Logic Gates

Resistor-Transistor Logic (RTL)

Resistor-transistor logic (RTL) is constructed from resistors and transistors.

One-Transistor RTL Gate

The figure below shows a simple 3-input RTL circuit which is a NOR gate. The circuit has each input combined through a coupling resistor (R₁, R₂, or R₃) and fed to the base of Q₁. The coupling resistors provide isolation that is less effective than that provided by transistors or diodes. This isolation makes the RTL sensitive to the value of the coupling resistors and the number of inputs (fan-in).

The operation of the circuit is quite simple. When all three inputs are zero (i.e., at ground level), the voltage V_B at the base of the transistor Q₁ is slightly more negative than ground since the base is tied to a negative voltage, -V_BB through resistor R₄. Q₁ is cut off and the output voltage V₀ tends to fall to the supply voltage +V_CC. If any of the three inputs are positive, the current flow through its corresponding resistor pulls the voltage V_B to some positive value. This drives Q₁ into saturation. Here, the output voltage V₀ is determined by the saturation voltage V_CE(SAT) of Q₁.

Multi-Transistor RTL Gate

The multi-transistor configuration is shown in the figure below. The configuration is the same as that of the corresponding DCTL NOR circuit, except for the addition of an input resistor, R₂. The addition of R₂ forces some of the input voltage to appear across R₂, and the base currents are more evenly divided between parallel inputs. By increasing R₂, high fan-out is achieved with no current-hogging; however, as R₂ is increased, the switching speed of the circuit is reduced. A design compromise must be made between high fan-out and high switching speed when using an RTL circuit.