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48-429: The DTL circuit shown in the first picture consists of three stages: an input diode logic stage (D1, D2 and R1), an intermediate level shifting stage (R3 and R4), and an output common-emitter amplifier stage (Q1 and R2). If both inputs A and B are high (logic 1; near V+), then the diodes D1 and D2 are reverse biased. Resistors R1 and R3 will then supply enough current to turn on Q1 (drive Q1 into saturation) and also supply

96-416: A multi-tool for augmenting the limited capabilities of regular CMOS 4000-series ICs , for instance by using a diode OR gate to add extra inputs on a flip-flop , or a diode AND gate to configure a divide-by-N counter. A variant approach suggests keeping a supply of 1N914 diodes with inverting Schmitt trigger ICs to provide hysteresis and functional completeness . An active-low OR diode logic gate

144-553: A short circuit between power and ground. Such outputs, however, may be used as inputs to passive AND or OR diode logic gates. This avoids the costs of adding active logic gates. However, diode logic will degrade voltage levels and result in poor noise rejection, so designers should be aware of the interfaced logic family's voltage ranges and limitations, to prevent failures. The humorously-named "Micky Mouse Logic" described in Don Lancaster 's CMOS Cookbook suggests using diodes as

192-456: A small voltage drop , while reverse-biased diodes have a very high impedance approximating an open circuit. The diode symbol 's arrow shows the forward-biased direction of conventional current flow . Each input of a diode logic gate connects through a diode connected to a shared wired logic output. Depending on the voltage level of each input and direction of the diode, each diode may or may not be forward-biased. If any are forward-biased,

240-449: A transient response that might be of concern. The capacitance between anode and cathode is inversely proportional to the reverse voltage, growing as it approaches 0 volts and into forward bias. There is also a recovery concern: a diode's current will not decrease immediately when switching from forward-biased to reverse-biased, because discharging its stored charge takes a finite amount of time (t rr or reverse recovery time ). In

288-442: 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

336-426: A diode OR gate, if two or more of the inputs are high and one switches to low, recovery issues will cause a short-term dip in the output voltage or increase current in the diodes that remain high. If a diode–transistor logic gate drives a transistor inverter of similar construction, the transistor will have a similar base-collector capacitance that is amplified by the transistor gain, so that it will be too slow to pass

384-423: 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

432-502: A low noise margin . 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. Various companies have applied

480-419: 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. A low-voltage diode arranged to prevent saturation of

528-420: A precise voltage range, provided that their input voltages were within a somewhat wider valid input voltage range . This level restoration allows more cascaded logic stages and removes noise, facilitating very large scale integration . However, passive diode logic gates accumulate the following voltage losses when gates are cascaded: Thus the feasible amount of cascading is limited by the value of V F and

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576-422: A voltage divider that makes Q1's base voltage negative and consequently turns off Q1. Q1's collector current will be essentially zero, so R2 will pull the output voltage Q high (logic 1; near V+). Up until 1952, IBM manufactured transistors by modifying off-the-shelf germanium diodes , after which they had their own alloy-junction transistor manufacturing plant at Poughkeepsie . In the mid 1950s, diode logic

624-941: Is additionally required to provide logical inversion (NOT) for functional completeness and amplification for voltage level restoration , which diode logic alone can't provide. Since voltage levels weaken with each diode logic stage, multiple stages can't easily be cascaded, limiting diode logic's usefulness. However, diode logic has the advantage of utilizing only cheap passive components . Logic gates evaluate Boolean algebra , typically using electronic switches controlled by logical inputs connected in parallel or series . Diode logic can only implement OR and AND, because inverters (NOT gates) require an active device. Main article: Logic level § 2-level logic Binary logic uses two distinct logic levels of voltage signals that may be labeled high and low . In this discussion, voltages close to +5 volts are high, and voltages close to 0 volts ( ground ) are low. The exact magnitude of

672-400: Is formed by a keypad containing diodes at each switch, all connected to a shared pull-up resistor. When no switch is closed, the pull-up keeps the output high. But when the switch for any key connects to ground, the output goes low. This OR result can be used as an interrupt signal to indicate that any key has been pressed. Then a microcontroller can wake from power-saving standby and scan

720-465: 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. It has

768-418: Is logical 1 while high is logical 0. The following diode logic gates work in both active-high or active-low logic, however the logical function they implement is different depending on what voltage level is considered active . Switching between active-high and active-low is commonly used to achieve a more efficient logic design. Forward-biased diodes have low impedance approximating a short circuit with

