![]() To minimize the size of the necessary capacitor, it is placed between the input and an output which swings in the opposite direction. One popular case where this is done deliberately is the Miller compensation technique for providing a low-frequency pole inside an integrated circuit. its impedance appears reduced) by negative feedback. A well-known example of this is the Miller effect, in which an unavoidable feedback capacitance appears increased (i.e. This is seldom done deliberately, however, and is normally an unwanted result of a particular circuit design. Negative feedback may alternatively be used to bootstrap an input impedance, causing the apparent impedance to be reduced. Because the feedback is positive, such circuits can suffer from poor stability and noise performance compared to ones that don't bootstrap. This was often necessary in the early days of bipolar transistors, which inherently have quite a low input impedance. ![]() Usually it is intended to increase the impedance, by using a small amount of positive feedback, usually over two stages. In analog circuit designs, a bootstrap circuit is an arrangement of components deliberately intended to alter the input impedance of a circuit. ![]() Input impedance Bootstrap capacitors C1 and C2 in a BJT emitter follower circuit 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. 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". The same term has been used somewhat more generally for dynamically altering the operating point of an operational amplifier (by shifting both its positive and negative supply rail) in order to increase its output voltage swing (relative to the ground). In the domain of MOSFET circuits, bootstrapping is commonly used to mean pulling up the operating point of a transistor above the power supply rail. When applied deliberately, the intention is usually to increase rather than decrease the impedance. In the field of electronics, a technique where part of the output of a system is used at startup can be described as bootstrapping.Ī 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.
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