Transistor Bootstrapping Circuit Using BJT

Bootstrapping is a technique used in the design of transistor amplifier circuits to increase the input impedance and thereby reduce the loading effects on the input source. It typically involves the use of a bootstrap capacitor, which provides positive feedback of ac signals to the base junction of a transistor in an emitter follower circuit. Application of this feedback increases the effective value of the base resistance. The amount of increase is a factor determined by the voltage gain of the circuit.

A bipolar junction transistor (BJT) has an inherently low input impedance of typically 1 Ω to 50 kΩ, and this reduces further after the addition of biasing transistors such as a potential divider network connected to the base junction of a transistor. Whilst it improves dc stability, these resistors have the effect of shunting the input thereby reducing the input resistance. These types of circuits usually pose a problem where high input impedance is required, and therefore bootstrapping is a technique that helps increase the input impedance.

A high impedance input is very useful for example in radio circuits where the base junction of a transistor connects to a ferrite coil to amplify the radio signal. Usually the amplifier stage is required to minimize the loading on the coil for better amplification. It is also useful in instrumentation where you need to take accurate measurements using probes such as bio-probes.

A common emitter follower circuit such as the one shown above will achieve a one hundred-fold increase in the effective value of Rb, when the voltage gain is 0.99. For a further increase in impedance, usually a two stage direct-coupled circuit is used.

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