816-552: Is named for Richard H. Baker, who described it in his 1956 technical report "Maximum Efficiency Switching Circuits". In 1964, James R. Biard filed a patent for the Schottky transistor . In his patent the Schottky diode prevented the transistor from saturating by minimizing the forward bias on the collector–base transistor junction, thus reducing the minority carrier injection to a negligible amount. The diode could also be integrated on

864-411: 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 ). With two or more base resistors (R 3 and R 4 ) instead of one,

912-553: Is the earliest class of transistorized digital logic circuit; it was succeeded by diode–transistor logic (DTL) and transistor–transistor logic (TTL). RTL circuits were first constructed with discrete components , but in 1961 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 began in 1961 and which first flew in 1966. A bipolar transistor switch

960-404: Is the simplest RTL gate ( inverter 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

1008-566: The Shockley diode equation , which has an more complicated exponential current–voltage relationship called the diode law . Designers must rely on a diode's specification sheet, which primarily provides a maximum forward voltage drop at one or more forward currents, a reverse leakage current (or saturation current ), and a maximum reverse voltage limited by Zener or avalanche breakdown . Effects of temperature and process variation are usually included. Typical examples: Diodes also have

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1056-448: The 930-series DTμL micrologic family that had a better noise immunity, smaller die, and lower cost. It was the most commercially successful DTL family and copied by other IC manufacturers. The DTL propagation delay is relatively large. When the transistor goes into saturation from all inputs being high, charge is stored in the base region. When it comes out of saturation (one input goes low) this charge has to be removed and will dominate

1104-506: The DTL gate, R3 is replaced by two level-shifting diodes connected in series. Also the bottom of R4 is connected to ground to provide bias current for the diodes and a discharge path for the transistor base. The resulting integrated circuit runs off a single power supply voltage. In 1962, Signetics introduced the SE100-series family, the first high-volume DTL chips. In 1964, Fairchild released

1152-447: The current needed by R4. There will be a small positive voltage on the base of Q1 (V BE , about 0.3 V for germanium and 0.6 V for silicon). The turned on transistor's collector current will then pull the output Q low (logic 0; V CE(sat) , usually less than 1 volt). If either or both inputs are low, then at least one of the input diodes conducts and pulls the voltage at the anodes to a value less than about 2 volts. R3 and R4 then act as

1200-426: The following circuits have two inputs for each gate and thus use two diodes, but can be extended with more diodes to allow for more inputs. At least one input of every gate must be connected to a strong-enough high or low voltage source. If all inputs are disconnected from a strong source, the output may not fall within a valid voltage range. Each input connects to the anode of a diode. All cathodes are connected to

1248-439: 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

1296-472: The glitch. But when the diode is much slower, recovery will become a concern: In one unusual design, small selenium diode discs were used with germanium transistors. The recovery time of the very slow selenium diodes caused a glitch on the inverter output. It was fixed by placing a selenium diode across the base-emitter junction of the transistor making it think it was a selenium transistor (if there could ever be one). Active logic gates output voltages within

1344-433: The high-low voltage difference. With special designs, two-stage systems are sometimes achieved. In order to compensate for the voltage drop and provide sufficient current to drive the next circuit(s) load, the pull-up resistors may be connected to a supply higher than the nominal high voltage level and similarly the pull-down resistors may be connected to a supply lower than the nominal low voltage. Historically, diode logic

1392-418: 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. The limitations of

1440-435: The inverter becomes a two-input RTL NOR gate (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

1488-436: The key matrix to determine which key specifically was pressed. During the 1960s the use of tunnel diodes in logic circuits was an active research topic. When compared to transistor logic gates of the time, the tunnel diode offered much higher speeds. Unlike other diode types, the tunnel diode offered the possibility of amplification of signals at each stage. The operating principles of a tunnel diode logic rely on biasing of

Diode–transistor logic - Misplaced Pages Continue

1536-437: The more nearly unilateral nature of transistor amplifiers overtook the tunnel diode gate, resulting in it no longer being used in modern computers. Resistor%E2%80%93transistor logic Resistor–transistor logic ( RTL ), sometimes also known as 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

1584-458: 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 in 1963 at Sylvania Electric Products ), since diodes and transistors were no more expensive than resistors in the IC. The disadvantage of RTL

1632-460: The number of transistors used. The IBM 1401 (announced in 1959) used DTL circuits similar to the circuit shown in the first picture. IBM called the logic "complemented transistor diode logic" (CTDL). CTDL avoided the level shifting stage (R3 and R4) by alternating NPN and PNP based gates operating on different power supply voltages. NPN based circuits used +6V and -6V and the transistor switched at close to -6V, PNP based circuits used 0V and -12V and

