Dual bootstrap preamplifier with high input impedance

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Dual bootstrapped preamplifer

The voltage follower Q2 bootstraps R5 to give it a greatly increased apparent resistance which both increases gain and reduces distortion. Q1 is biased through R7 and R8.  The voltage at the emitter of Q1 follows that at the input C2 therefore C2 bootstraps R8 and greatly increases its apparent resistance and also stops undesirable negative feedback through R7 and R8.  R6 by itself increases the input impedance somewhat and improves linearity. Negative feedback from the collector of Q2 controls the gain of the circuit and improves the input impedance still further.  The overall gain can be controlled by adjusting R6. 

Dual bootstrap preamplifier with CFP

Some further improvement can be obtained by replacing the PNP emitter follower by a complimentary feedback pair (CFP) follower. 


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