Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4870 has two operational am-

plifiers internally, allowing for a few different amplifier con-

figurations. The first amplifier’s gain is externally config-

urable, while the second amplifier is internally fixed in a

unity-gain, inverting configuration. The closed-loop gain of

the first amplifier is set by selecting the ratio of R

the second amplifier’s gain is fixed by the two internal 40 k

Ω

resistors.

Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both am-

plifiers producing signals identical in magnitude, but out of

phase 180˚. Consequently, the differential gain for the IC is:

A

By driving the load differentially through outputs V

V

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Consequently, four times the output power is

possible as compared to a single-ended amplifier under the

same conditions. This increase in attainable output power

assumes that the amplifier is not current limited or clipped. In

order to choose an amplifier’s closed-loop gain without caus-

Supply Current Distribution

vs Temperature

DS100094-19

Power Dissipation

vs Output Power

DS100094-20

Output Power vs

Load Resistance

DS100094-21

Output Power vs

Supply Voltage

DS100094-22

Output Power vs

Supply Voltage

DS100094-27

Open Loop

Frequency Response

DS100094-23

Power Supply

Rejection Ratio

DS100094-24

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