![]() This is often implemented as a current mirror ( Figure 3, below). To avoid sacrificing gain, a differential to single-ended converter can be utilized. The gain is half that of the stage with differential output. If the differential output is not desired, then only one output can be used (taken from just one of the collectors (or anodes or drains), disregarding the other output this configuration is referred to as single-ended output. An amplifier with differential output can drive a floating load or another stage with differential input. The two bases (or grids or gates) are inputs which are differentially amplified (subtracted and multiplied) by the transistor pair they can be fed with a differential (balanced) input signal, or one input could be grounded to form a phase splitter circuit. With two inputs and two outputs, this forms a differential amplifier stage (Figure 2). DC-coupled circuitry became the norm after the first generation of vacuum-tube computers.Ī differential (long-tailed, emitter-coupled) pair amplifier consists of two amplifying stages with common ( emitter, source or cathode) degeneration. Many computers of this time tried to avoid this problem by using only AC-coupled pulse logic, which made them very large and overly complex ( ENIAC: 18,000 tubes for a 20-digit calculator) or unreliable. One disadvantage is that the output voltage swing (typically ☑0–20 V) was imposed upon a high DC voltage (200 V or so), requiring care in signal coupling, usually some form of wide-band DC coupling. The long-tailed pair has many favorable attributes if used as a switch: largely immune to tube (transistor) variations (of great importance when machines contained 1,000 tubes or more), high gain, gain stability, high input impedance, medium/low output impedance, good clipper (with a not-too-long tail), non-inverting ( EDSAC contained no inverters!) and large output voltage swings. The long-tailed pair was very successfully used in early British computing, most notably the Pilot ACE model and descendants, Maurice Wilkes’ EDSAC, and probably others designed by people who worked with Blumlein or his peers. By the end of the 1930s the topology was well established and had been described by various authors, including Frank Offner (1937), Otto Schmitt (1937) and Jan Friedrich Toennies (1938), and it was particularly used for detection and measurement of physiological impulses. ![]() The earliest definite long-tailed pair circuit appears in a patent submitted by Alan Blumlein in 1936. An early circuit which closely resembles a long-tailed pair was published by British neurophysiologist Bryan Matthews in 1934, and it seems likely that this was intended to be a true long-tailed pair but was published with a drawing error. ![]() The long-tailed pair was developed from earlier knowledge of push–pull circuit techniques and measurement bridges. The bias points of “long-tail” resistor circuit are largely determined by Ohm's law and less so by active-component characteristics. ![]() The circuit works the same way for all three-terminal devices with current gain. This circuit was originally implemented using a pair of vacuum tubes. Modern differential amplifiers are usually implemented with a basic two-transistor circuit called a “long-tailed” pair or differential pair. Note that a differential amplifier is a more general form of amplifier than one with a single input by grounding one input of a differential amplifier, a single-ended amplifier results. It is an analog circuit with two inputs V in − is zero, and the CMRR is infinite. V s+ and V s− are the power-supply voltages they are often omitted from the diagram for simplicity but must be present in the actual circuit.Ī differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. The inverting and non-inverting inputs are distinguished by "−" and "+" placed in the amplifier triangle.
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