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Bootstrapped circuit


 
12/11/1997 12:58 AM
Karl Zwengel
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Bootstrapped circuit
I've been rooting around in some older circuit books lately and have come aross a term that I haven't heard before. Bootstrapping.  
 
What is it and what is the purpose? It looks like it may have something to do with high frequency positive feedback. Anybody know what's going on?  
thanks  
Karl
 
12/11/1997 3:08 AM
Dave Harris
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Bootstrapping is positive feedback but it does not have to be at high frequency. I have seen it used in old bipolar transistor AF input stages to increase the input impedance. Newer circuits usually use a FET. It is still used in transistor power amp output stages. The class A drive transistor is bootstrapped half way up its load resistor with a capacitor from the output. The collector load then looks like a constant current so the gain is increased and the amp output can get closer to the supply rail. I have that output stage in my home-brew bass amp.
 
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