3.3. THE ”BUFFER” GATE45• Gate circuits with totem pole output stages are able to both source and sink current. Gatecircuits with open-collector output stages are only able to sink current, and not source current.Open-collector gates are practical when used to drive TTL gate inputs because TTL inputsdon’t require current sourcing.3.3The ”buﬀer” gateIf we were to connect two inverter gates together so that the output of one fed into the inputof another, the two inversion functions would ”cancel” each other out so that there would be noinversion from input to ﬁnal output:0100 inverted into a 1Logic state re-invertedto original statusDouble inversionWhile this may seem like a pointless thing to do, it does have practical application. Rememberthat gate circuits are signal ampliﬁers, regardless of what logic function they may perform. A weaksignal source (one that is not capable of sourcing or sinking very much current to a load) may beboosted by means of two inverters like the pair shown in the previous illustration. The logic level isunchanged, but the full current-sourcing or -sinking capabilities of the ﬁnal inverter are available todrive a load resistance if needed.For this purpose, a special logic gate called a buﬀer is manufactured to perform the same functionas two inverters. Its symbol is simply a triangle, with no inverting ”bubble” on the output terminal:InputOutput"Buffer" gateOutputInput0110The internal schematic diagram for a typical open-collector buﬀer is not much diﬀerent fromthat of a simple inverter: only one more common-emitter transistor stage is added to re-invert theoutput signal.