Test & Soldering Equipment On-line Store

D.E.V.I.C.E.
(Wiki)Calculators
Services

Operational amplifier

An operational amplifier (op-amp) is an electronic amplifier of direct current, with a differential input and usually one output. Such an amplifier has a very high gain.

History

The operational amplifier was originally developed in the 1930s. The operational amplifier was designed to perform mathematical operations (hence the name - operational amplifier) as part of the first analog electronic computers (with mathematical operations - addition, subtraction, integration, differentiation, etc.) In the 60s an American Fairchild Semiconductor company developed the world's first op-amp in the form of a microcircuit.

Operating principle

The differential input (from the Latin differentia - "difference") of the device consists of a non-inverting (V_in+) and inverting (V_in-) inputs. See Fig. 1.

Fig. 1. Operational amplifier - conventional graphic designation

An ideal op-amp amplifies the difference in voltage at the inputs of the operational amplifier, while the common-mode signal (that is, coinciding in magnitude and phase at both inputs) does not affect the resulting voltage.

To calculate the output voltage V_out at the output of an ideal operational amplifier, the formula is used:

V_out = K × (V₁ − V₂)

where:

V₁ and V₂ are the voltage at the non-inverting and inverting inputs, respectively;
K is the gain in the absence of feedback.

In practice, such a circuit (without feedback) is not used, with the exception of the comparator circuit. Most commonly used in 741 and LM308 chips.

Comparison of parameters of ideal and real operational amplifier

Parameter	Ideal operational amplifier	Real operational amplifier
Operational amplifier gain without feedback (K)	Equals infinity	Is in the range 10⁴ ÷ 10⁵
Output voltage (V_out)	In the range from minus infinity to plus infinity	Vout is limited by the supply voltage range
Input resistance	Equals infinity	The resistance is very high and has little effect on the input signal
Input current and offset voltage	The zeros	They have small but real meanings
Bandwidth	Infinitely large	It is impossible to implement this in practice
Output voltage slew rate	Infinitely large	It is impossible to implement this in practice
Intrinsic noise	Complete absence of own noise	It is impossible to implement this in practice
Common mode rejection ratio	Infinitely large	It has great importance, but not infinite
Influence of power supply noise and ripple	Complete absence	Little influence
Output resistance	Equals zero	In a real op amp the output resistance is not zero, so the output current affects the output voltage (as the load resistance decreases, the output voltage decreases)