DCTL- What is Direct coupled Transistor Logic?

DCTL- What is Direct coupled Transistor Logic?

Resistance RB (like RTL) is not used to feed the base in DCTL logic family. A DCTL NOR gate is shown in the photo below. Let all the inputs be in LOW (0) state-
In this situation no current will flow in transistor T1, T2 or T3. Hence the value of output Y will be almost equal to the supply voltage (Vcc) ie HIGH. The transistors T4 and T5 of this load gate will be in saturation.

DCTL positive NOR gate
DCTL positive NOR gate

Thus when all the inputs are in LOW state, the output will be in HIGH state.

Now suppose that any one input (v1) is in HIGH (1) state. In this case transistor T1 will drive into saturation and the value of output Y will be VCE(sat) ie V(0), ie LOW. Thus this gate satisfies the NOR function.

Disadvantages of DCTL

Although DCTL circuit is simpler than RTL, DCTL is not widely used due to the drawback of current hogging. When the output is 1 in the NOR gate shown in the photo, the transistors T4 and T5 of the load gate should be driven into saturation by the NOR gate. This would not be a problem if the input characteristics of all transistors were the same.

What is Current Hogging ?

But since different packages of ICs have different tolerances at different temperatures, the input characteristics of transistors also differ. Due to which the saturation voltage of the load transistors also varies. For example, on saturation, the base emitter voltage of the transistors is 0.78, 0.79 and 0.80 V. The transistor whose Vbe value is 0.78 V, then when it comes into saturation, it will not allow other transistors to come into saturation and will itself take all the current supplied from the drive gate. This action is called current hogging.

DCTL-Direct coupled Transistor Logic Hope you liked this article.

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