CMOS output buffer with negative feedback dynamic-drive control and dual P,N active-termination transmission gates

摘要

An output buffer for a line driver uses transmission gates for active termination. A large p-channel driver is pulsed on during a low-to-high output transition, but this driver is turned off once the output voltage reaches a threshold. A feedback circuit includes a sensing inverter that has its input connected to the output node. The sensing inverter causes the gate of the p-channel driver to be driven high once the output swings past the threshold. A similar n-channel driver transistor is pulsed on during a low-going output transition but is disabled by a feedback circuit that senses the output voltage falling below a threshold. A pullup transmission gate is also connected between the output and the power supply, while a pulldown transmission gate is connected between the output and ground. Each transmission gate contains a p-channel and a n-channel transistor in parallel. The sizes of the p-channel and n-channel transistors in the transmission gate is sufficiently small to provide a resistance of 25 to 30 ohms. Both transistors in the pullup transmission gate are turned on when the output is driven high, while both transistors of the pulldown transmission gate are turned on when the output is driven low. Having both n and p transistors of the transmission gate on provides a more linear resistance across the voltage swing of the output. Since termination of about 25-30 ohms is provided by the transmission gate, an external series resistor is not needed for dampening. When driving large capacitive loads, several nanoseconds of R-C delay can be saved.