Resistor–transistor logic (RTL) (sometimes also transistor–resistor logic (TRL)) is a class of digital circuits built using resistors as the input network and

bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...

s (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit; it was succeeded by

diode–transistor logic Diode–transistor logic (DTL) is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function (e.g., AND) is performed by a diode network and the amplifying function ...

(DTL) and

transistor–transistor logic Transistor–transistor logic (TTL) is a logic family built from bipolar junction transistors. Its name signifies that transistors perform both the logic function (the first "transistor") and the amplifying function (the second "transistor"), as o ...

(TTL). RTL circuits were first constructed with discrete components, but in 1961 it became the first digital

logic family In computer engineering, a logic family is one of two related concepts: * A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compati ...

to be produced as a

monolithic integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny M ...

. RTL integrated circuits were used in the Apollo Guidance Computer, whose design begun in 1961 and which first flew in 1966.

Implementation

RTL inverter

A bipolar transistor switch is the simplest RTL gate (

inverter A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...

or NOT gate) implementing logical negation. It consists of a common-emitter stage with a base resistor connected between the base and the input voltage source. The role of the base resistor is to expand the very small transistor input voltage range (about 0.7 V) to the logical "1" level (about 3.5 V) by converting the input voltage into current. Its resistance is settled by a compromise: it is chosen low enough to saturate the transistor and high enough to obtain high input resistance. The role of the collector resistor is to convert the collector current into voltage; its resistance is chosen high enough to saturate the transistor and low enough to obtain low output resistance (high

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 ...

One-transistor RTL NOR gate

With two or more base resistors (R₃ and R₄) instead of one, the inverter becomes a two-input RTL

NOR gate The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0); if one or both input is HIGH (1), a LOW output (0 ...

(see the figure on the right). The logical operation OR is performed by applying consecutively the two arithmetic operations addition and comparison (the input resistor network acts as a parallel ''voltage summer'' with equally weighted inputs and the following common-emitter transistor stage as a ''voltage comparator'' with a threshold about 0.7 V). The equivalent resistance of all the resistors connected to logical "1" and the equivalent resistance of all the resistors connected to logical "0" form the two legs of a composed voltage divider driving the transistor. The base resistances and the number of the inputs are chosen (limited) so that only one logical "1" is sufficient to create base-emitter voltage exceeding the threshold and, as a result, saturating the transistor. If all the input voltages are low (logical "0"), the transistor is cut-off. The pull-down resistor R₁ biases the transistor to the appropriate on-off threshold. The output is inverted since the collector-emitter voltage of transistor Q₁ is taken as output, and is high when the inputs are low. Thus, the analog resistive network and the analog transistor stage perform the logic function NOR. RTL 3-Input NOR Gate

Multi-transistor RTL NOR gate

The limitations of the one-transistor RTL NOR gate are overcome by the multi-transistor RTL implementation. It consists of a set of parallel-connected transistor switches driven by the logic inputs (see the figure on the right). In this configuration, the inputs are completely separated and the number of inputs is limited only by the small leakage current of the cut-off transistors at output logical "1". The same idea was used later for building DCTL, ECL, some

TTL TTL may refer to: Photography * Through-the-lens metering, a camera feature * Zenit TTL, an SLR film camera named for its TTL metering capability Technology * Time to live, a computer data lifespan-limiting mechanism * Transistor–transistor lo ...

(7450, 7460), NMOS and CMOS gates.

Transistor bias

To ensure stability and predictable output of the bipolar transistors their base-inputs (V_b or base-terminal voltage) is biased.

Advantages

The primary advantage of RTL technology was that it used a minimum number of transistors. In circuits using discrete components, before integrated circuits, transistors were the most expensive component to produce. Early IC logic production (such as Fairchild's in 1961) used the same approach briefly, but quickly transitioned to higher-performance circuits such as

and then

(starting in 1963 at Sylvania Electric Products), since diodes and transistors were no more expensive than resistors in the IC.

Limitations

The disadvantage of RTL is its high power dissipation when the transistor is switched on, by current flowing in the collector and base resistors. This requires that more current be supplied to and heat be removed from RTL circuits. In contrast, TTL circuits with " totem-pole" output stage minimize both of these requirements. Another limitation of RTL is its limited

fan-in Fan-in is the number of inputs a logic gate can handle. For instance the fan-in for the AND gate shown in the figure is 3. Physical logic gates with a large fan-in tend to be slower than those with a small fan-in. This is because the complexity o ...

: 3 inputs being the limit for many circuit designs, before it completely loses usable noise immunity. It has a low

noise margin In electrical engineering, noise margin is the maximum voltage amplitude of extraneous signal that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage l ...

. Lancaster says that integrated circuit RTL NOR gates (which have one transistor per input) may be constructed with "any reasonable number" of logic inputs, and gives an example of an 8-input NOR gate. A standard integrated circuit RTL NOR gate can drive up to 3 other similar gates. Alternatively, it has enough output to drive up to 2 standard integrated circuit RTL "buffers", each of which can drive up to 25 other standard RTL NOR gates.

Speeding up RTL

Various companies applied the following speed-up methods to discrete RTL. Transistor switching speed has increased steadily from the first transistorized computers through the present. The ''GE Transistor Manual'' (7th ed., p. 181, or 3rd ed., p. 97 or intermediate editions) recommends gaining speed by using higher-frequency transistors, or capacitors, or a diode from base to collector ( parallel negative feedback) to prevent saturation. Placing a capacitor in parallel with each input resistor decreases the time needed for a driving stage to forward-bias a driven stage's base-emitter junction. Engineers and technicians use "RCTL" (resistor-capacitor-transistor logic) to designate gates equipped with "speed-up capacitors". The Lincoln Laboratory TX-0 computer's circuits included some RCTL. However, methods involving capacitors were unsuitable for integrated circuits. Using a high collector supply voltage and diode clamping decreased collector-base and wiring capacitance charging time. This arrangement required diode clamping the collector to the design logic level. This method was also applied to discrete DTL (

). Another method that was familiar in discrete-device logic circuits used a diode and a resistor, a germanium and a silicon diode, or three diodes in a negative feedback arrangement. These diode networks known as various

Baker clamp Baker clamp is a generic name for a class of electronic circuits that reduce the storage time of a switching bipolar junction transistor (BJT) by applying a nonlinear negative feedback through various kinds of diodes. The reason for slow turn-off t ...

s reduced the voltage applied to the base as the collector approached saturation. Because the transistor went less deeply into saturation, the transistor accumulated fewer stored charge carriers. Therefore, less time was required to clear stored charge during transistor turn off. A low-voltage diode arranged to prevent saturation of the transistor was applied to integrated logic families by using

Schottky diode The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...

s, as in Schottky