THAT1420 ETC [List of Unclassifed Manufacturers], THAT1420 Datasheet - Page 4

• Manufacturer Part Number: THAT1420
• Description: Balanced Line Drivers
• Manufacturer: ETC [List of Unclassifed Manufacturers]

Page 4
These equations can be solved much like standard
op-amp loop equations, and for the differential case,
we can see that (using superposition) resistor feed-
back results in
Substituting and simplifying into the equation that
defines differential operation yields
Dividing through by A
simplifying yields
as one would expect for a +6dB line driver.
more complicated
D
D
D D
The derivation for the common mode equation is
D
IN
IN
D
+
-
OUT
Gnd
Vcc
OUT
In+
Vee
=
=
(
1
1
=
3
3
=
D
2
D
A
OUT
(
OUT
D
In
(
1
+
-
+
-
D
)
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
in that it is dependent on the at-
D
+
3
OUT
D
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
2
5k
5k
(assuming that A
3
In
+
+
3
2
)
In
and
+
Din+
Cin+
)
Cin-
Din-
Cap2
Figure 3. THAT 1430 Equivalent Circuit Diagram
R
EXT
D
A
10k
>> 3) and
10k
D
&
C
A
EXT
C
Dout+
Dout-
Cap1
10k
10k
tached load, and in any event doesn't yield much
insight into the device's operation.
combination of negative feedback and high open-loop
gain results in the open-loop gain "dropping out" of
the equation, and the differential inputs being forced
to the same potential. If we start with that assump-
tion, we can intuitively discern the operation of the
common-mode feedback loop as follows:
mon-mode input actually senses the sum of the IC's
output currents by way of two 50 ohm resistors and
the bridge network (the 10pF capacitor simply limits
the maximum frequency at which this action occurs).
The resulting error signal is amplified and then
summed into both outputs, with the net effect being
to force the sum of the currents to be zero, and thus
the common mode output current to zero. Since this
is negative feedback, the common-mode loop can
raise the effective output impedance at audio fre-
quencies without the side effects of circuits that use
positive feedback to implement this function.
In op-amp analysis or in the above derivation, the
Referring again to Figures 2 and 3, the com-
10k
10k
50
50
THAT1420/1430 Balanced Line Driver
THAT 1430
10p
7k
7k
Preliminary Information
Out+
Out-