The term
bootstrap has a number of meanings in
electronicsElectronics is a branch of science and technology that deals with the controlled flow of electrons. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation,...
.
Origin of Terminology
The origin of this descriptive phrase is largely unknown but theorised to come from the stories about
Baron von MünchausenKarl Friedrich Hieronymus, Freiherr von Münchhausen was a German baron born in Bodenwerder, who in his youth was sent to serve as page to Anthony Ulrich II of Brunswick-Wolfenbüttel, and later joined the Russian military...
who got stuck in
quicksandQuicksand is a colloid hydrogel consisting of fine granular matter , clay, and salt water. In the name, as in that of quicksilver , "quick" does not mean "fast," but "living" ....
and lifted himself by grabbing his bootstraps; hence, "Pulling yourself up by your bootstraps". It is of course to do with boots and their straps (or laces) and the imagined feat of lifting oneself (from an undesirable state) off the ground (to a desirable state) by pulling on one's bootstraps. This seemingly impossible task is supposed to exemplify the feat of getting out of a difficult situation by one's own efforts.
It was known by the early 20th century. James Joyce alluded to it in Ulysses, 1922:
"There were others who had forced their way to the top from the lowest rung by the aid of their bootstraps."
A more explicit use of the phrase comes a little later, from Kunitz & Haycraft's British Authors of the 19th Century:
"A poet who lifted himself by his own boot-straps from an obscure versifier to the ranks of real poetry."
In relation to electronics it is said that the output transistor "lifts its own control voltage" (since it has no boots or bootstraps so speak of) and therefore this is considered to be "bootstrap" functionality.
Electronics
A) In analog circuit designs a bootstrap circuit is an arrangement of components used to boost the
input impedanceThe input impedance, load impedance, or external impedance of a circuit or electronic device is the Thévenin equivalent impedance looking into its input.-Audio systems:...
of a circuit by using a small amount of positive
feedbackFeedback describes the situation when output from an event or phenomenon in the past will influence the same event/phenomenon in the present or future....
, usually over two stages. This was often necessary in the early days of
bipolarA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes...
transistorA transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece 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...
s, which inherently have quite a low input impedance. The need for such arrangements has largely been alleviated by the use of modern
field effect transistorThe field-effect transistor relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with the dual-carrier-type...
designs, except when ultra-high input impedances are required. Note that because the feedback is positive, such circuits usually suffer from poor stability and noise performance compared to ones that don't bootstrap.
B) A N-
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
/IGBT needs a significantly positive charge (
VGS > Vth) applied to the gate in order to turn on. Using only N-channel MOSFET/IGBT devices is a common cost reduction method due largely to
dieA die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon through processes such as photolithography. The...
size reduction (there are other benefits as well). However, using nMOS devices means that a voltage higher than the power rail supply (V+) is needed in order to saturate the transistor and thus avoid significant heat loss.
A bootstrap capacitor is connected from the supply rail (V+) to the output voltage. If the capacitor is polarized then the orientation of the capacitor is as follows:
AnodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
(marked with ‘+’)→(V+) and
CathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
(marked with ‘-’)→Output. In other words, the
capacitorA capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force...
should be between the output (source of an N-
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
) and (V+). Usually the source terminal of the N-
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
is connected to the
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
of a recirculation
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
allowing for efficient management of stored energy in the typically inductive load (See
Flyback diodeA flyback diode is a diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.- Induction at the Opening of a Contact:According to Lenz's law, if the current through an inductance changes, this...
). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage.
A
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
/IGBT is a voltage controlled device which, in theory, will not have any gate current. This makes it possible to utilize the charge inside the capacitor for control purposes. However, eventually the capacitor will lose its charge (due to parasitic gate current and non ideal (i.e. infinite) internal resistance), so this scheme is only used where there is a steady pulse present. This is because the pulsing action allows for the capacitor to discharge (at least partially if not completely). Most control schemes that use a bootstrap capacitor force the high side driver (N-MOSFET) off for a minimum time to allow for the capacitor to refill. This means that the
duty cycleIn telecommunications and electronics, the duty cycle is the fraction of time that a system is in an "active" state. In particular, it is used in the following contexts:...
will always need to be less than 100% to accommodate for the parasitic discharge unless the leakage is accommodated for in another manner.
C) Also in SMPS (Switch Mode Power Supplies) a leakage resistance can be used to trickle charge supply rail for the control circuit to start it oscillating. From then on it sources power from its own output.
