TOTEM POLE OUTPUT FOR TTL LOGIC FAMILIES CODE
Low-power Schottky TTL designed as 74LS or 54LSĪdvanced low-power Schottky TTL designed as 74ALS or 54ALSĪdvanced Schottky TTL designed as 74AS or 54ASĪn alphabetic code preceding this indicates the name of the manufacturer. The totem-pole output stage for TTL, shown in Figure. In this way, we could get quick charging and very low power dissipation. Ideally we would like to have a very low resistance pull-up when the output is high, but a very high pull-up resistance when the output is low. The problem arises when the output is high and the pull-up resistor is too large. If we look at the circuit, we observe that when the transistor is saturated, it presents a very low effective resistance to ground. One way to improve the rise time is to reduce the resistance value as is often done, but this also increases the power dissipation when the output is low. One of the problems with the TTL gate circuit is that the pull-up resistor on the output transistor will prevent rapid charging of any wiring capacitance on the output.
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The collector of Q2 is HIGH and turns Q3 into saturation. Hence there is no current into the base of Q2 and making it into cur-off. Then there is a current from Vcc through R1 ti the base emitter junction of Q1 and into the LOW input, which provides a path to the ground for the current. If A or B is low, the base-emitter junction of Q1 is forward biased and its base-collector junction is reverse biased. the output transistors Q3 and Q4 form a totem-pole output arrangement.
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For the most part, we will consider the input transistor, act just like two diodes.Ī two input standard TTL NAND gate is a multiple emitter transistor for the inputs A and B. The analysis of this circuit follows very much the same path as the analysis of the DTL gate. Later on, we will make additional modifications to this circuit to improve its performance further. The input diodes are replaced by the multi-emitter NPN transistor. The p-n junction of diode is replaced by the BE junction of transistor and with the current gain of the transistor, the current going into the base of transistor is greatly increased, increasing the fanout. For example, the diode in the DTL can be replaced by a transistor whose collector is pulled up to the power supply. The evolution from Diode transistor Logic to transistor transistor Logic can be seen by observing the placement of p-n junctions.