Datasheet
SLOS315B − JUNE 2000 − REVISED OCTOBER 2002
16
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
application schematics (continued)
It is important to note that using the additional R
F
resistor connected between IN+ and BYPASS causes V
DD
/2
to shift slightly, which could influence the THD+N performance of the amplifier. Although an additional external
op-amp could be used to buffer BYPASS from R
F
, tests in the lab have shown that the THD+N performance is
only minimally affected by operating in the fully differential mode as shown in Figure 27. The following sections
discuss the selection of the components used in Figures 26 and 27.
component selection
gain setting resistors, R
F
and R
I
The gain for each audio input of the TPA731 is set by resistors R
F
and R
I
according to equation 5 for BTL mode.
(5)
BTL gain +*2
ǒ
R
F
R
I
Ǔ
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 TPA731 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 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, 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)
−3 dB
f
c
f
c
+
1
2p R
F
C
F
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 the audio range.