Download RF Filtering for Audio Amplifier Circuits

RF Filtering for Audio Amplifier Circuits
Summary
Today, as more EMC regulations are imposed by governments, or self-imposed
by industry, the need for cost effective filtering for audio amplifier circuits is more
important than ever. Audio amplifiers are used in everything from car and home
stereos to portable CD devices and MP3 players. Any device that outputs sound
to a speaker requires an audio amplifier and their use has increased dramatically
along with the growing numbers of consumer electronic devices.
Changes in the electromagnetic interference (EMI) filter technology, combined
with pressures to keep consumer electronics low cost, have altered the way that
audio amplifier circuits are filtered. What may have been overlooked as these
changes occurred was the DC resistance added to the circuit as designers moved
from a standard through-hole type ceramic capacitor to the multilayer chip
capacitor (MLCC) feedthrough device. Prohibitive costs for the through-hole type
capacitors and the frequency limitations of standard MLCC capacitors have forced
engineers to use the newer chip feedthrough components to bridge the
price/performance gap.
The next generation of audio filtering technology has arrived. This solution will
lower overall system cost and boost performance. One single X2Y® component
can replace multiple standard filtering devices and offer a higher and broader
frequency filtering band with no DC resistance added to the circuit.
Introduction to
Car Audio
In the past, many in the automotive industry used ceramic through-hole
capacitors to filter the audio amplifier in car radios. These popular devices
provided high insertion loss with no effect on the power output of the audio
amplifier because wire was fed through a hole in the center of the device, which
was mounted to a grounded plate (Figure 1). As DC current traveling through the
wire passed through the filter device, EMI (RF) noise was shunted to a circuit
ground reference.
Figure 1.
Standard “through-hole” feedthrough capacitors.
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Note# 2003, v3.0, 4/20/05
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RF Filtering for Audio Amplifier Circuits
Consumer and Original Equipment Manufacturer (OEM) price pressures fuel the
demand for alternative solutions. Ongoing research by passive component
manufacturers has led to the development of the four terminal surface mount
chip feedthrough capacitor (Figure 2).
Figure 2.
Chip “feedthrough” capacitors in a circuit.
The newer type chip feedthrough seemed a natural replacement for its cousin,
the through-hole feedthrough capacitor and could offer lower costs in a number
of ways.
1. A simplified component design lowers production costs.
2. No need for a mounting plate for the chip feedthrough, which can be
surface mounted on a PCB.
3. Labor costs are lowered through the use of automated pick and place
machines during production.
Although both capacitors are called “feedthrough”, the method by which the DC
current is fed through the two devices differs dramatically. The chip feedthrough
does not have a hole-through for the wire lead; instead it must carry the current
through the internal electrode plates. In doing so, DC resistance is added to the
circuit.
DC Resistance
When current is fed through a chip feedthrough device, the DC resistance
associated with the component reduces the power output of the audio amplifier.
This is not just true for a chip feedthrough capacitor, but for any filter component
that offers resistance to the circuit.
The DC resistance of a typical ceramic chip feedthrough is approximately ≤ 0.6 Ω
for 0805 to 1206 and ≤ 0.4 Ω for a 1608 size chip ferrite bead device (both rated
for 300mA current). The Bridged-output amplifier Power Maximum Equation
(National Semiconductor Lm 4862 Boomer) describes the relationship of power to
resistance1.
PwrMax =
Note# 2003, v3.0, 4/20/05
4(Vdd ) 2 2(Vdd ) 2
= 2
2π 2 R L
π RL
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RF Filtering for Audio Amplifier Circuits
Car Audio Power Example
Given:
LM 4862 Amplifier
Vdd = 3V
RSpeaker = 8Ω
RFilter = 0.4Ω per filter (Typical)
Using the Power Maximum Equation, first find the maximum power supplied by
the audio amplifier without filters (Figure 3). PwrMax Amp w/o Filter represents the
power out of the speaker because there is no intermediate circuitry between the
amplifier and the speaker.
PwrMax Amp w/o Filter = PSpeak w/o Filter =
Figure 3.
2(Vdd ) 2 2(3) 2
=
= 228.0mW
π 2 R L π 2 (8)
LM 4862 Amp without filters connected to 8Ω speaker.
