Resistor–transistor logic (RTL) is a class of digital circuit

Digital circuit

Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...

s built using resistor

Resistor

A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

s as the input network and bipolar junction transistor

Bipolar junction transistor

|- align = "center"| || PNP|- align = "center"| || NPNA 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...

s (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and transistor–transistor logic (TTL).
A bipolar transistor switch is the simplest RTL gate (inverter

Inverter (logic gate)

In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. The truth table is shown on the right.This represents perfect switching behavior, which is the defining assumption in Digital electronics. In practice, actual devices have electrical characteristics that...

 or NOT gate) implementing logical negation.