Direct-coupled transistor logic (DCTL) is similar to resistor–transistor logic (RTL) but the input

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

bases are connected directly to the collector outputs without any base resistors. Consequently, DCTL gates have fewer components, are more economical, and are simpler to fabricate onto integrated circuits than RTL gates. Unfortunately, DCTL has much smaller signal levels, has more susceptibility to ground noise, and requires matched transistor characteristics. The transistors are also heavily overdriven; that is a good feature in that it reduces the saturation voltage of the output transistors, but it also slows the circuit down due to a high stored charge in the base. Gate

fan-out In digital electronics, the fan-out is the number of gate inputs driven by the output of another single logic gate. In most designs, logic gates are connected to form more complex circuits. While no logic gate input can be fed by more than one ...

is limited due to "current hogging": if the transistor base-emitter voltages () are not well matched, then the base-emitter junction of one transistor may conduct most of the input drive current at such a low base-emitter voltage that other input transistors fail to turn on. DCTL is close to the simplest possible digital logic family, using close to fewest possible components per logical element. A similar logic family, direct-coupled transistor-transistor logic, is faster than ECL. John T. Wallmark and Sanford M. Marcus described direct-coupled transistor logic using

JFET The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifier ...

s. It was termed direct-coupled unipolar transistor logic (DCUTL). They published a variety of complex logic functions implemented as integrated circuits using

s, including complementary memory circuits.

References

* Digital electronics Logic families {{electronics-stub