TTL Output Voh Clamp
Original Publication Date: 1988-Jan-01
Included in the Prior Art Database: 2005-Feb-14
Background: The nominal output voltage swing of a modern Schottky TTL output device is from .2 volts to 3.6 volts. The output high level voltage (3.6 volts) is controlled by a Darlington output transistor in a totem pole output stage of a TTL circuit. The Darlington turns on when the emitter-base junctions are forward biased (1.4 volts). The output terminal of the TTL circuit is connected to the emitter of the Darlington transistor. The base of the Darlington is connected to a resistor which is connected to Vcc, +5.0 volts. The Darlington will conduct as long as the output node is approximately 1.4 volts less than the base of the device. As the output nears the final high voltage, the base current to the Darlington approaches zero.