BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 3, the LM4883 consists of two pairs of

operational amplifiers, forming a two-channel (channel A and

channel B) stereo amplifier. External feedback resistors

R

of Amp A (-out) and Amp B (-out) whereas two internal 20k

Ω

resistors set Amp A’s (+out) and Amp B’s (+out) gain at 1.

The LM4883 drives a load, such as a speaker, connected

between the two amplifier outputs, −OUTA and +OUTA.

Figure 3 shows that Amp A’s (-out) output serves as Amp A’s

(+out) input. This results in both amplifiers producing signals

identical in magnitude, but 180˚ out of phase. Taking advan-

tage of this phase difference, a load is placed between

−OUTA and +OUTA and driven differentially (commonly re-

ferred to as “bridge mode”). This results in a differential gain

of

A

(1)

or

A

Bridge mode amplifiers are different from single-ended am-

plifiers that drive loads connected between a single amplifi-

er’s output and ground. For a given supply voltage, bridge

mode has a distinct advantage over the single-ended con-

figuration: its differential output doubles the voltage swing

across the load. This produces four times the output power

when compared to a single-ended amplifier under the same

conditions. This increase in attainable output power as-

sumes that the amplifier is not current limited or that the

output signal is not clipped. To ensure minimum output sig-

nal clipping when choosing an amplifier’s closed-loop gain,

refer to the Audio Power Amplifier Design section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

channel A’s and channel B’s outputs at half-supply. This

eliminates the coupling capacitor that single supply, single-

ended amplifiers require. Eliminating an output coupling ca-

pacitor in a single-ended configuration forces a single-supply

amplifier’s half-supply bias voltage across the load. This

increases internal IC power dissipation and may perma-

nently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful single-ended or bridged amplifier. Equation (2)

states the maximum power dissipation point for a single-

ended amplifier operating at a given supply voltage and

driving a specified output load.

P

Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is higher internal

power dissipation for the same conditions.

The LM4883 has two operational amplifiers per channel. The

maximum internal power dissipation per channel operating in

the bridge mode is four times that of a single-ended ampli-

fier. From Equation (3), assuming a 5V power supply and a

4

Ω load, the maximum single channel power dissipation is

1.27W or 2.54W for stereo operation.

P

Bridge Mode

(3)

The LM4883SQ’s power dissipation is twice that given by

Equation (2) or Equation (3) when operating in the single-

ended mode or bridge mode, respectively. Twice the maxi-

mum power dissipation point given by Equation (3) must not

exceed the power dissipation given by Equation (4):

P

θ

JA

(4)

The LM4883’s T

to a DAP pad that expands to a copper area of 5in

200887A4

* Refer to the section Selecting Proper External Components, for a detailed discussion of C5 size.

FIGURE 3. Typical Audio Amplifier Application Circuit

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