In the field of

electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, a technique where part of the output of a system is used at startup can be described as bootstrapping. A bootstrap circuit is one where part of the output of an amplifier stage is applied to the input, so as to alter the input impedance of the amplifier. When applied deliberately, the intention is usually to increase rather than decrease the impedance. In the domain of

MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

circuits, bootstrapping is commonly used to mean pulling up the

operating point {{unreferenced, date=April 2013 The operating point is a specific point within the operation characteristic of a technical device. This point will be engaged because of the properties of the system and the outside influences and parameters. In el ...

of a transistor above the power supply rail. The same term has been used somewhat more generally for dynamically altering the operating point of an

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 ...

(by shifting both its positive and negative supply rail) in order to increase its output voltage swing (relative to the ground). In the sense used in this paragraph, bootstrapping an operational amplifier means "using a signal to drive the reference point of the op-amp's power supplies". A more sophisticated use of this rail bootstrapping technique is to alter the non-linear C/V characteristic of the inputs of a JFET op-amp in order to decrease its distortion.

Input impedance

analog circuit Analogue electronics ( en-US, analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional relati ...

designs, a bootstrap circuit is an arrangement of components deliberately intended to alter the

input impedance The input impedance of an electrical network is the measure of the opposition to current ( impedance), both static ( resistance) and dynamic ( reactance), into the load network that is ''external'' to the electrical source. The input admittance (the ...

of a circuit. Usually it is intended to increase the impedance, by using a small amount of positive

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

, usually over two stages. This was often necessary in the early days of bipolar

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...

s, which inherently have quite a low input impedance. Because the feedback is positive, such circuits can suffer from poor stability and noise performance compared to ones that don't bootstrap. ''Negative'' feedback may alternatively be used to bootstrap an input impedance, causing the apparent impedance to be reduced. This is seldom done deliberately, however, and is normally an unwanted result of a particular circuit design. A well-known example of this is the

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. The virtually increased inpu ...

, in which an unavoidable feedback capacitance appears increased (i.e. its impedance appears reduced) by negative feedback. One popular case where this ''is'' done deliberately is the Miller compensation technique for providing a low-frequency pole inside an integrated circuit. To minimize the size of the necessary capacitor, it is placed between the input and an output which swings in the opposite direction. This bootstrapping makes it act like a larger capacitor to ground.

Driving MOS transistors

An N-

IGBT An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...

needs a significantly positive charge (''V_GS > V''_th) 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

die Die, as a verb, refers to death, the cessation of life. Die may also refer to: Games * Die, singular of dice, small throwable objects used for producing random numbers Manufacturing * Die (integrated circuit), a rectangular piece of a semicondu ...

size reduction (there are other benefits as well). However, using nMOS devices in place of pMOS devices means that a voltage higher than the power rail supply (V+) is needed in order to bias the transistor into linear operation (minimal current limiting) and thus avoid significant heat loss. A bootstrap capacitor is connected from the supply rail (V+) to the output voltage. Usually the source terminal of the N-

is connected to the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

of a recirculation

diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...

allowing for efficient management of stored energy in the typically inductive load (See

Flyback diode A flyback diode is any diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. It is used in circuits in which ind ...

). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage. A bootstrap circuit is often used in each half-bridge of an all-N-MOSFET

H-bridge A H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. The name is derived from its common sch ...

. When the low-side N-FET is on, current from the power rail (V+) flows through the bootstrap diode and charges the bootstrap capacitor through that low-side N-FET. When the low-side N-FET turns off, the low side of the bootstrap capacitor remains connected to the source of the high-side N-FET, and the capacitor discharges some of its energy driving the gate of the high-side N-FET to a voltage sufficiently above V+ to turn the high-side N-FET fully on; while the bootstrap diode blocks that above-V+ voltage from leaking back to the power rail V+. A

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. finite) 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 cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a formu ...

will always need to be less than 100% to accommodate for the parasitic discharge unless the leakage is accommodated for in another manner.

Switch-mode power supplies

In switch-mode power supplies, the control circuits are powered from the output. To start the power supply, a leakage resistance can be used to trickle-charge the supply rail for the control circuit to start it oscillating. This approach is less costly and simpler than providing a separate linear power supply just to start the regulator circuit.

Output swing

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.

Digital integrated circuits

Within an integrated circuit a bootstrap method is used to allow internal address and clock distribution lines to have an increased voltage swing. The bootstrap circuit uses a coupling capacitor, formed from the gate/source capacitance of a transistor, to drive a signal line to slightly greater than the supply voltage. Some all-pMOS integrated circuits such as the

Intel 4004 The Intel 4004 is a 4-bit central processing unit (CPU) released by Intel Corporation in 1971. Sold for US$60, it was the first commercially produced microprocessor, and the first in a long line of Intel CPUs. The 4004 was the first signific ...

and the

Intel 8008 The Intel 8008 ("''eight-thousand-eight''" or "''eighty-oh-eight''") is an early byte-oriented microprocessor designed by Computer Terminal Corporation (CTC), implemented and manufactured by Intel, and introduced in April 1972. It is an 8-bit CP ...

use that 2-transistor "bootstrap load" circuit.

References