LM4950

SNAS174E – JULY 2003 – REVISED MAY 2013

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APPLICATION INFORMATION

HIGH VOLTAGE BOOMER WITH INCREASED OUTPUT POWER

Unlike previous 5V Boomer amplifiers, the LM4950 is designed to operate over a power supply voltages range of

9.6V to 16V. Operating on a 12V power supply, the LM4950 will deliver 7.5W into an 8

Ω BTL load with no more

than 10% THD+N.

Figure 65. Typical LM4950 BTL Application Circuit

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 65, the LM4950 consists of two operational amplifiers that drive a speaker connected

between their outputs. The value of external input and feedback resistors determine the gain of each amplifier.

Resistors RINA and RFA set the closed-loop gain of AMPA, whereas two 20kΩ resistors set AMPB's gain to -1.

The LM4950 drives a load, such as a speaker, connected between the two amplifier outputs, VOUTA and

VOUTB. Figure 65 shows that AMPA's output serves as AMPB's input. This results in both amplifiers producing

signals identical in magnitude, but 180° out of phase. Taking advantage of this phase difference, a load is placed

between AMPA and AMPB and driven differentially (commonly referred to as "bridge mode"). This results in a

differential, or BTL, gain of

AVD = 2(Rf/ Ri)

(1)

Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single

amplifier's output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-

ended configuration: its differential output doubles the voltage swing across the load. Theoretically, this produces

four times the output power when compared to a single-ended amplifier under the same conditions. This increase

in attainable output power assumes that the amplifier is not current limited and that the output signal is not

clipped. To ensure minimum output signal clipping when choosing an amplifier's closed-loop gain, refer to AUDIO

POWER AMPLIFIER DESIGN.

Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by

biasing AMP1's and AMP2's outputs at half-supply. This eliminates the coupling capacitor that single supply,

single-ended amplifiers require. Eliminating an output coupling capacitor in a typical single-ended configuration

forces a single-supply amplifier's half-supply bias voltage across the load. This increases internal IC power

dissipation and may permanently damage loads such as speakers.

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