# Single logic gates--alternatives to ICs?

#### autorelease

Joined Jan 26, 2009
39
In my circuit I need a single OR gate. I don't want to use a quad-OR-gate IC because it would take up too much space. However, I'd still like performance (low propagation delay, low current draw) similar to a 74HC series chip.

Would I be good with an OR gate made from two diodes and a resistor? What kind of diodes would be fast enough? The voltage drop shouldn't be an issue, I'm using CMOS logic everywhere else so 4.3V should be properly recognized as high. Are there better alternatives when a single logic gate is needed?

#### KL7AJ

Joined Nov 4, 2008
2,208
In my circuit I need a single OR gate. I don't want to use a quad-OR-gate IC because it would take up too much space. However, I'd still like performance (low propagation delay, low current draw) similar to a 74HC series chip.

Would I be good with an OR gate made from two diodes and a resistor? What kind of diodes would be fast enough? The voltage drop shouldn't be an issue, I'm using CMOS logic everywhere else so 4.3V should be properly recognized as high. Are there better alternatives when a single logic gate is needed?
It depends on the application. What sort of switching speed do you need?

eric

#### Metalfan1185

Joined Sep 12, 2008
161
I have seen Single Logic Gates that come in a Surface mount 3 pin package, i cant remember what the part number was, but i do remember the sheet saying that one of these little 3 legged things was a NAND Gate, (as in 1/4 of a 7400). I remember there was other gates available as well.

correction, that is a 5 pin package, check out this part number:
74LX1G03

i would imagine that there should be something available for this with the OR function.

hope it helps

#### SgtWookie

Joined Jul 17, 2007
22,201
You could use a pair of diodes (1N914 or 1N4148) and a resistor to create an OR or AND gate.
http://en.wikipedia.org/wiki/Diode_logic

If you want a low Vf (which is a good thing) and a small package, you could use a BAT54C for an OR gate, and a BAT54A for an AND gate. Both come in SOT-23 packages.

Switching times are <= 5nS.

Don't use 1N400x or 1N540x series or other power switching diodes; they're much too slow and have too much capacitance.

#### beenthere

Joined Apr 20, 2004
15,808
Fairchild makes a line of single gates, called TinyLogic. The single two input OR is a NC7S32M5XCT from Digi-Key (their part number). It's in SOT-23-5 for $0.28. There's an ultra high speed device, the NC7SZ32M5XCT, also$0.28

#### Wendy

Joined Mar 24, 2008
21,840
I do it all the time, there are times for ICs, but if all you need is a single gate...

You can also speed things up by using Schottky diodes, which are inherently high speed.

#### autorelease

Joined Jan 26, 2009
39
A switching time of 10 nanoseconds or less would be great. The 1N914 datasheet indicates a "reverse recovery time" of 4 nanoseconds. I'd assume this is the time it takes for the output to change when the diode switches from reverse to forward bias. (or vice versa?)

This is a through-hole application, so unfortunately surface-mount components won't really work for me.

#### SgtWookie

Joined Jul 17, 2007
22,201
The 1N914 datasheet indicates a "reverse recovery time" of 4 nanoseconds. I'd assume this is the time it takes for the output to change when the diode switches from reverse to forward bias. (or vice versa?)
When switching from the conducting to the blocking state, a diode or rectifier has stored charge that must first be discharged before the diode blocks reverse current. This discharge takes a finite amount of time known as the Reverse Recovery Time, or trr. During this time, diode current may flow in the reverse direction.
This is a through-hole application, so unfortunately surface-mount components won't really work for me.
You can keep a pretty small footprint if your vertical clearance isn't limited.