A direct-coupled amplifier or DC amplifier is a type of
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
in which the output of one stage of the amplifier is
coupled
''Coupled'' is an American dating game show that aired on Fox from May 17 to August 2, 2016. It was hosted by television personality, Terrence J and created by Mark Burnett, of '' Survivor'', ''The Apprentice'', '' Are You Smarter Than a 5th G ...
to the input of the next stage in such a way as to permit signals with zero
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, also referred to as
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
, to pass from input to output. This is an application of the more general
direct coupling
In electronics, direct coupling or DC coupling (also called conductive coupling and galvanic coupling) is the transfer of electrical energy by means of physical contact via a conductive medium, in contrast to inductive coupling and capacitive coup ...
. It was invented by Harold J Paz and Francis P. Keiper Jr. in 1955. It displaced the triode vacuum tube amplifier designed by
Lee de Forest. Almost all vacuum tube circuit designs are now replaced with direct coupled transistor circuit design. It is the first transistor amplifier design that did not include coupling capacitors. The direct-coupled amplifier allowed analog circuits to be built smaller with the elimination of coupling capacitors and removed the lower frequency limitation that is dependent on capacitors.
History
Paz first started his career at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial Research and development, research and scientific developm ...
as an intern from December 1950 to April 1952 as an Engineering Aid. Paz worked on testing several transistor parameters, such as rise time, RC timing constant, alpha coefficient, to determine their effects on a transistor circuit design. He then went on to work at
RCA
The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
as a summer student engineering intern from June 1953 to September 1953. Paz was assigned to determine the effects of several variables on a transistor's noise factor at various radio frequencies. It was the result of this research that Paz designed the first transistor-based wireless microphone, called Phantom. RCA took interest in Paz's design and made their subsidiary
National Broadcasting Company aware of the new microphone. RCA decided to file patent US2,810,110 for the microphone on July 16, 1954 and was granted on October 15, 1957. The design was used for the ND-433 wireless microphone that NBC used in 1955.
It was In June 1954, thatPaz took an engineering position at
Philco
Philco (an acronym for Philadelphia Battery Company) is an American electronics industry, electronics manufacturer headquartered in Philadelphia. Philco was a pioneer in battery, radio, and television production. In 1961, the company was purchased ...
and was assigned to the Transistor Product Engineering Group to study the theory of operation of the direct-coupled switching circuits of R. Brown. The switching circuited used one resistor per transistor, which was revolutionary at the time for its low component count. Transistors were primarily used to be an on-off digital device that would be beneficial for making a solid-state digital computer. It was after understanding the theory of the switching circuits that Paz invented the direct-coupled "Triplet", which is a three-stage linear amplifier which uses only one resistor per transistor amplification stage. It was during this time that Paz was introduced to
Robert Noyce, who was also in the Transistor Research Department. The direct-coupled amplifier was influential in the development of Fairchild's uA709 operational amplifier by
Bob Widlar
Robert John Widlar (pronounced ''wide-lar''; November 30, 1937 – February 27, 1991) was an American electronics engineer and a designer of linear integrated circuits (ICs).
Early years
Widlar was born November 30, 1937 in Cleveland to par ...
, which Noyce knew about as he was one of the founders of
Fairchild Semiconductors.
The direct-coupled amplifier is also the basis for Philco's Mark I hearing aid, which used the circuit built with silicon alloy transistors. The hearing aid was listed as Figure 5 as an application of the Triplet on the US3030586 patent
Afterwards, Paz returned to work at RCA in August 1955. He worked on designing a power supply that used small transistors which could regulate and control a large amount of power. This research resulted in new circuitry that Paz designed and was featured in RCA's internal academic journal, the Industry Service Laboratory. Paz later on published his design with the International Radio Engineers and presented his findings at the IRE convention in 1957.
Paz continued working on various transistor amplifier designs for RCA, including a transistor circuit design that could be used in the rack mounted amplifier product line. This transistor circuit became a transistor preamplifier equalizer for professional turntables. Paz submitted his paper at the 1957 Annual Audio Engineering Convention. Additional products include the "hybrid" transistor power amplifier, which was the first 10 watt amplifier which had less than 0.333% distortion from 30 cycles to 15,000 cycles. It was also compatible to be used with any RCA 2N301 power transistors. The low distortion transistor power amplifier was published in the ''Electronics Industries'' magazine and Paz presented his findings at the National Convention of IRE in 1959.
Current
The common use of the term "DC amplifier" does not mean "direct current amplifier", as this type can be used for both direct current and
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
signals. The frequency response of the direct coupled amplifier is similar to low pass filter and hence it is also known as "Low-Pass Amplifier". The amplification of DC (zero frequency) is possible only by this amplifier, hence it later becomes the building block for
differential amplifier
A 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. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outpu ...
and
operational amplifier
An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
. Furthermore, monolithic integrated circuit technology does not allow the fabrication of large coupling capacitors.
Drift
Direct-coupled amplifiers constructed in the conventional form of
single-ended amplifiers connected in
cascade suffer from drift of the output voltage with time and temperature. In many high-gain applications it is necessary to provide offset adjustments to deal with drift. The drift problem can be overcome by using
differential amplifier
A 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. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outpu ...
s.
[Balbir Kumar, Shail B. Jain, ''Electronic Devices and Circuits'', pp. 431–432, PHI Learning, 2007 .]
Application
Direct-coupled amplifiers are used in
voltage regulator
A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...
s,
servo drive
A servo drive is an electronic amplifier used to power electric servomechanisms.
A servo drive monitors the feedback signal from the servomechanism and continually adjusts for deviation from expected behavior.
Function
A servo drive receives a c ...
s and other
instrumentation amplifier
An instrumentation amplifier (sometimes shorthanded as in-amp or InAmp) is a type of differential amplifier that has been outfitted with input buffer amplifiers, which eliminate the need for input impedance matching and thus make the amplifie ...
s. It also forms a building block for
differential amplifier
A 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. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outpu ...
s and
operational amplifier
An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
s.
See also
*
Capacitive coupling
References
{{DEFAULTSORT:Direct Coupled Amplifier
Electronic amplifiers