Microsoft Word - Digital Logic Design v_4_6a

9.2. Output Types

 Totem-Pole or Push-Pull Output
Totem-Pole output uses two complementary transistors to force the output to Vcc or ground. The
advantage is that the output is set to one value. Disadvantages are:

• Multiple outputs can not be connected together.


• Circuit is constantly using power since there is a path between Vcc and ground.

 Open Collector or Drain Output
This type of output will connect to low voltage when the output is 0, but it is not connected to anything
(High Impedance) when the output is 1. Therefore it needs a pull up resistor to make sure it is
connected to high, otherwise, it is floating resulting in an unknown value.

The advantage of this type of output is that the designer can connect multiple outputs together to

create a wired AND.

IC (TTL)

GND

Internal

Output

Vo

Vcc

IC (CMOS)

GND

Internal

Output

Vo

Vcc

p-Channel

n-Channel