Microsemi

Linfinity Microelectronics Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 9

Copyright

© 2000

Rev. 1.1, 2000-12-01

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LX1710 Filter Implementation, 1-stage vs. 2- stage

0.001

100

0.002

0.005

0.01

0.02

0.05

0.1

0.2

0.5

1

2

5

10

20

50

0.55626

0.12572

50m

30

100m

200m

500m

1

2

5

10

1.15

Watts(W)

2-Stage

1-Stage

A 1-stage or 2-stage filter may be used depending on

your application and performance targets. The main

tradeoff

in

this

selection

is

price

(number

of

components,

component

costs,

PCB

area)

vs.

performance.

The primary advantage of the single

stage filter is lower cost whereas the main benefit to a

2-stage filter is that it will provide steeper attenuation.

This allows the corner frequency to be selected further

outside of the audio band (to minimize the effects of

impedance variations in the passband) and still

provide adequate RF attenuation.

Single Stage Filter Advantages

•

Low Cost: The 1-stage LC filter uses one half

the number of inductors/capacitors resulting in

a substantial cost savings over a 2-stage

design.

Key parameters such as THD+N,

frequency response, and nose performance

do not change significantly.

•

Power Loss: Since current will flow in two

inductors and not four, the inductor power loss

will be less in the single stage design.

The

overall amplifier will have a wider dynamic

range and improved efficiency.

•

Filter Design: This easy-to-design filter can

limit audio signal changes within +/- 3dB

across

the

audio

band

with

impedance

vari

"  )  *+ ,!  

a

steeper

rolloff

with

the

2-stage

filter,

impedance changes could result in a +/- 6dB

change.

•

THD: There are minimal differences between

the 1-stage and 2-stage implementations with

other parameters such as THD+N as seen in

the above graph.

Single Stage Filter Disadvantages

•

EMI and Switching Frequency: For the 1-

stage, the switching frequency must be higher

than 400kHz to ensure the corner frequency

will provide adequate amplifier performance in

the high end of the audio frequency range. If

too close to the desired audio band. A higher

oscillation

frequency

could

translate

into

greater MOSFET switching losses, slightly

lower efficiency, and increased EMI effects.

With a 2-stage 4th order filter, the switching

oscillation frequency could help minimize EMI

issues.

The output filter helps to reconstruct the amplified

audio signal and filter out the switching frequency.

The design of the filter depends on the type of

attenuation and frequency response desired at the

output.

The output filter designed into the LXE1710