EUA5202
Application Information
Input Capacitor, CI
Gain Setting Resistors, RF and RI
In the typical application an input capacitor, CI, is
required to allow the amplifier to bias the input signal to
the proper dc level for optimum operation. In this case,
CI and RI form a high-pass filter with the corner
frequency determined in equation 4.
The gain for each audio input of the EUA5202 is set by
resistors by resistors RF and RI according to equation 1
for BTL mode.
R
R
F
-------------------------------- (1)
BTL Gain = −2
1
------------------- (4)
f
=
2 π R C
c(highpass)
I
I I
BTL mode operation brings about the factor 2 in the gain
equation due to the inverting amplifier mirroring the
voltage swing across the load. Given that the EUA5202
is a MOS amplifier, the input impedance is very high,
value of RF increases. In addition, a certain range of RF
values is required for proper start-up operation of the
amplifier. Taken together it is recommended that the
effective impedance seen by the inverting node of the
amplifier be set between 5kΩ and 20kΩ .The effective
impedance is calculated in equation 2.
The value of CI is important to consider as it directly
affects the bass (low frequency) performance of the
circuit. Consider the example where RI is 10kΩ and the
specification calls for a flat bass response down to 40Hz.
Equation 8 is reconfigured as equation 5.
R R
F
I
EffectiveImpedance =
-------------------- (2)
R
+ R
F
I
1
As an example consider an input resistance of 10kΩ and
a feedback resistor of 50kΩ. The BTL gain of the
amplifier would be -10 and the effective impedance at
the inverting terminal would be 8.3kΩ, which is well
within the recommended range. For high performance
applications metal film resistors are recommended
because they tent to have lower noise levels than carbon
resistors. For values of RF above 50kΩ the amplifier
tends to become unstable due to a pole formed from RF
and the inherent input capacitance of the MOS input
structure. For this reason, a small compensation
capacitor of approximately 5pF should be places in
parallel with RF when RF is greater than 50kΩ. This, in
effect, creates a low pass filter network with the cutoff
frequency defined in equation 3.
C
=
I
------------------------------------ (5)
2π R f
I C
In this example, CI is 0.40 µF so one would likely
choose a value in the range of 0.47µF to 1µF. A further
consideration for this capacitor is the leakage path from
the input source through the input network (RI, CI) and
the feedback resistor (RF) 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 that the source dc level. Please note that
it is important to confirm the capacitor polarity in the
application.
1
f (lowpass) =
-------------------- (3)
c
2 π R C
F F
For example, if RF is 100kΩ and CF is 5 pF then fC is 318
KHz, which is well outside of the audio range.
DS5202 Ver 1.6 May. 2005
18
EUTECH [ EUTECH MICROELECTRONICS INC ]
