Direct-coupled transistor logic

Direct-coupled transistor logic (DCTL) is similar to <a href="/facts/Resistor%25E2%2580%2593transistor_logic/V9JGuspy">resistor–transistor logic</a> (RTL), but the input <a href="/facts/Transistor/Bmr8zwL0">transistor</a> bases are connected directly to the collector outputs without any base <a href="/facts/Resistor/JrsGReM6">resistors</a>. Consequently, DCTL <a href="/facts/Gate/1pmqQj8G">gates</a> have fewer components, are more economical, and are simpler to fabricate onto <a href="/facts/Integrated_circuit/fjn6vg7C">integrated circuits</a> 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; this 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 <a href="/facts/Fan-out/lCm5joyw">fan-out</a> is limited due to "current hogging": if the transistor base–emitter voltages (VBE) 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 <a href="/facts/Emitter-coupled_logic/wAN1uEgH">ECL</a>.
<a href="/facts/J._Torkel_Wallmark/Wz12hV1K">John T. Wallmark</a> and Sanford M. Marcus described direct-coupled transistor logic using <a href="/facts/JFET/oo5QxmTK">JFETs</a>. It was termed direct-coupled unipolar transistor logic (DCUTL). They published a variety of complex logic functions implemented as integrated circuits using <a href="/facts/JFET/oo5QxmTK">JFETs</a>, including complementary memory circuits.

Direct-coupled transistor logic open-in-new

Direct-coupled transistor logic