LM4766 datasheet(13/14 Pages) NSC

13 / 14 page

Application Information (Continued)

verify that with the required power supply voltage and load

impedance, that the required heatsink value

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:

V ≥ 15.5.

By selecting a gain of 21, and with a feedback resistor, R

f =

20 k

Ω, the value of R

i follows from Equation (7).

i = Rf (AV − 1)

(7)

Thus with R

i = 1kΩ a non-inverting gain of 21 will result.

Since the desired input impedance was 47 k

Ω, a value of

47 k

Ω 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.

i ≥ 1/(2π * 1kΩ * 4 Hz) = 39.8 µF;

use 39 µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the gain, AV. With a

V = 21 and fH = 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.

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