Diode–Transistor Logic (
DTL) is a class of
digital circuitDigital electronics are systems that represent signals as discrete levels, rather than as a continuous range. In most cases the number of states is two, and these states are represented by two voltage levels: one near to zero volts and one at a higher level depending on the supply voltage in use...
s built from
bipolar junction transistorA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes...
s (BJT),
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
s and
resistor|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable
resistor|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"...
s; it is the direct ancestor of transistor–transistor logic. It is called
diode–transistor logic 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).
With the simplified circuit shown in the picture the negative bias voltage at the base is required to prevent unstable or invalid operation.
Diode–Transistor Logic (
DTL) is a class of
digital circuitDigital electronics are systems that represent signals as discrete levels, rather than as a continuous range. In most cases the number of states is two, and these states are represented by two voltage levels: one near to zero volts and one at a higher level depending on the supply voltage in use...
s built from
bipolar junction transistorA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes...
s (BJT),
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
s and
resistor|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable
resistor|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"...
s; it is the direct ancestor of transistor–transistor logic. It is called
diode–transistor logic 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).
Operation
With the simplified circuit shown in the picture the negative bias voltage at the base is required to prevent unstable or invalid operation. In an integrated circuit version of the gate, two diodes replace R3 to prevent any base current when one or more inputs are at low logic level. Alternatively to increase fan-out of the gate an additional transistor and diode may be used. 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.
Speed disadvantage
A major advantage over the earlier resistor–transistor logic is the increased
fan-inFan-in is the number of inputs of an electronic logic gate. For instance the 'fan-in' for the AND gate shown below is 3. Logic gates with a large fan-in tend to be slower than those with a small fan-in, because the complexity of the input circuitry increases the input capacitance of the device.A...
. However, the
propagation delayPropagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics. In general it is the length of time taken for the quantity of interest to reach its destination.- Networking :...
is still 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. One way to speed it up is to connect a resistor to a negative voltage at the base of the transistor which aids the removal of the minority carriers from the base.
The above problem is solved in TTL by replacing the diodes of the DTL circuit with a multiple-emitter transistor, which also slightly reduces the required area per gate in an
integrated circuitIn electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin substrate of semiconductor material...
implementation.
CTDL
Another way to speed up DTL is to add a capacitor across R3, and a small inductor in series with R2. This technique was used in the IBM 1401, where it was called CTDL (complemented transistor diode logic).
See also
- Diode logic
Diode logic or Diode-resistor logic, is a circuit style that uses diodes to construct Boolean logic gates for circuits. Only non-inverting functions may be implemented, so it is not a complete logic family. Furthermore, such circuits are non-restoring: the output of any gate is further from a...
(DL)
- Resistor–transistor logic (RTL)
- Transistor–transistor logic (TTL)
- Emitter-coupled logic (ECL)
- Integrated injection logic
Integrated injection logic is a class of digital circuits built with multiple collector bipolar junction transistors . When introduced it had speed comparable to TTL yet was almost as low power as CMOS, making it ideal for use in VLSI integrated circuits...
(I2L)