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The Famous Op-Amp

May 17th, 2006

Operational Amplifier or so called ‘op-amp’ is the famous solid-state IC that uses external feedbacks to contorl its functions. It has a very high gain with one inverting input (V-), one non-inverting input (V+), one output (Vout), positive and negative power supply (Vs+ and Vs-). Most of the time we look at op-amp as an ideal op-amp to simplify things. In real life applications, op-amp behave almost like an ideal op-amp usually in MOSFET type op-amp. In ideal op-amp, it has an infinite open-loop gain, infinite bandwidth, infinite input impedance and zero output impedance.

This is a picture of op-amps including the famous 741. These small ICs are cheap in the cents per piece. The DC behavior of such real op-amp for its open-loop gain usually in the 100k to millions. The real op-amp does not have infinite gain and bandwidth. When the two inputs are equal in voltages, then the output is zero volt. The current entering the two inputs are small or can be assume to be zero.

Op-amp is usually use for filter design. A simple sallen-key low pass filter is shown done on SPICE. It tooks less than 10 minutes to actually build the circuit using the computer aided design software and simulation shows almost everything than can be done hands on in laboratory. Simulation is a good way to check your work in laboratory.

The plot shown is the frequency response of the filter output. As we could see, this is a pretty simple low-pass filter that allow signals to be passed below the 5 kHz range and reject anything above 10 kHz. There are many other different filter techniques that can be used in filter design according to the specifications. Op-amp is widely used in analog signal processing systems and in many other applications.

Filed under: Analog Design