Difference Between 74x138 and 74x238 Decoders

The 74x138 and 74x238 are popular 3-to-8 line decoders/demultiplexers used in digital electronics. Both ICs take 3 binary inputs and generate 8 outputs, but they differ in output type and enable configuration, which makes them suitable for slightly different applications.

Let’s dive into their basic features, pinout comparison, and differences.

✅ 74x138: 3-to-8 Decoder with Active-Low Outputs

The 74x138 is a 3-to-8 decoder/demultiplexer with active-low outputs.

Inputs: 3 (A0, A1, A2)
Enable Pins: 3 (G1 active high, G2A and G2B active low)
Outputs: 8 (Y0–Y7), all active-low
Function: Only one output goes LOW based on the input combination, while others remain HIGH.

Common Uses:

Memory address decoding
Chip select in microprocessor systems
Logic minimization circuits

✅ 74x238: 3-to-8 Decoder with Active-High Outputs

The 74x238 is also a 3-to-8 decoder/demultiplexer, but it has active-high outputs.

Inputs: 3 (A0, A1, A2)
Enable Pins: 3 (G1 active high, G2A and G2B active low)
Outputs: 8 (Y0–Y7), all active-high
Function: Only one output goes HIGH based on the input combination, while others remain LOW.

Common Uses:

Driving LEDs
Selecting one of many logic devices
Data routing and demultiplexing

Pinout Similarities

Both ICs (74x138 and 74x238) have the same pin configuration:

Inputs: A0, A1, A2
Enable: G1, G2A, G2B
Outputs: Y0–Y7
Power: Vcc and GND

⚡ Key Differences Between 74x138 and 74x238

Feature 74x138 74x238

Output Type Active-Low (only one goes LOW) Active-High (only one goes HIGH)

Default State All outputs HIGH All outputs LOW

Best for Memory decoding, chip select LED driving, logic control

Pin Count 16 pins (same as 74x238) 16 pins (same as 74x138)

Enable Pins 3 (same logic configuration) 3 (same logic configuration)

Summary

The 74x138 and 74x238 are both 3-to-8 decoders/demultiplexers.
74x138 outputs are active-low, while 74x238 outputs are active-high.
Functionally, they are identical except for output polarity, which determines their usage.
74x138 is widely used for memory decoding and chip select logic, while 74x238 is more suitable for driving indicators and active-high logic circuits.