TPA6100A2D
www.ti.com
SLOS269B–JUNE 2000–REVISED SEPTEMBER 2004
APPLICATION INFORMATION
GAIN SETTING RESISTORS, RF, RI,and R
The voltage gain for the TPA6100A2D is set by resistors RF and RI according to Equation 1.
R
R
F
F
Gain + * ǒ Ǔor Gain (dB) + 20 log ǒ Ǔ
R
R
I
I
(1)
Given that the TPA6100A2D is an 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 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 5 kΩ
and 20 kΩ. The effective impedance is calculated in Equation 2.
R R
F
I
Effective Impedance +
R ) R
F
I
(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 RF above 50 kΩ, 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 5 pF should be placed in parallel with RF. In effect, this creates a
low-pass filter network with the cutoff frequency defined in Equation 3.
1
f
+
c
2pR C
F
F
(3)
For example, if RF is 100 kΩ and CF is 5 pF, then fc is 318 kHz, which is well outside the audio range.
For maximum signal swing and output power at low supply voltages like 1.6 V to 3.3 V, BYPASS is biased to
VDD/4. However, to allow the output to be biased at VDD/2, a resistor, R, equal to RF must be placed from the
negative input to ground.
INPUT CAPACITOR, CI
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.
1
f
+
c
2pR C
I
I
(4)
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 20 kΩ and the specification calls for a flat bass response down to 20 Hz.
Equation 4 is reconfigured as Equation 5.
1
C +
I
2pR f
c
I
(5)
In this example, CI is 0.4 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 (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 (>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 VDD/4, which is likely higher
than the source dc level. It is important to confirm the capacitor polarity in the application.
5
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