"[9], Instead of alternating NPN and PNP stages, another coupling method employed Zener diodes and resistors to shift the output logic levels to be the same as the input logic levels.[17]. [6] In addition, the essentially constant current draw of the differential amplifiers minimises delays and glitches due to supply-line inductance and capacitance, and the complementary outputs decrease the propagation time of the whole circuit by reducing inverter count. This meant that interoperation between ECL and other logic families, such as the popular TTL family, required additional interface circuits. As illustrated, the differential amplifiers 26 and 28 are connected in a balanced-type configuration. This is a common emitter amplifier with R E . The transfer characteristics for drain current Id1 and Id2 are shown in the figure. The dc collector voltage of the first stage given as base bias voltage to the second stage circuitry. Notice furthermore that the common-mode input component does not affect the output current. The importance of this observation and how it can be put to good use will become apparent in the next chapter (13) on transimpedance amplifiers. The two inputs at the bases or gates can be fed with a differential or balanced input signal and the two outputs from the collectors or drains remain balanced, or one input could be grounded to convert a single ended input signal to a differential output. Motorola introduced their first digital monolithic integrated circuit line, MECL I, in 1962. Difference- and common-mode signals. At low input voltage (logical "0") or at high input voltage (logical "1") the differential amplifier is overdriven. ADALM2000 Lab Activity 12m, MOS Differential Amplifier. 1 The two transistors Q 1 and Q It has a emitter-degeneration bias with a voltage divider. [27] The MIPS R6000 computers also used ECL. Marilyn Wolf, in Embedded System Interfacing, 2019. This circuit is particularly favored in integrated circuits, because matched transistors are much easier to construct than precession matched high value resistors. [13][14][15], Yourke's current switch was a differential amplifier whose input logic levels were different from the output logic levels. Emitter follower and darlington amplifier are the most common examples for feedback amplifiers. Other noteworthy characteristics of the ECL family include the fact that the large current requirement is approximately constant, and does not depend significantly on the state of the circuit. The output voltages at the collector load resistors RC1 and RC3 are shifted and buffered to the inverting and non-inverting outputs by the emitter followers T4 and T5 (shaded blue). [20] Some cookies are required for secure log-ins but others are optional for functional activities. Dual Input Balanced Output Long-tailed pairs are frequently used in circuits that implement linear amplifiers with feedback, as in operational amplifiers, and in other circuits that require a differential amplifier. This family is particularly suited to monolithic fabrication techniques because logic levels are function of resistor ratios. Due to this, R E1 appears in parallel with R E2 and the combination can be replaced by a single resistance denoted as R E . Emitter-coupled logic Last updated September 20, 2019 Motorola ECL 10,000 basic gate circuit diagram. This circuit was originally implemented using a pair of vacuum tubes. "The disadvantages are that more different power supply voltages are needed, and both pnp and npn transistors are required. Question 1Figure 5.16 shows an emitter coupled pair differential amplifierwith Vi1 and Vi2 as the input and VO as the output. current-mode logic (CML)[4] Linear equivalent half-circuits They share common positive supply V CC, common emitter resistor R E and common negative supply V EE. Fairchild introduced the F100K family.[when? However, two reasons for this is to increase input impedance and to reduce distortion due to the nonlinearity of the BJTs. This difference voltage will appear across Vbe and be amplified by gm*RL at Vout. Modern differential amplifiers are usually implemented with a basic two-transistor circuit called a “long-tailed” pair or differential pair. Cl. The ECL circuit operation is considered below with assumption that the input voltage is applied to T1 base, while T2 input is unused or a logical "0" is applied. A differential amplifier can be connected as a single-input, single-output device; a single-input, differential-output device; or a differential-input, differential-output device. Differential Pair Small Signal Up: Differential Amplifiers Previous: Differential Amplifiers Differential Pair DC Bias The differential amplifier is also called the emitter coupled pair, because, as shown in figure 5.1, it has two transistors … A.C. Differential Gain (A d). It is the fundamental building block of analog circuit. The bias condition assumes equal voltages at Vpos and Vneg, forcing the bias current Itail (set by Rtail) to split equally between the transistors resulting in IC1 = IC2. The differential amplifier is probably the most widely used circuit building block in analog integrated circuits, principally op amps. ADALM1000 Lab Activity 12m, MOS Differential Amplifier, ADALM2000 Lab Activity 12, BJT Differential Amplifier John F. Wakerly. The two bases or gates are inputs which are differentially amplified by the pair. The input resistance is high again since the base-emitter junction is cutoff. At least one manufacturer, IBM, made ECL circuits for use in the manufacturer's own products. The information that follows summarizes the important points of this chapter. You can see that there is no coupled capacitor exits in this circuitry. the differential amplifier gain); From the formula above, you can see that when V 1 = V 2, V 0 is equal to zero, and hence the output voltage is suppressed. The right figure shows the transfer characteristic of the differential amplifier (REF=40VT/IEE). Fig.2 (i) shows the basic circuit of a differential amplifier. Putting it all together you get a single-ended output at each collector, Subtracting the two outputs gets you a differential output of, An example to set the bias: Rtail sets the bias at Ie = (-0.6V - VDD) / Rtail = (-0.6 V - (-15 V)) / 7.2 kΩ = 2 mA which divides equally between Q1 and Q2 giving. The residual gain is low (K = RC/RE < 1). These are the mostly used ones with a number of applications. In the previous introduction to the amplifier tutorial, we saw that a family of curves known commonly as the Output Characteristic Curves, relate the transistors