Datasheet

TPA0202
2-W STEREO AUDIO POWER AMPLIFIER
SLOS205B – FEBRUARY 1998 – REVISED DECEMBER 2000
29
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
gain setting resistors, R
F
and R
I
The gain for each audio input of the TPA0202 is set by resistors R
F
and R
I
according to equation 5 for BTL mode.
(5)
BTL Gain 2
R
F
R
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 TPA0202 is a MOS amplifier, the input impedance is very 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.
(6)
Effective Impedance
R
F
R
I
R
F
R
I
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 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, a small
compensation capacitor of approximately 5 pF should be placed in parallel with R
F
when R
F
is greater than
50 kΩ. This, in effect, creates a low pass filter network with the cutoff frequency defined in equation 7.
(7)
f
c(lowpass)
1
2 R
F
C
F
–3 dB
f
c
For example, if R
F
is 100 kΩ and Cf is 5 pF then f
c
is 318 kHz, which is well outside of the audio range.