't be driving more than a 1 amp load.
     
  5. MikeML

    AAC Fanatic!

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    That still leaves you with a shoot-through current of 24A (purple trace) as the inverter swings through the center point...
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Other than fabrication of the base MOSFET transistors, how does CMOS chips do it?

    To the OP:

    Give us better information as to the load. We can help you make drivers that won't have as much shoot through. It is a problem with any circuit that can go rail to rail.

    With the right drivers you can even use two N-ch MOSFETs to do the same thing.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Answer is: it is not possible to completely eliminate shoot-through. In CMOS processes, the Vth of the P and the N devices are carefully controlled, and are tailored to the nominal Vdd voltage for the process, and the W/L of the transistors are also chosen in part to reduce it. Every CMOS gate exhibits it to some extent. The total current of the chip is a function of how many gates are switching per unit time. The shoot-through is minimized by having internal nodes slew through the mid-point as fast as possible. Lower voltage processes ~1.5V minimize it because the sum of VthP and VthN can be greater than Vdd.
     
  8. surfline

    Thread Starter Active Member

    Aug 12, 2009
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    By shoot through, I'm assuming you're talking about the time when the NMOS and the PMOS are both on? That's why, my original question was asking are there any good NMOS & PMOS transistor combo's that have pretty identical Vt's.

    Specifically, I'm trying to use the inverter as a driver from A PWM output from PIC micro controller (0-5V PWM, 25mA max output) which is going to drive 4 power mosfet's. The four power mosfets are turning on / off 30 amp load. I shouldn't need more than one amp from the driver to charge the four power mosfets gate capacitance's.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Mosfets have thousands of paralleled "cells". Each cell is a tiny Mosfet.
    Cmos ICs are made of individual Mosfet Cells that have a low current. When boith the N-mos and P-mos transistors are turned on then the shoot through current is not high.
     
  10. surfline

    Thread Starter Active Member

    Aug 12, 2009
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    What about if you're not using an IC?
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Then you can use separate Mosfets that contain thousands of Mosfet cells so that they conduct 40A or more when they are both turned on.
    A Cmos IC draws maybe 20mA when both Mosfets are turned on. A huge difference.
     
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