Diode–transistor logic

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Diode–transistor logic

Diode–transistor logic (DTL) is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function (e.g., AND) is performed by a diode network and the amplifying function is performed by a transistor (contrast this with RTL and TTL).

Implementations

Schematic of basic two-input DTL NAND gate. R3, R4 and V- shift the positive output voltage of the input DL stage below the ground (to cut off the transistor at low input voltage).

The DTL circuit shown in the picture consists of three stages: an input diode logic stage (D1, D2 and R1), an intermediate level shifting stage (R3, R4 and V-) and an output common-emitter switching-transistor stage (Q1 and R2). The two resistors R3 and R4 form a resistive summing circuit with weighted inputs that adds the negative bias voltage V- to the positive diode logic output voltage. As a result, the unipolar (positive) diode output voltage (about V+ for logical one and 1.0 V for logical zero) is converted into a bipolar voltage (a few volts above and below ground) to drive the output transistor.

The IBM 1401 (announced in 1959[1]) used DTL circuits similar to the simplified circuit, but avoided the level shifting stage (R3, R4, and V-) by alternating NPN and PNP based gates operating on different power supply voltages. The 1401 used germanium transistors and diodes in its basic gates[2]. The 1401 also added an inductor in series with R2[2][3].

In an integrated circuit version of the DTL gate, two series connected level shifting diodes replace R3. Also the bottom of R4 is connected to ground to provide bias current for the diodes and a discharge path for the transistor base. The resulting integrated circuit runs off a single power supply voltage.[4][5][6]

Speed acceleration

The DTL propagation delay is relatively large. When the transistor goes into saturation from all inputs being high, charge is stored in the base region. When it comes out of saturation (one input goes low) this charge has to be removed and will dominate the propagation time. A Baker clamp can be used to keep the transistor from saturating.

Another way to speed up DTL is to add a small capacitor across R3. The capacitor helps turn off the transistor by removing the stored base charge; the capacitor also helps turn on the transistor by increasing the initial base drive.[7]

Interfacing considerations

A major advantage over the earlier resistor–transistor logic is the increased fan-in. Alternatively, to increase fan-out of the gate, an additional transistor and diode may be used.[8]

References

1. ^ http://www.computermuseum.li/Testpage/IBM-1401.htm
2. ^ a b IBM 1401 logic. Retrieved on 2009-06-28.
3. ^ Form 223-688, IBM (1960). Form 223-6889-Transistor Component Circuits. IBM. Retrieved 2008-04-23.
4. ^ Delham, Louis A. (1968), Design and Application of Transistor Switching Circuits, Texas Instruments Electronics Series, McGraw-Hill , page 188 states resistor is replaced with one or more diodes; figure 10-43 shows 2 diodes; cites to Schulz 1962.
5. ^ Schulz, D. (August 1962), "A High Speed Diode Coupled NOR Gate", Solid State Design 1 (8): 52, OCLC 11579670
6. ^ ASIC world: "Diode Transistor Logic"
7. ^ Roehr, William D., ed. (1963), High-Speed Switching Transistor Handbook, Motorola, Inc. . Page 32 states, "As the input signal changes, the charge on the capacitor is forced into the base of the transistor. This charge can effectively cancel the transistor stored charge, resulting in a reduction of storage time. This method is very effective if the output impedance of the preceding stage is low so that the peak reverse current into the transistor is high."
8. ^ Jacob Millman, (1979). Microelectronics Digital and Analog Circuits and Systems. New York: McGraw-Hill Book Company. pp. 141–143. ISBN 007042327X.

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