Thus, a “high” input gives a “high” output.
The output terminal will be floating (neither connected to ground nor Vcc), and this will be equivalent to a “high” state on the input of the next TTL gate that this one feeds in to. Thus, Q4 will be in cutoff mode, conducting no current. Having Q2 saturated causes Q3 to be saturated as well, resulting in very little voltage dropped between the base and emitter of the final output transistor Q4. All of R1‘s current goes through the base of transistor Q2, saturating it: Let’s analyze this circuit for two conditions: an input logic level of “1” and an input logic level of “0.” First, a “high” (1) input:Īs before with the inverter circuit, the “high” input causes no conduction through the left steering diode of Q1 (emitter-to-base PN junction). The internal schematic diagram for a typical open-collector buffer is not much different from that of a simple inverter: only one more common-emitter transistor stage is added to re-invert the output signal. Its symbol is simply a triangle, with no inverting “bubble” on the output terminal:īuffer Circuit with Open-Collector Output
The logic level is unchanged, but the full current-sourcing or sinking capabilities of the final inverter are available to drive a load resistance if needed.įor this purpose, a special logic gate called a buffer is manufactured to perform the same function as two inverters.
Remember that gate circuits are signal amplifiers, regardless of what logic function they may perform.Ī weak signal source (one that is not capable of sourcing or sinking very much current to a load) may be boosted by means of two inverters like the pair shown in the previous illustration. While this may seem like a pointless thing to do, it does have practical application. If we were to connect two inverter gates together so that the output of one fed into the input of another, the two inversion functions would “cancel” each other out so that there would be no inversion from input to final output:
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