Recomendation for a digital inverter made of discrete components

Question

I have a digital circuit in which I need only one logic inverter, and both PCB size and power consumption are serious constrains. So I thought about using a pair of complementary SMD MOSFETs (the classic coupled gate configuration) instead of an IC with many ready-made inverters. Which MOSFET would be more recommended in this case (both for small size and low gate-drain current)?

Answer 1

You can now get individual SMD inverters like this one, which is just one of the six gates found in a classic 74AHCT04 style package.

Answer 2

You'll never beat an IC for size with discrete components; tcrosley's link only draws a few µA and the footprint is a SOT23-5, which is only 2.1x2.4mm.

However, if simplicity or education is your goal, you can do fairly well with a simple FET inverter. With a logic-level PNP and NPN, you can make a push-pull totem-pole circuit that will have similar power requirements to an inverter IC. You can also replace the top transistor with a resistor:

This creates a simple inverter in a small space. When the input voltage is above $V_{TO}$, the output will be connected to ground through the transistor. When the input voltage is small, the transistor will be off, and the output will be pulled up to $V_{CC}$ through the resistor.

There will be power dissipation in the resistor when 'driving high' (really a passive operation for this circuit), and current will flow through the resistor when you're driving low, so make the resistor as large as your load capacitance, switching speed, and input impedance allow. This circuit can sink lots of current, but sourcing current requires a smaller resistor. If you need to source current, use a PNP and flip the design upside down. For best performance, use two transistors.

Answer 3

When I have a digital circuit and I discover I need "just one more inverter", I usually put in an entire chip -- perhaps a 74HC132 -- because usually a few minutes after I get that inverter wired up, I discover that I need yet another "just one more inverter".

Answer 4

A pair of complementary MOSFETs in the standard static CMOS inverter arrangement should work fine. There are literally hundreds of different kinds of individual transistors and transistor arrays currently on the shelf of my favorite suppliers that would work. (By "work", I mean "has a threshold Vgs is small enough that you can drive it with digital logic, rather than something that needs 6 V or more to turn on").

Randomly-picked examples:

• BSS84 p-channel FET; and 2N7002 n-channel FET, each in a discrete SOT-23 package
• Diodes Inc. DMC2004DWK-7 dual complementary MOSFET pair in a single SOT-363 package