Chapter 9.2

Operating Without Feedback

The common configuration of an Op Amp is to include a feedback circuit. Without any feedback the amplification function, as we know it, ceases and the device operated more like a switch with three output states, low-level, mid-level and high-level output voltage.

General Output Voltage States with Feedback

 1) The low output voltage state is when
    the non-inverting input voltage is
    below the inverting voltage.  This
    causes the output voltage to
    decrease, moving very rapidly to a
    point close to the V- voltage.  I
    will call this State-1.

 2) The mid-point output voltage is set somewhere between the rails
    and the inputs are at the same potential. I will call this State-2.

 3) The high output voltage state occurs anytime the non-inverting
    input voltage is above the inverting voltage. This causes the
    output voltage to increase, moving very rapidly to a point
    close to the V+ voltage. I will call this State-3.

Unity Gain Amplifier:

The simplest configuration is the unity gain amplifier which has a gain of one(1). What that means is that for every change in the non-inverting input voltage, an identical change occurs in the output voltage level.

 Unity Gain Amplifier Characteristics:
  1) The output voltage is tied directly back to the inverting pin.
  2) The input impedance is very high at over 1M ohm.
     Impedance is a resistance load for changing voltages.  This 
     means that this circuit will not consume much current from
     the circuit feeding it.
  3) The output impedance is low.  This means this circuit can
     supply power to the circuit it feeds.

This amplifier is generally used as a buffer amplifier. It is placed between some input device, like a microphone, and some output device, like a power amplifier. It provides the same output voltage as the input voltage but can deliver more current thus it can boost the power of the signal in order to drive down stream devices without consuming to much power from the input device or input signal.

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