Fan-Out in Logic Families
Fan-out refers to the maximum number of standard logic gate inputs that a single output can drive without affecting proper operation.
When one logic gate sends its output to multiple other gates, each input draws a small amount of current. If too many inputs are connected, the output may fail to maintain correct logic levels.
Fan-out = Number of inputs one output can safely drive
Formula
Fan-out is calculated as:
Fan-out = Output current / Input current
There are two types:
- High-level fan-out (logic 1)
- Low-level fan-out (logic 0)
The smaller value is taken as the actual fan-out.
Example
If a gate output can supply 4 mA current and each input requires 1 mA:
Fan-out = 4 mA / 1 mA = 4
So, one output can drive 4 inputs.
Logic Family Comparison
- TTL (Transistor-Transistor Logic): Fan-out ≈ 10
- CMOS (Complementary MOS): Fan-out is higher (often 50+)
Why Fan-Out Matters
- Prevents signal degradation
- Ensures correct voltage levels
- Avoids timing issues