Output Power vs Supply Voltage

Output Power vs Supply Voltage

10104231

10104232

Supply Current vs Supply Voltage

10104233

Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4875 consists of two opera-

tional amplifiers internally. An external DC voltage sets the

closed-loop gain of the first amplifier, whereas two internal

20k

Ω resistors set the second amplifier’s gain at -1. The

LM4875 can be used to drive a speaker connected between

the two amplifier outputs or a monaural headphone con-

nected between V

Figure 1 shows that the output of Amp1 serves as the input

to Amp2. This results in both amplifiers producing signals

that are identical in magnitude, but 180˚ out of phase.

Taking advantage of this phase difference, a load placed

between V

referred to as “bridge mode“ ). This mode is different from

single-ended driven loads that are connected between a

single amplifier’s output and ground.

Bridge mode has a distinct advantage over the single-ended

configuration: its differential drive to the load doubles the

output swing for a specified supply voltage. This results in

four times the output power when compared to a single-

ended amplifier under the same conditions. This increase in

attainable output assumes that the amplifier is not current

limited or the output signal is not clipped. To ensure mini-

mum output signal clipping when choosing an amplifier’s

closed-loop gain, refer to the Audio Power Amplifier De-

sign section.

Another advantage of the differential bridge output is no net

DC voltage across load. This results from biasing V

V

that single supply, single-ended amplifiers require. Eliminat-

ing an output coupling capacitor in a single-ended configu-

ration forces a single supply amplifier’s half-supply bias volt-

age across the load. The current flow created by the half-

supply bias voltage increases internal IC power dissipation

and may permanently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful bridged or single-ended amplifier. Equation (1)

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

(1)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation point for a bridge amplifier oper-

ating at the same given conditions.

P

Bridge Mode

(2)

The LM4875 has two operational amplifiers in one package

and the maximum internal power dissipation is 4 times that

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