SA572
Buffer Amplifier
In audio systems, it is desirable to have fast attack time
and slow recovery time for a tone burst input. The fast
attack time reduces transient channel overload but also
causes low-frequency ripple distortion. The low-frequency
ripple distortion can be improved with the slow recovery
time. If different attack times are implemented in
corresponding frequency spectrums in a split band audio
system, high quality performance can be achieved. The
buffer amplifier is designed to make this feature available
with minimum external components. Referring to
Figure 5, the rectifier output current is mirrored into the
input and output of the unipolar buffer amplifier A
3
through
Q
8
, Q
9
and Q
10
. Diodes D
11
and D
12
improve tracking
accuracy and provide common-mode bias for A
3
. For a
positive-going input signal, the buffer amplifier acts like a
voltage-follower. Therefore, the output impedance of A
3
makes the contribution of capacitor C
R
to attack time
insignificant. Neglecting diode impedance, the gain Ga(t)
for
DG
can be expressed as follows:
Ga(t)
+
(Ga
INT
*
Ga
FNL
) e
t
A
)
Ga
FNL
Ga
INT
= Initial Gain
Ga
FNL
= Final Gain
t
A
= R
A
•
C
A
= 10 k
W
•
C
A
*t
where
t
A
is the attack time constant and R
A
is a 10 kW
internal resistor. Diode D
15
opens the feedback loop of A
3
for a negative-going signal if the value of capacitor C
R
is
larger than capacitor C
A
. The recovery time depends only
on C
R
•
R
R
. If the diode impedance is assumed negligible,
the dynamic gain G
R
(t) for
DG
is expressed as follows:
G
R
(t)
+
(G
RINT
*
G
RFNL
) e
t
R
)
G
RFNL
G
R
(t)
+
(G
RINT
*
G
RFNL
) e
t
R
)
G
RFNL
t
R
= R
R
•
C
R
= 10 kW
•
C
R
*t
*t
where
t
R
is the recovery time constant and R
R
is a 10 kW
internal resistor. The gain control current is mirrored to the
gain cell through Q
14
. The low level gain errors due to input
bias current of A
2
and A
3
can be trimmed through the
tracking trim pin into A
3
with a current source of
"3.0
mA.
V+
Q
8
Q
9
Q
10
Q
17
I
Q
= 2IR
2
IR
2
X2
Q
16
I
R
+
V IN
R
10kW
−
A3
+
10kW
IR
1
D
15
D
13
Q
14
X2
Q
18
D
11
D
12
C
A
TRACKING
TRIM
C
R
Figure 5. Buffer Amplifier Schematic
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