Operational Amplifier – Notes – 1

1. The Op Amp is a widely used amplifier abstraction that forms the foundations of much of electronic circuit design. Op Amp devices are constructed using primitive elements such as transistors and resistors.

2. The Op Amp is a four-ported device. The ports include an input port with terminals usually labeled v+ and v , an output port with one terminal labeled vo, and the other being ground, a positive power supply port with a +VS voltage applied with respect to ground, and a negative power supply port with a −VS voltage applied with respect to ground. Although the ground terminal is not explicitly shown in the Op Amp symbol, it is very much a part of all Op Amp circuits.

3. The Op Amp behaves like a voltage-dependent voltage source. Its input output relationship can can be expressed mathematically as 

vo=A(v+−v)

where A is a large number called the open loop gain of the amplifier. In most practical Op Amp applications, A is treated as infinity.

4. Most useful Op Amp circuits are built using the negative feedback connection, in which a portion of the output signal of the Op Amp is fed back to the v input of the Op Amp. Examples of Op Amp circuits built this way include inverting and non-inverting amplifiers, buffers, adders, integrators, and differentiators.

5. We commonly apply the constraint : 

v+≈v 

in analyzing Op Amp circuits, if the Op Amp is not saturated and the feedback is negative.

6. Op Amp circuits are sometimes built using the positive feedback connection, in which a portion of the output signal of the Op Amp is fed back to the v+ input of the Op Amp. Examples of such Op Amp circuits include oscillators and comparators.

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