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 transistors 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 circuits were first constructed with discrete components , but in it became the first digital logic family to be produced as a monolithic integrated circuit.
RTL integrated circuits were used in the Apollo Guidance Computer , whose design was begun in and which first flew in The role of the base resistor is to expand the very small transistor input voltage range about 0. 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. 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.
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 R 1 biases the transistor to the appropriate on-off threshold.
The output is inverted since the collector-emitter voltage of transistor Q 1 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.
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 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 used the same approach briefly, but quickly transitioned to higher-performance circuits such as diode—transistor logic and then transistor—transistor logic starting at Sylvania , since diodes and transistors were no more expensive than resistors in the IC.
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 : 3 inputs being the limit for many circuit designs, before it completely loses usable noise immunity.
Advantages of RTL, DTL and TTL Logic circuit
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. Transistor switching speed has increased steadily from the first transistorized computers through the present.
The GE Transistor Manual 7th ed. 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". 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.
Diode Transistor Logic NAND Gate
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 clamps 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. From Wikipedia, the free encyclopedia.
Rtl and dtl circuits pdf merge
Customer Engineering Manual of Instruction. Form Retrieved The logical function is performed by the input resistor network and the invert function is accomplished by the common emitter transistor configuration Morton Jr.
Electronics: The Life Story of a Technology. JHU Press. Lancaster RTL cookbook. Bobbs-Merrill Co.
GE Transistor Manual third through seventh editions ed. MIT Lincoln Laboratory. The Digital Logic Handbook. Digital Equipment Corporation.
Logic families. Static Dynamic Domino logic Four-phase logic.
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