OP-AMP

why the point P is referred to as a virtual earth 3M
  gain of op-amp is VERY LARGE / infinite
  non-inverting input is at earth / 0V
  for amplifier not to saturate, P must be at about earth /0V
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By reference to an amplifier, explain what is meant by negative feedback 2M
  (part of) the output is added to /returned to / mixed with the input
  and is out of phase with the input / fed to inverting input
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N09 42 N9 suggest a use for the circuit
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State two effects of negative feedback on an amplifier 2M
   reduces gain
   increases bandwidth
   less distortion
   greater stability
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An operational amplifier (op-amp) may be used as a comparator, State the function of a comparator 2M
  to compare two potentials / voltages
  output depends upon which is greater
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J11 43 Describe  the  structure  of  a  metal  wire  strain  gauge. You  may  draw  a  diagram  if  you wish 3M
 THIN / FINE metal wire
 lay-out shown as a grid
 encased in plastic
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State three characteristics of an ideal operational amplifier (op-amp) 3M
  infinite (voltage) gain
  infinite input impedance
  zero output impedance
  infinite bandwidth
  infinite slew rate
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J11 41 The output of the amplifier circuit of Fig. 9.1 may be connected to a relay, State and explain one purpose of a relay 2M
  either  relay can be used to switch a large current/voltage
          output current of op-amp is a few mA/very small
  or      relay can be used as a remote switch
          for inhospitable region/avoids using long heavy cables
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