Datasheet
SLOS230D − NOVEMBER 1998 − REVISED OCTOBER 2002
24
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
component selection (continued)
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(lowpass)
+
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.
input capacitor, C
I
In the typical application an 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.
(8)
f
c(highpass)
+
1
2p R
I
C
I
−3 dB
f
c
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 10 kΩ and the specification calls for a flat bass response down to 40 Hz.
Equation 8 is reconfigured as equation 9.
(9)
C
I
+
1
2p R
I
f
c