Now find the maximum power supplied by the audio amplifier with the filters
added. Note by adding a 0.4Ω DC filter to each conductor (Figure 4) will increase
the resistance as seen by the amplifier.
R OUT_TOTAL = R Speaker + (2) R Filter = 8 + (2) (0.4) = 8.8Ω
PwrMax Amp w/ Filter
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2(Vdd ) 2
2(3) 2
= 2
=
=
π R L π 2 (8.8)
207.2mW
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RF Filtering for Audio Amplifier Circuits
Figure 4.
LM 4862 Amp with feedthrough filters connected to 8Ω speaker.
Now the output power loss and output power loss percentage from the amplifier
can be calculated.
PwrLOSS = PwrMax Amp w/o Filter - PwrMax Amp w/ Filter
= 228mW - 207.2mW
= 20.8mW
⎛ PwrMax Amp w/ Filter
PwrLOSS % = ⎜1 ⎜ PwrMax
Amp w/o Filter
⎝
⎛ 207.2mW ⎞
<OR>
= ⎜1 ⎟ × 100
228mW ⎠
⎝
= 9.12%
⎞
⎟ × 100
⎟
⎠
The filters are an intermediate between the amplifier and the speaker, so now
calculate output power loss and output power loss percentage of the speaker.
PSpeaker w/Filter =
R Speaker
R OUT_TOTAL
× PwrMaxAmp w/ Filter
(8)
× 207.2mW
(8.8)
= 188.36mW
=
⎛
PwrMax Amp w/ Filter ⎞
⎟ × 100
PwrLOSS % = ⎜1 −
⎜ PwrMax
⎟
Amp w/o Filter ⎠
⎝
⎛ 188.36mW ⎞
<OR>
= ⎜1 ⎟ × 100
228mW ⎠
⎝
= 17.5%
Another factor to consider by adding DC resistance is damping of the loudspeaker
which affects the sound quality of the bass. When resistance is added to
R OUT_TOTAL , it decreases the damping factor. A high series resistance and a low
damping factor will give a ‘boomy’ bass, in contrast with a "tense" sounding bass
for a high damping factor. The Damping Factor is calculated by the following
formula:
Damping Factor =
Note# 2003, v3.0, 4/20/05
R Speaker + R OUT_TOTAL
R OUT_TOTAL
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RF Filtering for Audio Amplifier Circuits
X2Y® Bypass
Filter
An X2Y® component supplies no new DC resistance to the circuit over the trace
or wire resistance. A single component is placed in bypass between the two lines
and the ground terminals of the component are attached to the circuit ground
reference, resulting in no power loss to the speaker circuit (Figure 5).
Figure 5.
Example showing application of a single X2Y® bypass capacitor.
Since the X2Y® chip filter is in bypass (Figure 5), the series DC resistance in the
circuit is 0. Hence the bridged-output amplifier power equation is the same as the
example shown earlier without a filter in the circuit.
PwrMax Amp w/ X2Y Filter
Comparing Filter
Performance
2(Vdd ) 2 2(3) 2
= 2
=
= 228.0mW
π R L π 2 (8)
A natural question is ‘how can a bypass capacitor attenuate noise in a manner
equal to a feedthrough capacitor?’ Consider for a moment a standard two
terminal capacitor and the flux field it creates (Figure 6). By placing two standard
capacitors in opposite directions some flux cancellation can occur and improve
circuit performance2, 3.
Figure 6.
Two closely placed standard capacitors. A partial flux cancellation occurs on the
outside of the components.
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RF Filtering for Audio Amplifier Circuits
An X2Y® component takes the concept of flux cancellation a step further. The
internal electrode design forces opposing current to flow internally in a single
component (Figure 7). The spacing from opposing electrodes to a nearby shared
ground layer is only a dielectric layer apart. Flux cancellation reduces the internal
mutual inductance and is achieved whether X2Y® is applied single ended or
differentially. The net result is a bypass capacitor with improved broadband and
high frequency performance yielding significant attenuation in common mode
noise.
Figure 7.
Flux cancellation happens internally in an X2Y®.
An added benefit to the X2Y® design is that differential mode noise is also filtered
simultaneously along with the common mode noise. This is due to the internal
“X” capacitor connection between the power and return lines. Feedthrough
capacitors can only filter the common mode noise and requires an additional “X”
capacitor for differential mode filtering.
