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Resistor-transistor logic

Resistor-transistor logic

Overview
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.
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Encyclopedia
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).

RTL inverter


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. It consists of a common-emitter stage
Common emitter
In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor amplifier topologies, typically used as a voltage amplifier...

 with a base resistor connected between the base and the input voltage source. The role of the base resistor is to expand the negligible 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).

One-transistor RTL NOR gate


By connecting additional base resistors (R3 and R4) to the inverter it is expanded to the simplest RTL NOR gate
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 results if both the inputs to the gate are LOW . If one or both input is HIGH , a LOW output results. NOR is the result of the negation of the OR operator...

 (see the figure on the right). It is interesting fact that the basic input logical operation OR
Logical disjunction
In logic and mathematics, a two-place logical connective or, is a logical disjunction, also known as inclusive disjunction or alternation, that results in true whenever one or more of its operands are true. E.g. in this context, "A or B" is true if A is true, or if B is true, or if both A and B are...

 is performed by applying consecutively the two arithmetic operations addition
Addition
Addition is a mathematical operation that represents combining collections of objects together into a larger collection. It is signified by the plus sign . For example, in the picture on the right, there are 3 + 2 apples—meaning three apples and two other apples—which is the same as five apples....

 and comparison (the input resistor network acts as a parallel voltage summer with equally weighted inputs and the next 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 provides reliable cut-off of the transistor (it is not absolutely necessary in the case of a silicon transistor). The output is inverted since the voltage drop across the collector-emitter junction of the transistor Q1 is taken as a grounded output instead the voltage drop across the floating collector resistor R2.
Thus, the analog resistive network and the analog transistor stage perform the logic function NOR.

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 reverse saturation current of the cut-off transistors at output logical "1". The same idea is used later for building DCTL
Direct-coupled transistor logic
A special type of resistor–transistor logic , a direct-coupled transistor logic or DCTL gate is one wherein the bases of the transistors are connected directly to inputs without any base resistors. Without the base resistors, DCTL gates are more economical and simpler to fabricate onto integrated...

, ECL
Emitter-coupled logic
In electronics, emitter-coupled logic , is a logic family that achieves high speed by using an overdriven BJT differential amplifier with single-ended input, whose emitter current is limited to avoid the slow saturation region of transistor operation....

, some TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...

 (7450, 7460), NMOS
NMOS logic
N-type metal-oxide-semiconductor logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits...

 and CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 gates.

Advantages


The primary advantage of RTL technology was that it involved a minimum number of transistors, which was an important consideration before integrated circuit technology (that is, in circuits using discrete components), as 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 diode–transistor logic and then transistor–transistor logic (starting 1963 at Sylvania), since diodes and transistors were no more expensive than resistors in the IC.

Limitations


The obvious disadvantage of RTL is its high power dissipation when the transistor is switched on (the power is dissipated mainly by the base resistors connected to logical "1" and by the collector resistor). 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.
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
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...

 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
TX-0
The TX-0, for Transistorized Experimental computer zero, but affectionately referred to as tixo , was an early fully transistorized computer and contained a then-huge 64K of 18-bit words of magnetic core memory. The TX-0 was built in 1955 and went online in 1956 and was used continually through the...

 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 (diode–transistor logic).

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
Baker clamp is a generic name for a class of electronic circuits that reduce the storage time of a switching bipolar junction transistor by applying a nonlinear negative feedback through various kinds of diodes. The reason for slow turn-off times of saturated BJTs is the stored charge in the base...

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
Schottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...

s, as in Schottky TTL.