![]() ![]() Transistor switching speed has increased steadily from the first transistorized computers through the present. Various companies applied the following speed-up methods to discrete RTL. Ī standard integrated circuit RTL NOR gate can drive up to 3 other similar gates.Īlternatively, 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. 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. In contrast, TTL circuits with " totem-pole" output stage minimize both of these requirements.Īnother limitation of RTL is its limited fan-in: 3 inputs being the limit for many circuit designs, before it completely loses usable noise immunity. This requires that more current be supplied to and heat be removed from RTL circuits. The disadvantage of RTL is its high power dissipation when the transistor is switched on, by current flowing in the collector and base resistors. 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 in 1963 at Sylvania Electric Products), since diodes and transistors were no more expensive than resistors in the IC. In circuits using discrete components, before integrated circuits, transistors were the most expensive component to produce. The primary advantage of RTL technology was that it used a minimum number of transistors. To ensure stability and predictable output of the bipolar transistors their base-inputs (V b or base-terminal voltage) is biased. Thus, the analog resistive network and the analog transistor stage perform the logic function NOR. ![]() 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. The pull-down resistor R 1 biases the transistor to the appropriate on-off threshold. If all the input voltages are low (logical "0"), the transistor is cut-off. ![]() 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. 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 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). With two or more base resistors (R 3 and R 4) instead of one, the inverter becomes a two-input RTL NOR gate (see the figure on the right). Schematic of a one-transistor RTL NOR gate. 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). 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 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. It consists of a common-emitter stage with a base resistor connected between the base and the input voltage source. Implementation RTL inverter Ī bipolar transistor switch is the simplest RTL gate ( inverter or NOT gate) implementing logical negation. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |