Bootstrapping (electronics)
Encyclopedia
In the field of electronics
, a bootstrap circuit is one where part of the output of an amplifier stage is applied to the input, so as to increase the input impedance
of the amplifier. Generally, any technique where part of the output of a system is used at startup is described as bootstrapping.
of a circuit by using a small amount of positive feedback
, usually over two stages. This was often necessary in the early days of bipolar
transistor
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.
/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 die
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 saturate the transistor 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-MOSFET
is connected to the cathode
of a recirculation diode
allowing for efficient management of stored energy in the typically inductive load (See Flyback diode
). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage.
A MOSFET
/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. 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
will always need to be less than 100% to accommodate for the parasitic discharge unless the leakage is accommodated for in another manner.
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 capacitor
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 transistor
s or field effect transistors respectively.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, a bootstrap circuit is one where part of the output of an amplifier stage is applied to the input, so as to increase the input impedance
Impedance
Impedance may refer to:*Electrical impedance, the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit**Characteristic impedance of a transmission line...
of the amplifier. Generally, any technique where part of the output of a system is used at startup is described as bootstrapping.
Input impedance
In analog circuit designs a bootstrap circuit is an arrangement of components used to boost the input impedanceInput impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
of a circuit by using a small amount of positive feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
, usually over two stages. This was often necessary in the early days of bipolar
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA 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...
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...
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.
Driving MOS transistors
A N-MOSFETMOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented 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 die
Die (integrated circuit)
A 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 or other semiconductor through processes such as...
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 saturate the transistor 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-MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
is connected to the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
of a recirculation diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
allowing for efficient management of stored energy in the typically inductive load (See Flyback diode
Flyback diode
A 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.- Working principle :In its most simplified form with a voltage source connected to an inductor with a switch, we...
). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage.
A MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented 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. 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
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
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 regulation 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 more efficient 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.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 capacitor
Electrolytic capacitor
An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics"...
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 transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA 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...
s or field effect transistors respectively.
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-drain capacitance of a transistor, to drive a signal line to slightly greater than the supply voltage.See also
- BootingBootingIn computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
of a computer - BootstrappingBootstrappingBootstrapping or booting refers to a group of metaphors that share a common meaning: a self-sustaining process that proceeds without external help....
generally - Black startBlack startA black start is the process of restoring a power station to operation without relying on the external electric power transmission network.Normally, the electric power used within the plant is provided from the station's own generators...
of a electric power system - Miller theorem applications (creating a virtual infinite impedance)