Datasheet

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COMPONENT SELECTION
Gain Setting Resistors, R
F
and R
I
BTL Gain  A
V
  2

R
F
R
I

(5)
Effective Impedance 
R
F
R
I
R
F
 R
I
(6)
−3 dB
f
c
f
c

1
2 R
F
C
F
(7)
Input Capacitor, C
I
TPA321
SLOS312C – JUNE 2000 – REVISED JUNE 2004
The gain for each audio input of the TPA321 is set by resistors R
F
and R
I
according to Equation 5 for BTL mode.
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 TPA321 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 6 .
As an example, consider an input resistance of 10 kΩ and a feedback resistor of 50 kΩ. The BTL gain of the
amplifier would be –10 V/V, and the effective impedance at the inverting terminal would be 8.3 kΩ, which is well
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, place a small
compensation capacitor (C
F
) of approximately 5 pF in parallel with R
F
when R
F
is greater than 50 kΩ. In effect,
this creates a low-pass filter network with the cutoff frequency defined in Equation 7 .
For example, if R
F
is 100 kΩ and C
F
is 5 pF then f
c
is 318 kHz, which is well outside of 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 8 .
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