3DQDVRQLF
33
The signal gain : G =
Vo p
Vi p
( )
( )
=
−
+
d p A p
A p K p
( ) ( )
( ) ( )
1
Generally, at low frequencies, A(p) >>
1
K p
( )
is valid, we can approximate this formula
G =
−
= −
d p
K p
Zf
Zi
( )
( )
For this formula, we can define :
* Attenuation factor d(p)=
V
p
V
p
V p
i
−
+
−
( )
( )
( )
at V
o
(p)=0
=
Z
Z
Z
Z
Z
f
a
f
a
i
/ /
/ /
+
* Feedback factor K(p)=
V
p
V
p
V
p
o
−
+
−
( )
( )
( )
at V
i
(p)=0
=
Z
Z
Z
Z
Z
i
a
i
a
f
/ /
/ /
+
b – Z-421 implementation
•
Zi impedance consists of input resistors connected between TDA6106Q pin 3 and the TDA8844
output : R90l, R902, R903 and R520, R521, R522.
•
Za impedance consists of bias resistors : R904, R905, R906 arid parasitic capacitance Ca which
appears between TDA6106Q input and ground.
•
Zf impedance consists of feedback resistors : R907, R908, R909 and parasitic capacitance Cf which
appears between TDA6106Q input and output.
Then we can summaries the video amplifier function with the following figure 31
V
output
V
+
V
-
-
+
A
C
f
R907
R908
R909
R904
R905
R906
C
a
R901
R902
R903
R511
R512
R513
T
D
A
8
374
11
12
13
Fig. 31
