Diode-transistor logic
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Image:DTL npn nand.png Diode-Transistor Logic (DTL) is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors; it is the direct ancestor of transistor-transistor logic.
A major advantage over the earlier resistor-transistor logic is the increased fanin. The propagation delay is still relatively big, mainly because the base charge having to leak out before the transistor can stop conducting. In practice this delay is often decreased by connecting the base of the transistor through a resistor to ground or a negative voltage.
With the simplified circuit shown in the picture the voltage at the base will be near 0.7 volts even when one input is held at ground level, which results in unstable or invalid operation. Two diodes in series are commonly used to lower the voltage and prevent any base current when one or more inputs are at low logic level. The IBM 1401 used DTL circuits almost identical to this simplified circuit, but solved the base bias level problem mentioned above by alternating NPN and PNP based gates operating on different power supply voltages instead of adding extra diodes.
See also: resistor-transistor logic (RTL), transistor-transistor logic (TTL), emitter coupled logic (ECL), Integrated injection logic (I2L).