SA572 Datasheet, PDF (6/12 Pages) NXP Semiconductors – Programmable analog compandor

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 A₃ through
Q₈, Q₉ and Q₁₀. Diodes D₁₁ and D₁₂ improve tracking
accuracy and provide common-mode bias for A₃. For a
positive-going input signal, the buffer amplifier acts like a
voltage-follower. Therefore, the output impedance of A₃
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:
*t
Ga(t) + (Ga_INT * Ga_FNL) e ^tA ) Ga_FNL
Ga_INT = Initial Gain
Ga_FNL = Final Gain
t_A = R_A • C_A = 10 kW • C_A
where t_A is the attack time constant and R_A is a 10 kW
internal resistor. Diode D₁₅ opens the feedback loop of A₃
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:
*t
G_R(t) + (G_RINT * G_RFNL) e ^tR ) G_RFNL
*t
G_R(t) + (G_RINT * G_RFNL) e ^tR ) G_RFNL
t_R = R_R • C_R = 10 kW • C_R
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 Q₁₄. The low level gain errors due to input
bias current of A₂ and A₃ can be trimmed through the
tracking trim pin into A₃ with a current source of "3.0 mA.
Q₈
Q₉
V+
Q₁₀
Q₁₇
IR
+
VIN
R
10kW
−
A3
+
IR₁
D₁₁
D₁₂
I_Q = 2IR₂
IR₂
X2
Q₁₆
10kW
D₁₅
D₁₃
Q₁₄
X2
Q₁₈
C_A
C_R
TRACKING
TRIM
Figure 5. Buffer Amplifier Schematic
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