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APPLICATION BULLETIN
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DOUBLE THE OUTPUT CURRENT TO A LOAD
WITH THE DUAL OPA2604 AUDIO OP AMP
By Morgan Monks
Headphones typically have an impedance of 40鈩?to 300鈩?
By using the dual OPA2604 and four resistors one can
economically drive a 2.8V peak signal into the 40鈩?head-
phones.
Figure 1 illustrates a circuit that can be used to drive loads
that exceed the output current capabilities of an operational
amplifier, but not enough to require the use of a power
operational amplifier. The OPA2604 used in this application
is a dual, FET-input operational amplifier that can typically
sink or source 35mA on the output. By taking advantage of
the fact that the OPA2604 is a dual, this circuit will sink or
source 70mA. In addition, each operational amplifier has its
own short circuit protection of
鹵40mA
(typ), which makes
the overall typical short circuit current of this application
鹵80mA.
One side of the dual OPA2604, A
2
, is in the feedback loop
of the other side of the dual, A
1
. The current, I
1
, which is
supplied by A
1
is matched by the current I
2
, the output
current of A
2
. The load will receive a total current of I
1
+ I
2
.
The ratio between the output currents, I
1
and I
2
, is equal to:
I
2
= I
1
(R
3
/R
4
)
Resistors R
3
and R
4
are set equal for equal output currents.
Resistors R
1
and R
2
set the overall gain of the circuit. The
transfer function is:
V
OUT
= V
IN
(1 + R
2
/R
1
)
The OPA2604 is a dual, FET-input operational amplifier
designed for enhanced AC performance. Very low distor-
tion, low noise and wide bandwidth provide superior perfor-
mance in high quality audio applications. The OPA2604 is
available is plastic 8-pin DIP and plastic 8-pin SOIC.
1/2 OPA2604
A
2
I
2
R
4
51鈩?/div>
1/2 OPA2604
A
1
V
IN
R
2
R
3
51鈩?/div>
I
1
I
L
= I
1
+ I
2
V
OUT
R
1
Load
FIGURE 1. Using the Dual OPA2604 Op Amp to Double the Output Current to a Load.
漏
1993 Burr-Brown Corporation
AB-051
Printed in U.S.A. March, 1993
1
