ResistorTransistor

Resistortransistor is not a standard formal name in electronics; it is sometimes used to describe circuits that combine resistors and transistors, particularly the early digital logic family called resistor-transistor logic (RTL). In RTL, transistors act as switches controlled by currents through resistor networks that feed their bases. The basic gates are built by arranging resistors to provide input current and a transistor to pull a node low when the input is asserted. Outputs are typically taken from the collector or emitter via a load resistor.

RTL emerged in the late 1950s and reached prominence in the 1960s as one of the first practical methods for implementing combinational logic using discrete components before integrated circuits became prevalent.

Technology: A typical RTL gate uses an NPN transistor with several input resistors that form the input network; the base current depends on the logical inputs; when enough current flows, the transistor saturates, delivering a low on the output. Power consumption is relatively modest for its era, but speed is limited by resistor charging times and transistor switching speeds. RTL logic offered straightforward design rules and low component counts but suffered from limited fan-out, noise margins, and variability.

Applications and legacy: RTL was widely used in early computers and control circuits and influenced subsequent families such as diode-transistor logic (DTL) and transistor-transistor logic (TTL). Modern digital design relies primarily on MOS-based logic, but RTL remains a useful historical term and a descriptive shorthand for resistor-plus-transistor circuit layouts.

See also: Resistor-transistor logic, Diode-transistor logic, Transistor-transistor logic, Early integrated circuits.