Fully balanced operational amplifier

A fully balanced operational amplifier ( english Fully differential amplifier , shortly FDA ) is a variant of an operational amplifier , are made differentially at the input and output. Like a normal operational amplifier, it has a very high gain and is designed for the use of an external negative feedback . The two outputs result in two negative feedback paths.

The fully symmetrical design has the advantage of being less sensitive to common-mode interference , such as can come from the operating voltages. In addition, the required operating voltage for a certain signal level is reduced. Both are advantages in highly complex mixed-signal circuits, which is why fully symmetrical operational amplifiers are primarily found as circuit elements in complex integrated circuits . However, they are also offered as individual components and are mainly used for analog signal processing together with analog-digital converters and digital-analog converters .

General

Circuit symbol

If the FDA is assumed to be ideal, the input voltage is formed as the difference between the two input voltages: ${\ displaystyle U_ {id}}$

{\ displaystyle U_ {id} = U_ {in +} - U_ {in-}}

From this, a finite gain for the respective application and the output voltage as the difference between the two outputs are formed via the external negative feedback : ${\ displaystyle A_ {g}}$ ${\ displaystyle U_ {od}}$

{\ displaystyle U_ {od} = U_ {out +} - U_ {out -} = U_ {id} \ cdot A_ {g}}

The common-mode voltage of the two outputs in relation to the reference potential can be selected within certain limits with many FDA via a separate control input. The common-mode voltage from the output is independent of the common-mode voltage at the input and symmetrical to the two voltage outputs. In practical applications, such as in analog-digital converters, the common-mode voltage at the output is selected via the external control input to be, for example, half the reference voltage of the AD converter. ${\ displaystyle U_ {oc}}$

literature

Jim Karki, Texas Instruments: Fully differential amplifiers . ( PDF ).
Jim Karki, Texas Instruments: Analysis of fully differential amplifiers . ( PDF ).

Individual evidence

↑ High-speed fully differential I / O amplifiers. Texas Instruments corporate publication, THS414x data sheet, accessed January 13, 2018 .

[ths41-1] High-speed fully differential I / O amplifiers. Texas Instruments corporate publication, THS414x data sheet, accessed January 13, 2018 .