Tai-1 Microelectronics

TMPA401DM

www.taimec.com.tw

May 25, 2005

Copyright

©2004,Tai-1 Microelectronics Corp.

7

Voltage gain

The voltage gain can be set through input resistance connecting to input pins. Lower input

resistance can be used for higher gain. The voltage gain is defined by (800k ohms)/(Ri+20k

ohms) at Vcc=15V without loading, where 20k ohms is internal resistance and Ri is external

series resistance of the input pin. If Ri is not used (Ri=0 ohm) the voltage gain is (800k

ohms)/(20k ohms) or 32dB. Insert Ri if lower gain is preferable. For example if Ri=30k ohms

then voltage is (800k ohms)/(30k+20k ohms) or 24dB. For best result the input resistors should

be well matched. Matched resistors are also required even for single ended input configuration

for low noise.

Input filter

AC coupling capacitors are required to block the DC voltage from the device. They also define

the

–3db frequency at the low frequency side.

The

–3db frequency of the low frequency side is

f-3db = 1/ (2πR C)

where C is the AC coupling capacitance and R is the total resistance in series with C.

Note that R=Zin(internal resistance) + Rext(external resistance)

Also note that the internal input resistance of INN/INP is 20K ohms.

In the following diagram Rext=22k ohms, Zin=20k ohms and C=C1=0.47uF. Thus the

–3db

frequency at the low frequency side is about 8Hz.

C1

0.47uF

R

22k

C1

0.47uF

Zin

20k

R

22k

Zin

20k

Class-Damplifier

inp

inn

C2

0.47nF

A bypass capacitor placed in between the positive signal path and negative signal path is to

attenuate the high frequencies. It defines the

–3bd frequency at the high frequency side. The

input filter becomes a band pass filter.

The

–3db frequency of the high frequency side is

f

–3db=1/(2πRC)

where C is the bypass capacitance and R is the total resistance in parallel with C.

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