one TTL load one CMOS load


Joined Jul 17, 2007
TTL (Transistor-Transistor Logic), in general, has a working voltage of between 4.75 and 5.25 for Vcc; a logic 0 is generally 0.8V to 0V and a logic 1 is 2.4V to Vcc. Current for a TTL load can vary due to the myriad TTL configurations available; IE: standard, (L)ow power, H, A, and various combinations of these and additional types. However, the current is in the neighborhood of 1 to 2mA.

CMOS (Complementary Metal Oxide Semiconductor) can generally function over a much broader range of VCC; typically from 5v to 15v. The input current required is far below that of TTL, on the order of fractions of microamps, or around 1/1000 that of TTL requirements. While being more energy efficient, CMOS is comparatively slow. Review some datasheets to get some ideas.


Joined Feb 24, 2006
Voltage levels although interesting is not what the OP was asking about.

TTL inputs are the emitters of multiemitter transistors. They source current. TTL outputs have active pulldown transistors and they sink current. They have passive pullups and have very little ability to source current. One standatd TTL load is a current of 1.6 mA One LS load is a current of 0.4 mA. A TTL output that can drive 10 loads or 16 mA will do that AND maintain the proper logic thresholds. A standard TTL output will also drive 40 LS loads AND maintain the proper thresholds.

A COMS input is the gate of a FET. It is high impedance and does not source or sink any appreciable current. A CMOS input does have capacitence to ground and Vdd so that limits the speed at which an input can be changed. CMOS outputs are complementary. That means they can both sink current in the low state and source current in the high state. The more current capability the output has the faster it can charge and discharge one or more CMOS inputs.

I hope this little essay answers your question.


Joined Dec 20, 2007
Hi PapaBravo,
With only 5V for its supply, ordinary Cmos has a minimum output current of only 0.5mA so cannot drive one TTL 1.6mA input. It can drive one LS-TTL 0.4mA input.

74HCxx high-speed Cmos has a higher output current of 4ma that can drive two TTL inputs or 10 LS-TTL inputs.

A TTL or LS-TTL output voltage is too low to be a valid Cmos high input so a pullup resistor is needed.
74HCTxx high-speed Cmos have TTL-compatible inputs and don't neeed pullup resistors.

Ron H

Joined Apr 14, 2005
Audioguru! Good to see you here! You'll be a valuable contributor.
SgtWookie, have a look at this document if you think CMOS is slow relative to TTL.

Ron, AKA Roff