Collector Current (Ic), to its Collector Voltage (Vce) for different values of the transistors Base Current (Ib). ECL uses an overdriven BJT differential amplifier with single-ended input and limited emitter current to avoid the saturated(fully on) region of operation and its slow turn-off behavior. Single Input Unbalanced Output 2. To improve linearity, we introduce emitter-degeneration resistors, which increase the linear range from a few VT to about ITailR. The emitter-follower output stages could also be used to perform wired-or logic. "Microelectronic Circuits". The below figure shows the circuit diagram of differential amplifier. amplified) by the differential amplifier gain A d. It consists of two transistors Q 1 and Q 2 that have identical (ideally) characteristics. A differential amplifier also tends to reject the part of the input signals that are common to both inputs (Vin+ + Vin-)/2 . As a result, the common emitter resistor RE acts nearly as a current source. In ECL, the transistors are never in saturation, the input/output voltages have a small swing (0.8 V), the input impedance is high and the output impedance is low. If we apply equal amplitude, in phase signals to Vpos and Vneg, such that Vpos-Vneg = 0 then there will be no varying signal across Vbe and the output signal at Vout will be zero. The NPN output could drive PNP inputs, and vice versa. [34] Low-voltage positive emitter-coupled logic (LVPECL) is a power-optimized version of PECL, using a positive 3.3 V instead of 5 V supply. [8], ECL was invented in August 1956 at IBM by Hannon S. Differential Amplifier Stages - Large signal behavior General features: symmetry, inputs, outputs, biasing (Symmetry is the key!) The differential pair with a small differential input signal vi, Increasing the linear differential input range of the diff pair. [7], A variation of ECL in which all signal paths and gate inputs are differential is known as differential current switch (DCS) logic. The small signal voltage gain from Vneg to Vout is: Likewise, the small signal voltage gain from Vpos to Vout is: The transistor amplifies the small signal voltage across its Vbe which in this case is Vpos-Vneg. In this connection, when the supply voltage varies, the voltage drops across the collector resistors change slightly (in the case of emitter constant current source, they do not change at all). In electronics, emitter-coupled logic (ECL) is a high-speed integrated circuit bipolar transistor logic family. On the other hand, if we apply equal amplitude signals that are 180º out of phase with each other, then Vpos-Vneg = twice the amplitude of the inputs. Section, common-emitter stage with emitter degeneration, "Differential Current Switch – High performance at low power", "Design of a High-Speed Transistor for the ASLT Current Switch", "Logic Blocks Automated Logic Diagrams SLT, SLD, ASLT, MST", "TND309: General Information for MECL 10H and MECL 10K", "Digital Electronics: Principles, Devices and Applications", "High Performance ECL Data: ECLinPS and ECLinPS Lite", ECL Logic Manufacturers – "Emitter Coupled Logic", "Improved performance of IBM Enterprise System/9000 bipolar logic chips", "IBM zEnterprise System Technical Introduction", Electronic Materials Handbook: Packaging (page 163), "LVDS, CML, ECL – differential interfaces with odd voltages", Interfacing Between LVPECL, VML, CML and LVDS Levels, "DECL test run - Differential emitter-coupled logic", Motorola MECL logic family datasheets, 1963, General Information for MECL 10H and MECL 10K, Current mode logic / Source-coupled logic, https://en.wikipedia.org/w/index.php?title=Emitter-coupled_logic&oldid=990729737, All articles with vague or ambiguous time, Articles with unsourced statements from December 2018, Creative Commons Attribution-ShareAlike License, This page was last edited on 26 November 2020, at 05:07. If we instead DC couple the now symmetric inputs the biasing resistors become unnecessary and our difference amplifier now takes on the look of the classic differential pair we will discuss in the next section. Another feature of ECL is that it provides two outputs which are always complements of each other (figure 1). "[16] In Yourke's design, the two logic reference levels differed by 3 volts. The base voltage of the right transistor T3 is held fixed by a reference voltage source, shaded light green: the voltage divider with a diode thermal compensation (R1, R2, D1 and D2) and sometimes a buffering emitter follower (not shown on the picture); thus the emitter voltages are kept relatively steady. In this case, the voltage divider R1-R2 compensates the voltage variations to some extent. [26]) From 1975 to 1991 Digital Equipment Corporation's highest performance processors were all based on multi-chip ECL CPUs—from the ECL KL10 through the ECL VAX 8000 and VAX 9000 until the 1991 single-chip CMOS NVAX when the attempt failed to develop a competitive, single-chip ECL processor. This is often implemented with an active current mirror load instead of the collector/drain resistors. The high power consumption of ECL meant that it has been used mainly when high speed is a vital requirement. The main advantage of using MOSFETs for a differential pair compared to BJTs is the nearly infinite input impedance, while the disadvantage is generally lower differential gain. Let us consider two emitter-biased circuits as shown in fig. The emitter bypass capacitor, C E can provide an AC ground path for the Emitter, shorting out the emitter resistor, R E leaving only the signal Emitter resistance, re in the Emitter leg. As the current is steered between two legs of an emitter-coupled pair, ECL is sometimes called current-steering logic (CSL), current-mode logic (CML) or current-switch emitter-follower (CSEF) … Based on the methods of providing input and taking output, differential amplifiers can have four different configurations as below. Ans: In amplifier circuit the coupling to the next stage using resistor and capacitor (RC) combination can be called as a RC coupling. It would be advantageous if our differential amplifier had more symmetric inputs where the input impedance for both the positive and negative inputs was as high as possible, ideally infinite. Emitter follower circuit has a prominent place in feedback amplifiers. Another reason for using a negative power supply is protection of the output transistors from an accidental short circuit developing between output and ground[33] (but the outputs are not protected from a short circuit with the negative rail). 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