Technical Note
BH7824FVM,BH7826FVM
7/14
www.rohm.com
2011.08 - Rev.B
© 2011 ROHM Co., Ltd. All rights reserved.
Ranges
Rin 10kΩ ≦ R1 ≦120kΩ
Rf 47kΩ ≦ R2 ≦120kΩ
Cf
Gain＞+6dB Cf≦560pF
0＜Gain≦+6dB Cf≦270pF
fcL fcL ≧ 4kHz
●Functions and Settings of external components
【Function of external components】
①R1
R1 is an input resistor of inverting amplifier which determines closed loop gain with R2.
②R2
R2 is a feedback resistor of inverting amplifier determine closed loop gain with R2. The gain is set by below
expression.
Gain = 20log [dB]
③Cf
Cf is feedback capacitor to cut high frequency signals.
It forms a low-pass filter with R2. The cut-off frequency is calculated as below.
f
cL
= [Hz]
(注) R1,R2,Cf and f
cL has limits in below table. Please set these component values in the ranges.
④Cb1 (Cb2 is only BH7824FVM.)
The capacitors stabilize DC bias voltage.
As Cb1 (or Cb2) becomes larger, power supply rejection ratio is improved but turn on time becomes longer.
As it becomes smaller, turn on time becomes shorter but power supply rejection ratio and cross talk become worse.
⑤Cin
It is Input coupling capacitor.
The value of the input capacitor directly affects the low frequency performance of the circuit.
The corner frequency of the high pass filter is determined in equation at ③ of next section.
【Configuration of external components 】
① Gain
Gain is designed by output power application ask for.
Output power is determined to below.
Po[W] = Vo
2
[Vrms] / RL[Ω]
Vo = Gv・Vin
Gv ≧
RLPo
・
/ Vin
② Values of R1 and R2
R1 and R2 are determined to Av.
Av = ( R2 / R1 )×2
Input impedance (R1) must be probable driven by signal source
③ Input coupling capacitor
Corner frequency of HPF is determined in below.
fc[Hz] = 1 / (2π・R1・Cin)
Input coupling is solved by equation of fc.
Cin ≧ 1 / (2π・R1・fc)
R2
R1
1
2π×R2×Cf
BH7824FVM BH7826FVM
Cb1 1µF 1µF
Cb2 0.1µF ―
