LM3876

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SNAS090D – APRIL 1995 – REVISED MARCH 2013

k

Ω–100 kΩ), and the other feedback resistance, Ri, is calculated using standard op amp configuration gain

equations. Most audio amplifiers are designed from the non-inverting amplifier configuration.

DESIGN A 40W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

40W

Load Impedance

8

Ω

Input Level

1V(max)

Input Impedance

100 k

Ω

Bandwidth

20 Hz–20 kHz ± 0.25 dB

Equation 6 and Equation 7 give:

40W/8

Ω

(10)

Therefore the supply required is: ±30.3V at 3.16A

With 15% regulation and high line the final supply voltage is ±38.3V using Equation 8. At this point it is a good

idea to check the Figure 25 to ensure that the required output power is obtainable from the device while

maintaining low THD + N. It is also good to check the Power Dissipation vs Supply Voltage to ensure that the

device can handle the internal power dissipation. At the same time designing in a relatively practical sized heat

sink with a low thermal resistance is also important. Refer to Typical Performance Characteristics graphs and

THERMAL CONSIDERATIONS for more information.

The minimum gain from Equation 9 is:

We select a gain of 21 (Non-Inverting Amplifier); resulting in a sensitivity of 894 mV.

unless an additional input impedance was placed in series with the 10 k

Ω potentiometer that is depicted in

Figure 1. Adding the additional 100 k

Ω resistor would ensure the minumum required input impedance.

(11)

The bandwidth requirement must be stated as a pole, i.e., the 3 dB frequency. Five times away from a pole gives

0.17 dB down, which is better than the required 0.25 dB. Therefore:

(12)

(13)

At this point, it is a good idea to ensure that the Gain-Bandwidth Product for the part will provide the designed

gain out to the upper 3 dB point of 100 kHz. This is why the minimum GBWP of the LM3876 is important.

GBWP

(14)

GBWP = 2.0 MHz (min) for the LM3876

(15)

Solving for the low frequency roll-off capacitor, Ci, we have:

Ci

(16)

Definition of Terms

Input Offset Voltage: The absolute value of the voltage which must be applied between the input terminals

through two equal resistances to obtain zero output voltage and current.

Input Bias Current: The absolute value of the average of the two input currents with the output voltage and

current at zero.

Input Offset Current: The absolute value of the difference in the two input currents with the output voltage and

current at zero.

Input Common-Mode Voltage Range (or Input Voltage Range): The range of voltages on the input terminals

for which the amplifier is operational.

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