1680-402: 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

1728-437: The output be low: This corresponds to logical OR in active-high logic, as well as simultaneously to logical AND in active-low logic. This circuit mirrors the previous gate: the diodes are reversed so that each input connects to the cathode of a diode and all anodes are connected together to the output, which has a pull-up resistor. If any input is low, its diode will be forward-biased and will conduct current, and thus pull

1776-618: The output voltage low . If all inputs are high, all diodes will be reverse-biased and so none will conduct current. The pull-up resistor will quickly pull the output voltage high. In summary, if any input is low, the output will be low, but only if all inputs are high will the output be high: This corresponds to logical AND in active-high logic, as well as simultaneously to logical OR in active-low logic. For simplicity, diodes may sometimes be assumed to have no voltage drop or resistance when forward-biased and infinite resistance when reverse-biased. But real diodes are better approximated by

1824-417: The output, which has a pull-down resistor. If any input is high, its diode will be forward-biased and conduct current, and thus pull the output voltage high . If all inputs are low, all diodes will be reverse-biased and so none will conduct current. The pull-down resistor will quickly pull the output voltage low. In summary, if any input is high the output will be high, but only if all inputs are low will

1872-406: The propagation time. One way to speed up DTL is to add a small "speed-up" capacitor across R3. The capacitor helps to turn off the transistor by removing the stored base charge; the capacitor also helps to turn on the transistor by increasing the initial base drive. Another way to speed up DTL is to avoid saturating the switching transistor. That can be done with a Baker clamp . The Baker clamp

1920-407: 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

1968-753: The same die, had a compact layout, no minority-carrier charge storage, and was faster than a conventional junction diode. His patent also showed how the Schottky transistor could be used in DTL circuits and improve the switching speed of other saturated logic designs, such as Schottky-TTL, at a low cost. A major advantage over the earlier resistor–transistor logic is increased fan-in . Additionally, to increase fan-out, an additional transistor and diode may be used. Diode logic Diode logic (or diode-resistor logic ) constructs AND and OR logic gates with diodes and resistors . An active device ( vacuum tubes with control grids in early electronic computers , then transistors in diode–transistor logic )

Diode–transistor logic - Misplaced Pages Continue

2016-459: The shared output wire will be one small forward voltage drop within the forward-biased diode's input. If no diode is forward-biased then no diode will provide drive current for the output's load (such as a subsequent logic stage). So the output additionally requires a pull-up or pull-down resistor connected to a voltage source, so that the output can transition quickly and provide a strong driving current when no diodes are forward-biased. Note:

2064-581: The transistor switched at close to 0V. Thus for example a NPN gate driven by a PNP gate would see the threshold voltage of -6V in the middle of the range of 0V to -12V. Similarly for the PNP gate switching at 0V driven by a range of 6V to -6V. The 1401 used germanium transistors and diodes in its basic gates. The 1401 also added an inductor in series with R2. The physical packaging used the IBM Standard Modular System . In an integrated circuit version of

2112-528: The tunnel diode and supply of current from inputs over a threshold current, to switch the diode between two states. Consequently, tunnel diode logic circuits required a means to reset the diode after each logical operation. However, a simple tunnel diode gate offered little isolation between inputs and outputs and had low fan in and fan out . More complex gates, with additional tunnel diodes and bias power supplies, overcame some of these limitations. Advances in discrete and integrated circuit transistor speed and

2160-405: The voltage is not critical, provided that inputs are driven by strong enough sources so that output voltages lie within detectably different ranges . For active-high or positive logic , high represents logic 1 ( true ) and low represents logic 0 ( false ). However, the assignment of logical 1 and logical 0 to high or low is arbitrary and is reversed in active-low or negative logic, where low

2208-730: Was used extensively in the construction of early computers , since semiconductor diodes could replace bulky and costly active vacuum tubes . The invention of the transistor allowed transistors to replace tubes as the active element in diode–transistor logic . Since early transistors were not reliable, the D-17B missile guidance computer, for instance, primarily used diode logic and only used transistors when necessary. Transistors quickly advanced to replace diode logic almost entirely. However, diode logic still finds some modern uses. Low-impedance push–pull outputs of conventional ICs shouldn't directly be connected to external circuitry, as they may create

2256-461: Was used in the IBM 608 which was the first all-transistorized computer in the world. A single card would hold four two-way circuits or three three-way or one eight-way. All input and output signals were compatible. The circuits were capable of reliably switching pulses as narrow as one microsecond. The designers of the 1962 D-17B guidance computer used diode-resistor logic as much as possible, to minimize

2304-403: Was used later for building DCTL , ECL , some TTL (7450, 7460), NMOS and CMOS gates. To ensure stability and predictable output of the bipolar transistors their base-inputs (V b or base-terminal voltage) is biased. 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

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