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

DD

In

IN

OUT

+-

+

=+

()

1

3

2

3

and

DD

IN

OUT

-+

Substituting and simplifying into the equation that

defines differential operation yields

D

D

DA

In

OUT

D

=+

-

+

()

3

2

3

simplifying yields

()

D DIn

+

2

as one would expect for a +6dB line driver.

The derivation for the common mode equation is

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.

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com

Page 4

THAT1420/1430 Balanced Line Driver

Preliminary Information

Out-

Cap1

Cap2

Out+

Vcc

In+

Vee

Gnd

10k

10k

50

5k

5k

10k

10k

10k

10k

7k

7k

10p

50

THAT 1430

Din+

Cin+

Cin-

Din-

Dout-

Dout+

D

C

AA

&

Figure 3. THAT 1430 Equivalent Circuit Diagram