ft690_DS_2.1
ft690-
0502273076
www.fangtek.com
11
which must be stated as a pair of
−
3dB frequency points.
Five times away from a
−
3dB point is 0.17dB down from
passband response which is better than the required
±0.25dB specified.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz * 5 = 100kHz
R
i
in conjunction with C
i
create a highpass filter.
C
i
≥
1/(2
π
*20k
Ω
*20Hz) = 0.397
μ
F; use 0.39
μ
F
The high frequency pole is determined by the product of
the desired frequency pole, f
H
, and the differential gain,
A
VD
. With a A
VD
= 3 and f
H
= 100kHz, the resulting
GBWP = 300kHz which is much smaller than the ft690
GBWP of 2.5MHz. This figure displays that if a designer
has a need to design an amplifier with a higher
differential gain, the ft690 can still be used without
running into bandwidth limitations
Figure 3. HIGHER GAIN AUDIO AMPLIFIER
The ft690 is unity-gain stable and requires no external
components besides gain-setting resistors, an input
coupling capacitor, and proper supply bypassing in the
typical application. However, if a closed-loop differential
gain of greater than 10 is required, a feedback capacitor
(C4) may be needed as shown in Figure 2 to bandwidth
limit the amplifier. This feedback capacitor creates a low
pass filter that eliminates possible high frequency
oscillations. Care should be taken when calculating the
-3dB frequency in that an incorrect combination of R
3
and C
4
will cause rolloff before 20kHz. A typical
combination of feedback resistor and capacitor that will
not produce audio band high frequency rolloff is R
3
=
20k
Ω
and C
4
= 25pf. These components result in a -3dB
point of approximately 320kHz.
Figure 4. DIFFERENTIAL AMPLIFIER CONFIGURATION FOR ft690
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