THAT1420 Datasheet, PDF (4/8 Pages) List of Unclassifed Manufacturers – Balanced Line Drivers

Page 4
THAT1420/1430 Balanced Line Driver
Preliminary Information
Vcc
In+
Gnd
Vee
Cap2
REXT
CEXT
Cap1
10k
5k _Din+
10k
Dout-
Cin+
Cin- ^A_D ^{& A}_C
5k _Din-
Dout+
10k
10k
THAT 1430
50
Out-
10k
7k
10p
10k
7k
50
Out+
Figure 3. THAT 1430 Equivalent Circuit Diagram
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
D_{IN +} = (¹³ D_OUT- + ²³ In₊) and
D_{IN -} = ¹³ D_{OUT +}
Substituting and simplifying into the equation that
defines differential operation yields
D
D_OUT
=
A_D
(
-
DD_OUT
3
+
2
3
In₊
)
Dividing through by A_D (assuming that A_D >> 3) and
simplifying yields
D D_OUT = 2(In₊ )
as one would expect for a +6dB line driver.
The derivation for the common mode equation is
more complicated¹ in that it is dependent on the at-
tached load, and in any event doesn't yield much
insight into the device's operation.
In op-amp analysis or in the above derivation, the
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:
Referring again to Figures 2 and 3, the com-
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.
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