Other advantages of a single X2Y® component solution include:
1. Using one bypass component versus two or more resistive devices
resulting in reduced component count on a PCB.
2. Better balance between power and return lines to ground: capacitance
tolerance between the internal line to ground capacitors is typically 13% or less. This means matched suppression of common mode noise4.
3. The capacitor maintains a balance over time; which means equal aging
and temperature tracking from side to side.
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RF Filtering for Audio Amplifier Circuits
Portable Audio
Devices
In consumer electronic devices, where smaller, better, and cheaper is desired,
chip feedthrough devices are used extensively. A typical MP3 portable music
device may use as many as 10 components for filtering the different internal
components such as audio amplifiers, ASIC devices, digital to analog signal
converters, etc. (Figure 8).
Figure 8.
Components contained in a typical MP3 device 5.
Stereo headphone jacks are another ideal application for X2Y® bypass filter
devices. Standard components would require one component per line (Figure 9).
Although there is a new device available to filter all three lines at once, it still
adds series resistance to the circuit.
Figure 9.
Standard resistive components applied to the headphone jack.
A single X2Y® component can be used in the same application by placing the end
terminals of the X2Y® device on the right and left speaker outputs and the center
ground terminals on the ground of the jack (Figure 10). In this manner the full
intended power goes to the headphone.
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RF Filtering for Audio Amplifier Circuits
Figure 10. A single X2Y® bypass capacitor applied to the headphone jack.
Figure 11 compares the different attributes of selected components used for
filtering.
Filter Devices
EIA
Size
DC Rated
Current
DC
Resistance
Per
Line/Device
Circuit
Power
Loss ?
Devices
Required
Cost
O603-2220
Bypass,
No Limit
None
None
One
Low/Medium
O805
Feedthru,
300mA
Yes,
0.6 Ohms
Yes
Mono two,
Stereo three
Low/Medium
Feedthru,
300mA
Yes,
0.6 Ohms
Yes
Mono two,
Stereo three
Low/Medium
1806
Feedthru,
2A
Yes,
0.04 Ohms
Yes
Mono two,
Stereo three
Medium
2220
Feedthru,
6A
Yes,
0.01 Ohms
Yes
Mono two,
Stereo three
High
2506
Feedthru,
2A
Yes,
0.04 Ohms
Yes
Mono two,
Stereo three
High
1206
Feedthru,
6A
Yes,
0.01 Ohms
Yes
Mono two,
Stereo three
High
1206
Feedthru,
100mA
Yes,
0.40 Ohms
Yes
One
High
1206
Figure 11. Comparison Chart.
Note# 2003, v3.0, 4/20/05
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RF Filtering for Audio Amplifier Circuits
Conclusion
Using the X2Y® bypass capacitor for filtering audio amplifiers will result in no
power loss because of DC resistance. Power loss is inherent in the design of
series feedthrough chip components and can degrade circuit performance. In
addition, X2Y® will function equally well in amplifier circuits where the speakers
are fed power differentially.
Note:
Performance results reported in this and other application notes can only
be achieved with patented X2Y® components sourced from X2Y® licensed
manufacturers or their authorized distribution channels.
References
1
Data Sheet, HNational Semiconductor LM4862 BoomerH.
2
Decoupling Strategies for Printed Circuit Boards Without Power Planes, Hwan W.
Shim,Theodore M. Zeef, Todd Hubing, EMC Labrotory, University Missouri-Rolla,
Presented at the August 2002 IEEE EMC Symposium, Minneapolis, MN - TU-PMG-5, Volume 1, page258.
3
Dell Patent #6,337,798
4
Differential-to-common-mode conversion, By Howard Johnson, PhD,
Hhttp://signalintegrity.com/Pubs/edn/DifftoCommonMode.htmH ,(Originally
published in EDN Magazine, October 17, 2002)
5
Source, “How Stuff Works”, Hhttp://static.howstuffworks.com/gif/mp3player.gifH.
Contact
Information
Direct inquiries and questions about this application note or X2Y® products to
[email protected] or telephone:
X2Y Attenuators, LLC
st
2730B West 21 Street
Erie, PA 16506-2972
Phone: 814.835.8180
Fax: 814.835.9047
To visit us on the web, go to http://www.x2y.com.
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