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Ideal Amplifier Approximation

The following are properties of an ideal amplifier, which to a good approximation are obeyed by an operational amplifier:

  1. large forward transfer function,
  2. virtually nonexistent reverse transfer function,
  3. large input impedance, (any signal can be supplied to the op-amp without loading problems),
  4. small output impedance, (the power supplied by the op-amp is not limited),
  5. wide bandwidth, and
  6. infinite gain, .

If these approximations are followed two rules can be used to analyze op-amp circuits:

Rule 1:
The input currents and are zero, ( ).
Rule 2:
The voltages and are equal, ( ).

To apply these rules requires negative feedback.

Feedback is used to control and stabilize the amplifier gain. The open-loop gain is too large to be useful since noise will causes the circuit to clip. Stabilization is obtained by feeding the output back into the input (closed negative feedback loop). In this way the closed-loop gain does not depend on the amplifier characteristics.

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999