EUA6019
In this example, Ci is 56nF so one would likely choose a
value in the range of 56nF to 1µF. A further consideration
for this capacitor is the leakage path from the input source
through the input network (Ci) and the feedback network
to the load. This leakage current creates a dc offset voltage
at the input to the amplifier that reduces useful headroom,
especially in high gain applications. For this reason, a
low- leakage tantalum or ceramic capacitor is the best
choice. When polarized capacitors are used, the positive
side of the capacitor should face the amplifier input in
most applications as the dc level there is held at VDD/2,
which is likely higher than the source dc level. Note that it
is important to confirm the capacitor polarity in the
application.
Output Coupling Capacitor, (CC)
For general signal-supply SE configuration, the output
coupling capacitor (CC) is required to block the dc bias at
the output of the amplifier thus preventing dc currents in
the load. As with the input coupling capacitor, the output
coupling capacitor and impedance of the load form a
high-pass filter governed by equation 6.
1
fc(high)=
-------------------(6)
2π R C
L
C
Decoupling Capacitor, (CS)
The EUA6019 is a high-performance CMOS audio
amplifier that requires adequate power supply decoupling
to ensure the output total harmonic distortion (THD) is as
low as possible. Power supply decoupling also prevents
oscillations for long lead lengths between
The main disadvantage, from a performance standpoint, is
the load impedances are typically small, which drives the
low-frequency corner higher, degrading the bass response.
Large values of CC are required to pass low frequencies
into the load. Consider the example where a
the amplifier and the speaker. The optimum decoupling is
achieved by using two capacitors of different types that
target different types of noise on the power supply leads.
For higher frequency transients, spikes, or digital hash on
the line, a good low equivalent-series-resistance (ESR)
ceramic capacitor, typically 0.1µF placed as close as
possible to the device VDD lead, works best. For filtering
CC of 330µF is chosen and loads vary from 3Ω, 4Ω, 8Ω,
32Ω, 10kΩ, to 47kΩ. Table 3 summarizes the frequency
response characteristics of each configuration.
Table4. Common Load Impedances vs Low Frequency
Output characteristics in SE Mode
lower-frequency noise signals,
a larger aluminum
electrolytic capacitor of 10µF or greater placed near the
RL
CC
Lowest
Frequency
161Hz
120Hz
60Hz
audio power amplifier is recommended.
3Ω
330µF
330µF
330µF
330µF
330µF
330µF
4Ω
Bypass Capacitor, (CB)
8Ω
The bypass capacitor, CB, is the most critical capacitor and
serves several important functions. During start-up or
recovery from shutdown mode, CB determines the rate at
which the amplifier starts up. The second function is to
reduce noise produced by the power supply caused by
coupling into the output drive signal. This noise is from the
midrail generation circuit internal to the amplifier, which
appears as degraded PSRR and THD+N. Bypass capacitor,
CB, values of 0.47µF to 1µF ceramic or tantalum low-ESR
capacitors are recommended for the best THD and noise
performance.
32Ω
15Hz
10000Ω
47000Ω
0.05Hz
0.01Hz
As Table 4 indicates, most of the bass response is
attenuated into a 4-Ω load and 8-Ω load is adequate,
headphone response is good, and drive into line level
inputs (a home stereo for example) is exceptional.
Using Low- ESR Capacitors
Low- ESR capacitors are recommended throughout this
applications section. A real (as opposed to ideal) capacitor
can be modeled simply as a resistor in series with an ideal
capacitor. The voltage drop across this resistor minimizes
the beneficial effects of the capacitor in the circuit. The
lower the equivalent value of this resistance the more the
real capacitor behaves like an ideal capacitor.
DS6019 Ver 1.1 May. 2007
18
EUTECH [ EUTECH MICROELECTRONICS INC ]
