XPost: comp.sys.mac.system   
   From: burns4@nowhere.com   
      
   On 2/10/14, 7:51 PM, Salmon Egg wrote:   
   > In article , J Burns    
   > wrote:   
   >   
   >> I'm not used to the phrase "negative current feedback." I supposed it   
   >> could be used to protect the power transistors in an amplifier: if more   
   >> output current were sensed, the signal would be reduced in the preamplifier.   
   >>   
   >> If you put a resistor in series with a loudspeaker and the signal   
   >> voltage coming from the amplifier were constant, and the speaker   
   >> impedance changed, the voltage at the speaker would vary inversely with   
   >> the current.  That sounds like the effect of negative current feedback.   
   >   
   > Consider an amplifier with high gain that has two input terminals for   
   > signal input. If a small fraction of the output 1/m of the output is fed   
   > back to subtract from the signal going into the amplifier terminals, the   
   > output would be about m times the input, irrespective of the actual high   
   > gain of the amplifier. That also means that the output voltage will be   
   > held almost constant even with load changes--low impedance output.   
   >   
   > If feedback is voltage from a sampling resister Rf times the current   
   > flowing in the load, then the output current would be almost Vin * Rf   
   > irrespective of load impedance.   
   >   
   If what you're sampling is output current (I), it will increase if load   
   impedance decreases.  So I/m, the negative feedback, will increase.   
   Because it's negative feedback, the gain will vary in proportion to load   
   impedance.   
      
   If the amplifier gain is constant but you put resistance in series with   
   a load whose impedance varies, the load will also see a voltage that   
   varies with its impedance.  In this case, voltage will be approximately   
   output voltage times load impedance divided by the sum of load impedance   
   and series resistance.  (It won't be exactly that unless the phase of   
   the load impedance is zero.)   
      
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