Bootstrapping (electronics)
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. [1] Generally, any technique where part of the output of a system is used at startup is described as bootstrapping.
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[edit] Input impedance
In analog circuit designs a bootstrap circuit is an arrangement of components used to boost the input impedance 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 transistors, 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.
[edit] Driving MOS transistors
A N-MOSFET/IGBT needs a significantly positive charge (VGS > VDS + 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 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-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.
[edit] 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. [2]
[edit] 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 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 transistors or field effect transistors respectively.
[edit] 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. [3]
[edit] See also
- Booting of a computer
- Bootstrapping generally
- Black start of a electric power system
- Miller theorem applications (creating a virtual infinite impedance)
[edit] References
- ^ IEEE Standard 100 Authoritative Dictionary of IEEE Standards Terms, Seventh Edition, IEEE Press, 2000 ISBN 0-07381-2601-2 page 123
- ^ Raymond A. Mack Demystifying switching power supplies Newnes, 2005 ISBN 0750674458, page 121
- ^ William J. Dally, John W. Poulton, Digital systems engineering, Cambridge University Press, 1998 ISBN 0521592925 pages 190-191
