Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
A tristate buffer could be implemented using a transmission gate. There are other ways also to do the same. A transmission gate introduces a source resistance in the path when switched on. So other methods are used when good current sourcing/sinking is required.
Transmission gates were mainly introduced to come over the effect of the ordinary pass transistor logic which caused one of the outputs (High to be VDD-Vth for NMOS or Low to be Vth for PMOS) to be weak. So, the combined effect of both was added to make the nice transmission gate which has strong output.
Transmission gates are usually used in conjunction with inverter chains to form very fast and compact logic circuits.
first the tristate buffer has its advantages over transmission gate(TG);the tristate buffer is a buffered transmission gate.
practically in design the TG has its constrains due to its bilateral nature as they conduct well in both directionsmaking the driving capability to decay.
TG has the glitch problem ( spikes),due to charge sharing,when driving a high fan out circuit.
these restrictions made the evolution of tristate buffers:
To prevent charge sharing the TG is drived by an inverting buffer which
then leaded to the standard tristate buffer circuit as in the attached picture.
This circuit has two advantages over the buffered transmission gate: the area is marginally lower, and since no current flows in the channels of the 'inverter', a change in state of the input no longer produces a current spike.
TeE ThE EdE