Op-amp Basics

An Operational Amplifier, or op-amp is a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. It is a high-gain electronic voltage amplifier with a differential input and usually a single-ended output. Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices.

However, an op-amp is just one type of differential amplifier. Other include,

· A fully differential amplifier which is like an op-amp, but with two outputs.

· The instrumentation amplifier which is usually built from three op-amps,

· The isolation amplifier which is like an instrumentation amplifier, but with tolerance to common-mode voltages that would destroy an ordinary op-amp

· A negative-feedback amplifier which is usually built from one or more op-amps and a resistive feedback network.

Op-amp operation

The amplifier’s differential inputs consist of a non-inverting input with voltage (V+) and an inverting input with voltage (V−). Ideally, an op-amp amplifies only the difference in voltage between the two, also called differential input voltage. The output voltage of the op-amp V_out is given by the equation,

V_out = A_OL (V₊ – V_–)

where A_OL is the open-loop gain of the amplifier. In a linear operational amplifier, the output signal is the amplification factor, known as the amplifiers gain (A) multiplied by the value of the input signal.

Op-amp parameters

· Open-loop gain is the gain without positive or negative feedback. Ideally, the gain should be infinite, but typical real values range from about 20,000 to 200,000 ohms.

· Input impedance is the ratio of input voltage to input current. It is assumed to be infinite to prevent any current flowing from the source to amplifiers.

· The output impedance of the ideal operational amplifier is assumed to be zero. This impedance is in series with the load, thereby increasing the output available for the load.

· Bandwidth of an ideal operational amplifier is infinite and can amplify any frequency signal from DC to the highest AC frequencies. However, typical bandwidth is limited by the Gain-Bandwidth product. GB product is equal to the frequency where the amplifiers gain becomes unity.

· The ideal output of an amplifier is zero when the voltage difference between the inverting and the non-inverting inputs is zero. Real world amplifiers do exhibit a small output offset voltage.

An op-amp only responds to the difference of the two voltages irrespective of the individual values at the inputs. External resistors or capacitors are often connected to the op-amp in many ways to form basic circuits including Inverting, Non-Inverting, Voltage Follower, Summing, Differential, Integrator and Differentiator type amplifiers. Op-amp is easily available in IC packaging, the most common os whom is the μA-741.