TS634ID STMicroelectronics, TS634ID Datasheet - Page 7

• Manufacturer Part Number: TS634ID
• Description: Dual Wide Band Operational Amplifier Adsl Line Interface
• Manufacturer: STMicroelectronics

Component calculation:
Let us consider the equivalent circuit for a single
ended configuration, Figure 4.
Figure 4 : Single ended equivalent circuit
Let us consider the unloaded system. Assuming
the currents through R1, R2 and R3
as respectively:
As Vo° equals Vo without load, the gain in this
case becomes :
The gain, for the loaded system will be (1):
As shown in figure5, this system is an ideal gener-
ator with a synthesized impedance as the internal
impedance of the system. From this, the output
voltage becomes:
with Ro the synthesized impedance and Iout the
output current. On the other hand Vo can be ex-
pressed as:
GL
1/2
Vi
Vo
R1
=
G
=
Vo withload
----------------------------------- -
=
Vo
Vi 1
---------------------------------------------- -
2Vi
-------- - Vi Vo
R1
Vo noload
------------------------------ -
R2
+
_
=
Vi
------------------------- - and Vi
+
R3
1
Vi
ViG
2R2
---------- -
–
R1
–
R2
R2
------ -
R3
–
+
=
Vo°
R2
------ -
R3
=
RoIout
1
-- -
2
1
---------------------------------- -
1
---------------------------------- - 1
Rs1
-1
–
+
+
----------------------- -
1
Rs1Iout
--------------------- 3
2R2
---------- -
1
2R2
---------- -
R1
1
R1
–
–
,
R3
–
+
R2
------ -
R3
2
R2
------ -
R3
R2
------ -
R3
Vo
Vo
+
+
R2
------ -
R3
R2
------ -
R3
,
1/2
,
RL
By identification of both equations (2) and (3), the
synthesized impedance is, with Rs1=Rs2=Rs:
Figure 5 : Equivalent schematic. Ro is the syn-
Unlike the level Vo° required for a passive imped-
ance, Vo° will be smaller than 2Vo in our case. Let
us write Vo°=kVo with k the matching factor vary-
ing between 1 and 2. Assuming that the current
through R3 is negligeable, it comes the following
resistance divider:
After choosing the k factor, Rs will equal to
1/2RL(k-1).
A good impedance matching assumes:
From (4) and (5) it becomes:
By fixing an arbitrary value for R2, (6) gives:
Finally, the values of R2 and R3 allow us to extract
R1 from (1), and it comes:
with GL the required gain.
GL (gain for the
loaded system)
R2 (=R4)
R3 (=R5)
R1
Rs
Vi.Gi
thesized impedance
R1
=
GL is fixed for the application requirements
GL=Vo/Vi=0.5(1+2R2/R1+R2/R3)/(1-R2/R3)
2R2/[2(1-R2/R3)GL-1-R2/R3]
Abritrary fixed
R2/(1-Rs/0.5RL)
0.5RL(k-1)
--------------------------------------------------------- 7
2 1
R2
------ -
R3
Ro
Ro
Ro
R o
R3
–
=
=
=
R2
------ -
R3
=
=
1
---------------------------
RL
---------------- - 4
1
-- - RL 5
1
2
2R2
–
-------------------
1
kVoRL
–
Rs
Iout
GL 1
+
2Rs
--------- - 6
–
R2
------ -
R3
RL
R2
2Rs1
2Rs
--------- -
,
RL
–
,
,
–
R2
------ -
R3
,
1/2
RL
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