Diode - Transistor Logic (DTL)

Diode - Transistor Logic (DTL)

• The Diode - Transistor Logic, DTL is somewhat more complex than RTL but because of its greater fan-out and improved noise margins it has replaced RTL.

1. DTL Circuit

• The Fig. 2.4.1 shows a discrete circuit for DTL NAND gate. It consists of input diodes and resistor RD forming an AND gate and following them is transistor inverter.

2. Circuit Operation

• When both inputs are LOW, diode D_A and D_B conduct resulting 0.7 volts at point P. This 0.7 voltage at point is not sufficient to drive transistor Q₁. Therefore, Q₁ is cut off giving output voltage V_o = V_CC logic 1. A LOW level on any input cause corresponding diode to conduct resulting voltage at point P = 0.7 V.

• This causes transistor to remain in cut-off and the output voltage is equal Vcc = logic 1. When both inputs are logic HIGH, diodes D_A and D_B are reversed biased. This causes the base current of transistor Q₁ to flow through R_D, D₁ , D₂ and the base of the transistor Q₁ This drives transistor Q₁ in saturation giving output voltage = V_CE(sat) = 0.2 V = logic 0.

• For driving transistor Q₁ is saturation we require more than 2.1 V (V_Dl (0.7) + V_D2 (0.7) + 0.7 (V_BE) ) at point P to drive transistor in saturation. Therefore, we can say that due to diodes D1 and D2 we need increased voltage level to drive transistor in saturation. This improves the noise margin for DTL gate.

• When A and B inputs are HIGH, transistor Q1 is driven in saturation and its base to emitter junction capacitance is charged. Now if any of the input goes low, voltage at point P becomes 0.7 V and transistor Qx will try to come out of saturation. To drive transistor from saturation to cut-off it is necessary to discharge the stored charge on the internal capacitance. The resistance, RB provides a discharge path for the charged stored in the transistor. Resistor RB is connected to the - 2 V supply to increase the rate of discharge.

3. Specifications

• Table 2.4.1 gives the specifications for DTL gate

• From the above specifications we can say that the DTL has the advantage of greater fan-out and improved noise margins, but it suffers from somewhat lower speed.

4. Wire – AND Connection

• Like RTL, the DTL also has a capacity called wire-AND. In DTL two outputs can be connected together as shown in the Fig. 2.4.2.

5. Modified Integrated DTL NAND Gate

AU : May-04, 06, 07, 08, 09, 15, 16, 17,

• To increase the fanout of DTL gate the base current of Tx has to be increased. It is increased by replacing diode Dx by a transistor, as shown in the Fig. 2.4.3.

Review Questions

1. Draw and explain the operation of 2-input DTL NAND gate.

2. Write a note on DTL family.

3. Draw the circuit diagram of modified DTL NAND gate.