Stereo 3W Audio Power Amplifiers with Headphone Drive and Input Mux
Component Selection
⎛ 10kΩ ⎞ | ⎛ | R | ⎞ | |||
AVIN = − ⎜ |
| ⎟ | , AVOUT = − ⎜ | F | ⎟ | |
RIN | 10kΩ | |||||
⎝ | ⎠ | ⎝ | ⎠ | |||
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Combining AVIN and AVOUT, RIN and RF set the single- ended gain of the device as follows:
⎛ 10kΩ ⎞ | ⎛ | R | ⎞ | ⎛ | R | ⎞ | ||
AV = AVIN ⋅ AVOUT = − ⎜ |
| ⎟ | ⋅ − ⎜ | F | ⎟ | = +⎜ | F | ⎟ |
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⎝ | RIN | ⎠ | ⎝ | 10kΩ | ⎠ | ⎝ RIN ⎠ | ||
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As shown, the
Input Filter The input capacitor (CIN), in conjunction with RIN, forms a highpass filter that removes the DC bias from an incoming signal. The
1
f−3dB = 2πRINCIN
Choose RIN according to the
Other considerations when designing the input filter include the constraints of the overall system, the actual frequency band of interest, and
the load impedance form a highpass filter with a
1
f−3dB = 2πRLCOUT
As with the input capacitor, choose COUT such that
f is well below the lowest frequency of interest.
Setting f too high affects the amplifier‘s
Load impedance is a concern when choosing COUT. Load impedance can vary, changing the
If layout constraints require a physically smaller output- coupling capacitor, decrease the value of COUT and add series resistance to the output of the MAX9777/MAX9778 (see Figure 9). With the added series resistance at the output, the cutoff frequency of the highpass filter is:
f−3dB | = | 1 | |
2π(RL + RSERIES )COUT | |||
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Since the cutoff frequency of the output highpass filter is inversely proportional to the product of the total load resistance seen by the outputs (RL + RSERIES) and
COUT, increase the total resistance seen by the MAX9777/MAX9778 outputs by the same amount COUT is decreased to maintain
AV _ COMP = 20log⎛⎜ RL + RSERIES ⎞⎟
⎝ RL ⎠
COUTRSERIES
OUT_+
RL
Figure 9. Reducing COUT by Adding RSERIES
MAX9777/MAX9778
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