LM4894 datasheet(11/19 Pages) NSC

11 / 19 page

Typical Performance Characteristics

Non-LD Specific Characteristics (Continued)

PSRR vs

DC Common-Mode Voltage

PSRR vs

DC Common-Mode Voltage

20013780

20013781

THD vs

Common-Mode Voltage

THD vs

Common-Mode Voltage

20013782

20013783

Application Information

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4894 is a fully differential audio amplifier that fea-

tures differential input and output stages. Internally this is

accomplished by two circuits: a differential amplifier and a

common mode feedback amplifier that adjusts the output

voltages so that the average value remains V

DD/2. When

setting the differential gain, the amplifier can be considered

to have "halves". Each half uses an input and feedback

resistor (R

i1and RF1) to set its respective closed-loop gain

(see Figure 1). With R

i1 =Ri2 and RF1 =RF2, the gain is set

at -R

F/Ri for each half. This results in a differential gain of

VD =-RF/Ri

(1)

It is extremely important to match the input resistors to each

other, as well as the feedback resistors to each other for best

amplifier performance. See the Proper Selection of Exter-

nal Components section for more information. A differential

amplifier works in a manner where the difference between

the two input signals is amplified. In most applications, this

would require input signals that are 180˚ out of phase with

each other. The LM4894 can be used, however, as a single

ended input amplifier while still retaining its fully differential

benefits. In fact, completely unrelated signals may be placed

on the input pins. The LM4894 simply amplifies the differ-

ence between them. Figures 2 and 3 show single-ended

applications of the LM4894 that still take advantage of the

differential nature of the amplifier and the benefits to PSRR,

common-mode noise reduction, and "click and pop" reduc-

tion.

All of these applications, either single-ended or fully differ-

ential, provide what is known as a "bridged mode" output

(bridge-tied-load, BTL). This results in output signals at V

and V

o2 that are 180˚ out of phase with respect to each

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