Amplifiers
AC amplifiers can use bootstrapping to increase output swing. A capacitor (usually referred as
bootstrap capacitor) is connected from the output of the amplifier to the bias circuit, providing bias voltages that exceed the power supply voltage. Emitter followers can provide rail-to-rail output in this way, which is a common technique in class AB audio amplifiers.
In class-A Thermionic Valve/Vacuum Tube amplifier stages a resistance is often placed in the cathode circuit and when the cathode current passes through it the voltage drop developed across it raises the cathode voltage relative to the control grid voltage which is effectively the same as placing a
negative voltage on the control grid relative to the cathode, a requirement of many vacuum tubes operating class A. This negative grid voltage
relative to the cathode is referred to as the
bias, and obtaining the bias by employing a series cathode resistor eliminates the requirement for a separate negative voltage source but reduces the A.C. gain of the stage. A capacitor, often an
electrolytic capacitorAn electrolytic capacitor is a type of capacitor that uses an ionic conducting liquid as one of its plates with a larger capacitance per unit volume than other types. They are valuable in relatively high-current and low-frequency electrical circuits...
is often placed in parallel with the cathode resistor to provide a low impedance bypass for the A.C. component of the cathode current effectively increasing the A.C. voltage swing between the control grid and cathode, increasing the overall A.C. gain of the stage.
This A.C. bypassing capacitor is often referred to as a
bootstrap capacitor. At radio frequencies, where capacitive reactance is less, the bootstrap capacitor is usually an non-electrolytic type with a value of less than 100nF, it often has some resistance placed in series to carefully set the A.C. gain characteristics of the amplifier stage. Some discrete solid state class A amplifier stages employ a similar technique in the emitter or source circuits of
bipolar junction transistorA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes...
s or
field effect transistorThe field-effect transistor relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with the dual-carrier-type...
s respectively.
Computer Systems
Another meaning is in connection with the
bootingIn computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. A boot sequence is the initial set of operations that the computer performs when power is switched on...
process of a
computerA computer is a machine that manipulates data according to a set of instructions.Although mechanical examples of computers have existed through much of recorded human history, the first electronic computers were developed in the mid-20th century . These were the size of a large room, consuming as...
or other complex system, where the underlying electronics must arrange for the orderly startup of the
CPUThe Central Processing Unit or processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s...
and related electronics components. This is done long before the CPU is in a state where it can begin to execute software. Nowadays the bootstrap is coordinated by special
integrated circuitIn electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin substrate of semiconductor material...
s that monitor the raw
power supplyPower supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU...
and provide the relevant signals to enable the CPU and other chips accordingly.
Automobiles
In a typical car
alternatorAn alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used...
, a
DCDirect current is the undirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through...
field current is required before the device can produce any output. Once running, a small portion of the output is fed back to keep the field alive — this can be seen as a form of bootstrapping. The initial priming current is usually obtained from the vehicle's
batteryAn electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial...
, through a warning light on the dashboard — if the battery is totally flat, or if the warning light is burned out, the alternator won't produce any output voltage to spark the plugs or recharge the battery (by push starting for example) since it will be unable to bootstrap itself. However, in practice, a battery will usually have enough residual charge to avoid the first case, and some alternators are designed to self-start if the engine is revved to sufficient speed, even if the warning light is burned out.
Power Grids
In reference to a regional or national power grid, bootstrapping refers to the process of restarting
generating stationsA power station is an industrial facility for the generation of electric power....
and restoring power to the grid after a large-scale
power outageA power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
or disaster. This is also referred to as
cold starting or
black startA black start is the process of restoring a power station to operation without relying on external energy sources.Normally, the electric power used within the plant is provided from the station's own generators. If all of the plant’s main generators are shut down, station service power is provided...
ing. The process is required for reasons not dissimilar to the automobile case above: many power stations have
AC generatorsIn electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities...
which do not contain
permanent magnetsA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials and attracts or repels other magnets.A permanent magnet is an object made from a...
, and are incapable of starting to produce electricity unless they are fed some from an outside source to energize the coils and produce a magnetic field. Once running, they use their own output power for this purpose and are self-sustaining. Some power plants are designated within the grid as being capable of a cold start (either because they have main generators which contain permanent magnets, or have smaller auxiliary generators which do and which are capable of bringing the main ones up), and these plants are brought on-line first, and their power output directed across the grid to other plants and used to start them. In this way, generating stations across the network can be progressively brought back up.