JNTU B.Tech III IC APPLICATIONS Previous Question Paper (September - 2021 - R13)

A standard TTL NAND gate consists of four stages: the input stage (multi-emitter transistor), the phase splitter, and the totem-pole output stage. The multi-emitter transistor acts as an AND gate at the input. When both inputs are at logic HIGH, the base-collector junction of the input transistor is forward-biased, turning on the phase splitter transistor. This drives the lower output transistor into saturation, pulling the output LOW. When either input is LOW, the input transistor conducts, starving the phase splitter of base current, which turns off the lower output transistor and turns on the upper Darlington pair, pulling the output HIGH.

An open drain configuration refers to a MOSFET output where the drain terminal is left unconnected (floating) internally. Unlike a standard push-pull output, the device can only pull the signal line to ground (logic LOW) or leave it in a high-impedance state (Hi-Z). To achieve a logic HIGH state, an external pull-up resistor must be connected between the drain and the supply voltage.

This configuration is primarily used for:

• Wired-AND logic: Multiple open-drain outputs can be tied together to a single pull-up resistor, allowing any device to pull the bus LOW.
• Level Shifting: It allows a device operating at one voltage level to interface with a bus operating at a different voltage by choosing an appropriate pull-up supply.
• Bus Protocols: It is the standard for buses like I2C, where multiple devices must share a single line without causing contention.