Resistor–transistor logic – Wikipedia

Resistor–transistor logic (RTL) (typically additionally transistor–resistor logic (TRL)) is a category of digital circuits constructed utilizing resistors because the enter community and bipolar junction transistors (BJTs) as switching gadgets. RTL is the earliest class of transistorized digital logic circuit; it was succeeded by diode–transistor logic (DTL) and transistor–transistor logic (TTL).

RTL circuits have been first constructed with discrete components, however in 1961 it turned the primary digital logic family to be produced as a monolithic integrated circuit. RTL built-in circuits have been used within the Apollo Guidance Computer, whose design begun in 1961 and which first flew in 1966.^[1]

Implementation[edit]

RTL inverter[edit]

A bipolar transistor switch is the best RTL gate (inverter or NOT gate) implementing logical negation.^[2] It consists of a common-emitter stage with a base resistor linked between the bottom and the enter voltage supply. The function of the bottom resistor is to develop the very small transistor enter voltage vary (about 0.7 V) to the logical “1” stage (about 3.5 V) by changing the enter voltage into present. Its resistance is settled by a compromise: it’s chosen low sufficient to saturate the transistor and excessive sufficient to acquire excessive enter resistance. The function of the collector resistor is to transform the collector present into voltage; its resistance is chosen excessive sufficient to saturate the transistor and low sufficient to acquire low output resistance (excessive fan-out).

One-transistor RTL NOR gate[edit]

Schematic of a one-transistor RTL NOR gate.

With two or extra base resistors (R₃ and R₄) as a substitute of 1, the inverter turns into a two-input RTL NOR gate (see the determine on the fitting). The logical operation OR is carried out by making use of consecutively the 2 arithmetic operations addition and comparison (the enter resistor community acts as a parallel voltage summer time with equally weighted inputs and the next common-emitter transistor stage as a voltage comparator with a threshold about 0.7 V). The equal resistance of all of the resistors linked to logical “1” and the equal resistance of all of the resistors linked to logical “0” kind the 2 legs of a composed voltage divider driving the transistor. The bottom resistances and the variety of the inputs are chosen (restricted) in order that just one logical “1” is ample to create base-emitter voltage exceeding the brink and, consequently, saturating the transistor. If all of the enter voltages are low (logical “0”), the transistor is cut-off. The pull-down resistor R₁ biases the transistor to the suitable on-off threshold. The output is inverted for the reason that collector-emitter voltage of transistor Q₁ is taken as output, and is excessive when the inputs are low. Thus, the analog resistive community and the analog transistor stage carry out the logic operate NOR.^[3]

Multi-transistor RTL NOR gate[edit]

Schematic of a multi-transistor RTL NOR gate, as used within the Apollo Guidance Computer built-in circuits.^[4]

{Photograph} of the twin 3-input NOR gate chip used to construct the Apollo Guidance Computer. Connections (clockwise from prime heart) floor, inputs (3), output, energy (V_cc), output, inputs (3). The six transistors (two teams of three) are within the heart. The skinny wires from the terminals to the transistors are resistors.

Flatpack RTL NOR gate built-in circuits within the Apollo guidance computer

The restrictions 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 pushed by the logic inputs (see the determine on the fitting). On this configuration, the inputs are fully separated and the variety of inputs is proscribed solely by the small leakage present of the cut-off transistors at output logical “1”. The identical concept was used later for constructing DCTL, ECL, some TTL (7450, 7460), NMOS and CMOS gates.

Transistor bias[edit]

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

Benefits[edit]

The first benefit of RTL know-how was that it used a minimal variety of transistors. In circuits utilizing discrete elements, earlier than built-in circuits, transistors have been the costliest element to provide. Early IC logic manufacturing (akin to Fairchild’s in 1961) used the identical method briefly, however shortly transitioned to higher-performance circuits akin to diode–transistor logic after which transistor–transistor logic (beginning in 1963 at Sylvania Electric Products), since diodes and transistors have been no dearer than resistors within the IC.^[5]

Limitations[edit]

The drawback of RTL is its excessive energy dissipation when the transistor is switched on, by present flowing within the collector and base resistors. This requires that extra present be provided to and warmth be faraway from RTL circuits. In distinction, TTL circuits with “totem-pole” output stage decrease each of those necessities.

One other limitation of RTL is its restricted fan-in: 3 inputs being the restrict for a lot of circuit designs, earlier than it fully loses usable noise immunity.^{[citation needed]} It has a low noise margin. Lancaster says that built-in circuit RTL NOR gates (which have one transistor per enter) could also be constructed with “any cheap quantity” of logic inputs, and offers an instance of an 8-input NOR gate.^[6]

An ordinary built-in circuit RTL NOR gate can drive as much as 3 different comparable gates.
Alternatively, it has sufficient output to drive as much as 2 commonplace built-in circuit RTL “buffers”, every of which may drive as much as 25 different commonplace RTL NOR gates.^[6]

Dashing up RTL[edit]

Varied firms utilized the next speed-up strategies to discrete RTL.

Transistor switching pace has elevated steadily from the primary transistorized computer systems by the current. The GE Transistor Handbook (seventh ed., p. 181, or third ed., p. 97 or intermediate editions) recommends gaining pace through the use of higher-frequency transistors, or capacitors, or a diode from base to collector (parallel negative feedback) to stop saturation.^[7]

Inserting a capacitor in parallel with every enter resistor decreases the time wanted for a driving stage to forward-bias a pushed stage’s base-emitter junction. Engineers and technicians use “RCTL” (resistor-capacitor-transistor logic) to designate gates outfitted with “speed-up capacitors”. The Lincoln Laboratory TX-0 computer’s circuits included some RCTL.^[8] Nonetheless, strategies involving capacitors have been unsuitable for built-in circuits.^{[citation needed]}

Utilizing a excessive collector provide voltage and diode clamping decreased collector-base and wiring capacitance charging time. This association required diode clamping the collector to the design logic stage. This technique was additionally utilized to discrete DTL (diode–transistor logic).^[9]

One other technique that was acquainted in discrete-device logic circuits used a diode and a resistor, a germanium and a silicon diode, or three diodes in a detrimental suggestions association. These diode networks generally known as varied Baker clamps lowered the voltage utilized to the bottom because the collector approached saturation. As a result of the transistor went much less deeply into saturation, the transistor accrued fewer saved cost carriers. Due to this fact, much less time was required to clear saved cost throughout transistor flip off.^[7] A low-voltage diode organized to stop saturation of the transistor was utilized to built-in logic households through the use of Schottky diodes, as in Schottky TTL.

See additionally[edit]

References[edit]

Additional studying[edit]

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