Distributed Amplifier
Encyclopedia
Distributed amplifiers are circuit design
s that incorporate transmission line
theory into traditional amplifier design
to obtain a larger gain-bandwidth product
than is realizable by conventional circuits
.
s of individual vacuum tube
s could be added linearly without lumping their element capacitances at the input and output, thus arriving at a circuit that achieved a gain-bandwidth product greater than that of an individual tube. Percival's design did not gain widespread awareness however, until a publication on the subject was authored by Ginzton, Hewlett, Jasberg, and Noe in 1948. It is to this later paper that the term distributed amplifier can actually be traced. Traditionally, DA design architectures were realized using vacuum tube
technology.
technologies, such as GaAs and InP have been used. These have superior performance resulting from higher bandgaps (higher electron mobility), higher saturated electron
velocity, higher breakdown voltages and higher-resistivity
substrates. The latter contributes much to the availability of higher quality-factor (Q-factor or simply Q) integrated passive
devices in the III-V semiconductor technologies.
To meet the marketplace demands on cost, size, and power consumption of monolithic microwave
integrated circuits (MMICs), research continues in the development of mainstream digital
bulk-CMOS processes for such purposes. The continuous scaling of feature sizes in current IC technologies has enabled microwave and mm-wave CMOS circuits to directly benefit from the resulting increased unity-gain frequencies of the scaled technology. This device scaling, along with the advanced process control available in today's technologies, has recently made it possible to reach a transition frequency (ft) of 170 GHz
and a maximum oscillation
frequency
(fmax) of 240 GHz in a 90 nm CMOS process.
s with characteristic impedance
s of Z0 independently connecting the inputs and outputs of several active devices. An RF signal is thus supplied to the section of transmission line connected to the input of the first device. As the input signal propagates
down the input line, the individual devices respond to the forward traveling input step by inducing an amplified complementary forward traveling wave on the output line. This assumes the delays of the input and output lines are made equal through selection of propagation
constants and lengths of the two lines and as such the output signals from each individual device sum in phase
. Terminating resistors Zg and Zd are placed to minimize destructive reflection
s.
The transconductive gain
of each device is gm and the output impedance
seen by each transistor
is half the characteristic impedance of the transmission line. So that the overall voltage gain of the DA is:
Neglecting losses, the gain demonstrates a linear dependence on the number of devices (stages). Unlike the multiplicative nature of a cascade of conventional amplifier
s, the DA demonstrates an additive quality. It is this synergistic property of the DA architecture that makes it possible for it to provide gain at frequencies beyond that of the unity-gain frequency
of the individual stages. In practice, the number of stages is limited by the diminishing input signal resulting from attenuation on the input line. Means of determining the optimal number of stages are discussed below. Bandwidth is typically limited by impedance
mismatches brought about by frequency dependent device parasitic
s.
The DA architecture introduces delay
in order to achieve its broadband
gain characteristics. This delay is a desired feature in the design of another distributive system called the distributed oscillator.
are made of lumped elements of L and C. The parasitic L and the C from the transistors are used for this and usually some L is added to raise the line impedance
. Because of the Miller effect
in the common source amplifier the input and the output transmission line are coupled. For example for voltage inverting and current amplifying the input and the output form a shielded balanced line
. The current is increasing in the output transmission line with every subsequent transistor, and therefore less and less L is added to keep the voltage constant and more and more extra C is added to keep the velocity constant. This C can come from parasitics of a second stage. These delay lines do not have a flat dispersion near their cut off, so it is important to use the same L-C periodicity in the input and the output. If inserting transmission lines, input and output will disperse away from each other.
For a distributed amplifier the input is feed in series into the amplifiers and parallel out of them. To avoid losses in the input, no input signal is allowed to leak through. This avoided by using a balanced input and output also known as push-pull amplifier. Then all signals which leak through the parasitic capacitances cancel. The output is combined in a delay line with decreasing impedance. For narrow band operation other methods of phase-matching are possible, which avoid feeding the signal through multiple coils and capacitors. This may be useful for power-amplifiers.
The single amplifiers can be of any class. There may be some synergy between distributed class E/F amplifiers and some phase-matching methods. Only the fundamental frequency is used in the end, so this is the only frequency, which travels through the delay line version.
