1646 データシートの表示（PDF） - THAT Corporation

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1646

Balanced Line Driver ICs

THAT Corporation 

THAT1606/1646 Balanced Line Driver ICs

Page 5 of 12

Document 600078 Rev 05

Theory of Operation

OutSmarts® technology

The THAT 1606 and 1646 family employs the

OutSmarts topology, a variation of circuitry originally

developed by Chris Strahm at Audio Teknology Inc.,

(and later acquired by Audio Toys, Inc.). THAT's

OutSmarts topology employs two negative-feedback

loops -- one to control the differential signal, and a

separate loop to control the common mode output

levels.

Figures 2 and 3 show the gain core common to

the 1606 and 1646. The gain core is a single ampli-

fier that includes two differential input pairs, Cin+/-

and Din+/-, and complementary outputs, Vout+ and

Vout-, related to each other by two gain expressions,

AD(s) and AC(s). The first pair of differential inputs,

Din+/-, is connected to the differential feedback net-

work between the outputs and the input signal. The

second differential input pair, Cin+/-, is connected to

a bridge circuit which generates an error signal used

to servo the common-mode behavior of the outputs.

The loop equations are then:

DOUT+ - DOUT- = D DOUT = AD(DIN + - DIN - ) ,

where AD is the differential open-loop gain, and

S DOUT+ + DOUT- = DOUT = AC(CIN + - CIN - ) ,

where AC is the common-mode open-loop gain.

These equations can be solved much like stan-

dard op-amp loop equations.

For the differential case, using superposition, we

can see that this results in:

DIN +

=

1

3

DOUT

-

+

2

3

In+

, and

DIN -

=

1

3

DOUT

+

+

2

3

In

-

.

Substituting and simplifying into the equation

that defines differential operation yields:

[ ] D DOUT =

AD

- DDOUT

3

+

2

3

(In+

- In-)

.

Dividing through by AD (assuming that AD >> 3) and

simplifying yields

D DOUT = 2(In+ - In- ) .

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

For the 1646, In- is hard-wired to ground (0v), so

the differential equation above simplifies to:

D DOUT = 2(In+ ) .

The common mode equation is more complicated

in that it is dependent on the attached load, and in

any event doesn't yield much insight into the device's

operation. For those who are interested, a more

complete discussion is given in the reference men-

tioned in note 1.

In op-amp analysis using negative feedback loops,

the combination of negative feedback and high

open-loop gain usually results in the open-loop gain

"dropping out" of the equation, and the differential in-

puts being forced to the same potential. This is true

for the core of the 1606 and 1646 ICs. If we start

with that assumption, the operation of the com-

mon-mode feedback loop can be intuited as follows:

Referring again to Figures 2 and 3, the common-

mode input actually senses the sum of each IC's out-

put currents by way of two 25 W resistors and the

bridge network4. The resulting error signal is ampli-

fied 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.

To see why this is important, consider what hap-

pens when the IC is loaded with a single-ended load,

which shorts one or the other output to ground.

Suppose Out- is grounded. In this case, the differen-

tial feedback loop increases the voltage at Dout+ to

make up for most of the signal lost to the short at

Out-. The common-mode feedback loop forces the

current from Out- to be equal and opposite to that

from Out+. But, during peak signals which drive

Dout+ into clipping (exceeding its maximum output

voltage capability), the differential loop is starved for

feedback. Without the common-mode feedback, the

result would be for the voltage at Dout- to decrease in

an attempt to satisfy the differential loop's demand

for feedback. This is one significant weakness of

conventional cross-coupled output designs – com-

mon-mode feedback is lost when one output is

clipped while the other is grounded.

With OutSmarts, however, the common mode

feedback loop senses this happening because of the

increase in current at Out- (compared to that at

Out+), and prevents the voltage at Out- from rising

4. The 10 pF capacitor can be ignored for the purposes of this analysis. It simply limits the maximum frequency at which the current-sensing action occurs

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