Application Information
(Continued)
verify that with the required power supply voltage and load
impedance, that the required heatsink value
θ
SA
is feasible
given system cost and size constraints. Once the heatsink
issues have been addressed, the required gain can be deter-
mined from
Equation (6).
(6)
From
Equation (6),
the minimum A
V
is: A
V
≥
15.5.
By selecting a gain of 21, and with a feedback resistor, R
f
=
20 kΩ, the value of R
i
follows from
Equation (7).
(7)
R
i
= R
f
(A
V
− 1)
= 1 kΩ a non-inverting gain of 21 will result.
Thus with R
i
Since the desired input impedance was 47 kΩ, a value of
47 kΩ was selected for R
IN
. The final design step is to ad-
dress the bandwidth requirements which must be stated as a
pair of −3 dB frequency points. Five times away from a −3 dB
point is 0.17 dB down from passband response which is bet-
ter than the required
±
0.25 dB specified. This fact results in
a low and high frequency pole of 4 Hz and 100 kHz respec-
tively. As stated in the
External Components
section, R
i
in
conjunction with C
i
create a high-pass filter.
use 39 µF.
C
i
≥
1/(2π
*
1 kΩ
*
4 Hz) = 39.8 µF;
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the gain, A
V
. With a
A
V
= 21 and f
H
= 100 kHz, the resulting GBWP is 2.1 MHz,
which is less than the guaranteed minimum GBWP of the
LM4766 of 8 MHz. This will ensure that the high frequency
response of the amplifier will be no worse than 0.17 dB down
at 20 kHz which is well within the bandwidth requirements of
the design.
13
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