Because of the Miller effect a common source transistor acts as a capacitor (non inverting) at high frequencies and has an inverting transconductance at low frequencies. The channel of the transistor has three dimensions. One dimension, the width, is chosen depending on the current needed. The trouble is for a single transistor parasitic capacitance and gain both scale linearly with the width. For the distributed amplifier the capacitance – that is the width – of the single transistor is chosen based on the highest frequency and the width needed for the current is split across all transistors.
Voltage can be amplified by a common gate transistor, which shows no miller effect and no unit gain frequency cut off. Adding this yields the cascode
configuration. The common gate configuration is incompatible with CMOS; it adds a resistor, that means loss, and is more suited for broadband than for high efficiency applications.
Circuit design
The process of circuit design can cover systems ranging from complex electronic systems all the way down to the individual transistors within an integrated circuit...
s that incorporate transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
theory into traditional amplifier design
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
to obtain a larger gain-bandwidth product
Gain-bandwidth product
The gain–bandwidth product for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured....
than is realizable by conventional circuits
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
.
History
The design of the distributed amplifiers was first formulated by William S. Percival in 1936. In that year Percival proposed a design by which the transconductanceTransconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
s of individual vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s could be added linearly without lumping their element capacitances at the input and output, thus arriving at a circuit that achieved a gain-bandwidth product greater than that of an individual tube. Percival's design did not gain widespread awareness however, until a publication on the subject was authored by Ginzton, Hewlett, Jasberg, and Noe in 1948. It is to this later paper that the term distributed amplifier can actually be traced. Traditionally, DA design architectures were realized using vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
technology.
Current technology
More recently, III-V semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
technologies, such as GaAs and InP have been used. These have superior performance resulting from higher bandgaps (higher electron mobility), higher saturated electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
velocity, higher breakdown voltages and higher-resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
substrates. The latter contributes much to the availability of higher quality-factor (Q-factor or simply Q) integrated passive
Passivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
devices in the III-V semiconductor technologies.
To meet the marketplace demands on cost, size, and power consumption of monolithic microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
integrated circuits (MMICs), research continues in the development of mainstream digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
bulk-CMOS processes for such purposes. The continuous scaling of feature sizes in current IC technologies has enabled microwave and mm-wave CMOS circuits to directly benefit from the resulting increased unity-gain frequencies of the scaled technology. This device scaling, along with the advanced process control available in today's technologies, has recently made it possible to reach a transition frequency (ft) of 170 GHz
GHZ
GHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state — a quantum entanglement of three particles.# Galactic Habitable Zone — the region of a galaxy that is favorable to the formation of life....
and a maximum oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
(fmax) of 240 GHz in a 90 nm CMOS process.
Theory of operation
The operation of the DA can perhaps be most easily understood when explained in terms of the traveling-wave tube amplifier (TWTA). The DA consists of a pair of transmission lineTransmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s with characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
s of Z0 independently connecting the inputs and outputs of several active devices. An RF signal is thus supplied to the section of transmission line connected to the input of the first device. As the input signal propagates
Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
down the input line, the individual devices respond to the forward traveling input step by inducing an amplified complementary forward traveling wave on the output line. This assumes the delays of the input and output lines are made equal through selection of propagation
Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
constants and lengths of the two lines and as such the output signals from each individual device sum in phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
. Terminating resistors Zg and Zd are placed to minimize destructive reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
s.
The transconductive gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
of each device is gm and the output impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
seen by each transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
is half the characteristic impedance of the transmission line. So that the overall voltage gain of the DA is:
- Av = ½ n·g·m·Z0, where n is the number of stages.
Neglecting losses, the gain demonstrates a linear dependence on the number of devices (stages). Unlike the multiplicative nature of a cascade of conventional amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s, the DA demonstrates an additive quality. It is this synergistic property of the DA architecture that makes it possible for it to provide gain at frequencies beyond that of the unity-gain frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the individual stages. In practice, the number of stages is limited by the diminishing input signal resulting from attenuation on the input line. Means of determining the optimal number of stages are discussed below. Bandwidth is typically limited by impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
mismatches brought about by frequency dependent device parasitic
Parasitic element (electrical networks)
In electrical networks, a parasitic element is a circuit element that is possessed by an electrical component but which it is not desirable for it to have for its intended purpose. For instance, a resistor is designed to possess resistance, but will also possess unwanted parasitic...
s.
