LP3921

SNVS580A – AUGUST 2008 – REVISED MAY 2013

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Differential Amplifier Explanation

Table 22. Register Address 8h'19 Audio_Amp

Bit

Name

Function

(if the powerup default is "amplifier disabled")

0

amp_en

Bit set to '0' - amplifier disabled

Bit set to '1' - amplifier enabled

The LP3921 contains a fully differential audio amplifier that features 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 VDD/2. When setting the differential gain, the amplifier can

be considered to have "halves". Each half uses an input and feedback resistor (Ri1 and RF1) to set its respective

closed-loop gain. (See Figure 10.) With Ri1 = Ri2 and RF1 = RF2, the gain is set at -RF / Ri for each half. This

results in a differential gain of:

AVD =

−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. 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 LP3921 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 LP3921 simply

amplifies the difference between them.

A bridged configuration, such as the one used in the LP3921, also creates a second advantage over single

ended amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists

across the load. This assumes that the input resistor pair and the feedback resistor pair are properly matched.

BTL configuration eliminates the output coupling capacitor required in single supply, single-ended amplifier

configurations. If an output coupling capacitor is not used in a single-ended output configuration, the half-supply

bias across the load would result in both increased internal IC power dissipation as well as permanent

loudspeaker damage. Further advantages of bridged mode operation specific to fully differential amplifiers like

the LP3921 include increased power supply rejection ratio, common-mode noise reduction, and click and pop

reduction.

Figure 10. Audio Block

28

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