XPost: sci.electronics.design   
   From: jeroen@nospam.please   
      
   Bitrex wrote:   
   > On 3/2/2011 8:06 PM, John Larkin wrote:   
   >> Actually, you can cascode a transistor into the source of a fet that   
   >> has a grounded gate. In that case, the source/collector voltage might   
   >> be a volt or two. You would have to look at the fet transfer curve,   
   >> and know the design operating current, to see exactly what that   
   >> voltage might be.   
   >   
   > I'm foggy on how such a cascode reduces noise - improved distortion,   
   > bandwidth, and PSRR I can understand but how does two transistors end up   
   > less noisy than one?  I know with tubes a cascode was considered a low   
   > noise alternative since two triodes in cascode would have lower noise   
   > than a single pentode, with similar gain.   
      
   As far as I can see, a cascode has the same noise as its bottom   
   transistor, near enough. The virtue of a cascode is that it   
   greatly reduces the effect of the Miller capacitance, so you   
   get more bandwidth and less input capacitance.   
      
   Jeroen Belleman   
      
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