The DA architecture introduces delay
Propagation delay
Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...
in order to achieve its broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
gain characteristics. This delay is a desired feature in the design of another distributive system called the distributed oscillator.
Lumped elements
Delay linesAnalog delay line
An analog delay line is a network of electrical components connected in series; where each individual element creates a time difference or phase change between its input signal and its output signal. An example is a bucket-brigade device....
are made of lumped elements of L and C. The parasitic L and the C from the transistors are used for this and usually some L is added to raise the line impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
. Because of the Miller effect
Miller effect
In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...
in the common source amplifier the input and the output transmission line are coupled. For example for voltage inverting and current amplifying the input and the output form a shielded balanced line
Balanced line
In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...
. The current is increasing in the output transmission line with every subsequent transistor, and therefore less and less L is added to keep the voltage constant and more and more extra C is added to keep the velocity constant. This C can come from parasitics of a second stage. These delay lines do not have a flat dispersion near their cut off, so it is important to use the same L-C periodicity in the input and the output. If inserting transmission lines, input and output will disperse away from each other.
For a distributed amplifier the input is feed in series into the amplifiers and parallel out of them. To avoid losses in the input, no input signal is allowed to leak through. This avoided by using a balanced input and output also known as push-pull amplifier. Then all signals which leak through the parasitic capacitances cancel. The output is combined in a delay line with decreasing impedance. For narrow band operation other methods of phase-matching are possible, which avoid feeding the signal through multiple coils and capacitors. This may be useful for power-amplifiers.
The single amplifiers can be of any class. There may be some synergy between distributed class E/F amplifiers and some phase-matching methods. Only the fundamental frequency is used in the end, so this is the only frequency, which travels through the delay line version.
Because of the Miller effect a common source transistor acts as a capacitor (non inverting) at high frequencies and has an inverting transconductance at low frequencies. The channel of the transistor has three dimensions. One dimension, the width, is chosen depending on the current needed. The trouble is for a single transistor parasitic capacitance and gain both scale linearly with the width. For the distributed amplifier the capacitance – that is the width – of the single transistor is chosen based on the highest frequency and the width needed for the current is split across all transistors.
Applications
Note that those termination resistors are usually not used in CMOS, but the losses due to these are small in typical applications. In solid state power amplifiers often multiple discrete transistors are used for power reasons anyway. If all transistors are driven in a synchronized fashion a very high gate drive power is needed. For frequencies at which small and efficient coils are available distributed amplifiers are more efficient.Voltage can be amplified by a common gate transistor, which shows no miller effect and no unit gain frequency cut off. Adding this yields the cascode
Cascode
The cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance,...
configuration. The common gate configuration is incompatible with CMOS; it adds a resistor, that means loss, and is more suited for broadband than for high efficiency applications.
- radioRadioRadio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
- Acousto-optic modulatorAcousto-optic modulatorAn acousto-optic modulator , also called a Bragg cell, uses the acousto-optic effect to diffract and shift the frequency of light using sound waves . They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric...
- time to digital converterTime to digital converterIn electronic instrumentation and signal processing, a time to digital converter is a device for converting a signal of sporadic pulses into a digital representation of their time indices. In other words, a TDC outputs the time of arrival for each incoming pulse...
See also
- Gunn diodeGunn diodeA Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions...
is a device without any parasitic C or L very suitable for broadband applications - Regenerative circuitRegenerative circuitThe regenerative circuit or "autodyne" allows an electronic signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback...
is circuit using the parasitics of a single transistor for a high frequency narrow band amplifier - Armstrong oscillatorArmstrong oscillatorThe Armstrong oscillator is named after its inventor, the electrical engineer Edwin Armstrong. It is sometimes called a tickler oscillator because the feedback needed to produce oscillations is provided using a tickler coil via magnetic coupling between coil L and coil T...
is circuit using the parasitics of a single transistor for a high frequency narrow band oscillator