Datasheet

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APPLICATION INFORMATION
GAIN SETTING RESISTORS, R
F
and R
i
Gain  

R
F
R
I

(1)
Effective Impedance 
R
F
R
I
R
F
 R
I
(2)
f
c(lowpass)

1
2 R
F
C
F
(3)
INPUT CAPACITOR, C
i
f
c(highpass)

1
2 R
I
C
I
(4)
C
I

1
2 R
I
f
c(highpass)
(5)
TPA6111A2
SLOS313B – DECEMBER 2000 – REVISED JUNE 2004
The gain for the TPA6111A2 is set by resistors R
F
and R
I
according to Equation 1 .
Given that the TPA6111A2 is a MOS amplifier, the input impedance is high. Consequently, input leakage
currents are not generally a concern, although noise in the circuit increases as the value of R
F
increases. In
addition, a certain range of R
F
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 5 kΩ
and 20 kΩ. The effective impedance is calculated in Equation 2 .
As an example, consider an input resistance of 20 kΩ and a feedback resistor of 20 kΩ. The gain of the amplifier
would be –1 and the effective impedance at the inverting terminal would be 10 kΩ, which is within the
recommended range.
For high-performance applications, metal film resistors are recommended because they tend to have lower noise
levels than carbon resistors. For values of R
F
above 50 kΩ, the amplifier tends to become unstable due to a pole
formed from R
F
and the inherent input capacitance of the MOS input structure. For this reason, a small
compensation capacitor of approximately 5 pF should be placed in parallel with R
F
. In effect, this creates a
low-pass filter network with the cutoff frequency defined in Equation 3 .
For example, if R
F
is 100 kΩ and C
F
is 5 pF, then f
c(lowpass)
is 318 kHz, which is well outside the audio range.
In the typical application, input capacitor C
I
is required to allow the amplifier to bias the input signal to the proper
dc level for optimum operation. In this case, C
i
and R
I
form a high-pass filter with the corner frequency
determined in Equation 4 .
The value of C
I
is important to consider, as it directly affects the bass (low-frequency) performance of the circuit.
Consider the example where R
I
is 20 kΩ and the specification calls for a flat bass response down to 20 Hz.
Equation 4 is reconfigured as Equation 5 .
In this example, C
I
is 0.40 µF, so one would likely choose a value in the range of 0.47 µF to 1 µF. A further
consideration for this capacitor is the leakage path from the input source through the input network (R
I
, C
I
) and
the feedback resistor (R
F
) 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 (> 10). 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 V
DD
/2, which is likely higher
than the source dc level. Note that it is important to confirm the capacitor polarity in the application.
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