SA572D データシートの表示（PDF） - Philips Electronics

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SA572D

Programmable analog compandor

Philips Electronics 

Philips Semiconductors RF Communications Products

Programmable analog compandor

Product specification

NE/SA572

VREF

+

A2

–

R2

VIN

V+

IR

+

VIN *

VREF

R2

Q5

D7

Q6

Figure 2. Simplified Rectifier Schematic

Q8 Q9

V+

Q10

Q17

IR

+

VIN

R

10k

–

A3

+

IR1

D11

D12

IQ = 2IR2

IR2

X2

Q16

10k

D15

D13

Q14

X2

Q18

CR

CA

TRACKING

TRIM

Figure 3. Buffer Amplifier Schematic

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 3, 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 CR to attack time

insignificant. Neglecting diode impedance,

the gain Ga(t) for ∆G can be expressed as

follows:

*t

Ga(t) + (GaINT * GaFNL e t A ) GaFNL

GaINT=Initial Gain

GaFNL=Final Gain

τA=RA • CA=10k • CA

where τA is the attack time constant and RA

is a 10k internal resistor. Diode D15 opens

the feedback loop of A3 for a negative-going

signal if the value of capacitor CR is larger

than capacitor CA. The recovery time

depends only on CR • RR. If the diode

impedance is assumed negligible, the

dynamic gain GR (t) for ∆G is expressed as

follows.

*t

GR(t) + (GRINT * GRFNL et R ) GRFNL

GR(t)=(GR INT–GR FNL) e +GR FNL

τR=RR • CR=10k • CR

where τR is the recovery time constant and

RR is a 10k 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 ±3µA.

Basic Expandor

Figure 4 shows an application of the circuit as

a simple expandor. The gain expression of

the system is given by

October 7, 1987

6

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