MC1490 Ver la hoja de datos (PDF) - Motorola => Freescale

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MC1490

RF/IF/audio amplifier

Motorola => Freescale 

DESCRIPTION OF SPEECH COMPRESSOR

MC1490

Table 1. Distortion versus Frequency

The amplifier drives the base of a PNP transistor operating

common–emitter with a voltage gain of approximately 20.

The control R1 varies the quiescent Q point of this transistor

so that varying amounts of signal exceed the level Vr. Diode

] D1 rectifies the positive peaks of Q1’s output only when these

peaks are greater than Vr 7.0 V. The resulting output is

filtered by Cx, Rx.

Rx controls the charging time constant or attack time. Cx is

involved in both charge and discharge. R2 (the 150 kΩ and

input resistance of the emitter–follower Q2) controls the

decay time. Making the decay long and attack short is

accomplished by making Rx small and R2 large. (A

Darlington emitter–follower may be needed if extremely slow

decay times are required.)

The emitter–follower Q2 drives the AGC Pin 5 of the

MC1490P and reduces the gain. R3 controls the slope of

signal compression.

Frequency

100 Hz

Distortion

10 mV ei

3.5%

100 mV ei

12%

Distortion

10 mV ei

15%

100 mV ei

27%

300 Hz

2%

10%

6%

20%

1.0 kHz

1.5%

8%

3%

9%

10 kHz

1.5%

8%

1%

3%

100 kHz

1.5%

8%

1%

3%

Notes 1 and 2

Notes 3 and 4

Notes:

(1) Decay = 300 ms

Attack = 20 ms

(2) Cx = 7.5 µF

Rx = 0 (Short)

(3) Decay = 20 ms

Attack = 3.0 ms

(4) Cx = 0.68 µF

Rx = 1.5 kΩ

Figure 24. Speech Compressor

+12 V

25 µF

0.001

1.0 k

1.0 k

15 µF

Input

2

5

1

4 MC1490P

8

6

3

10 µF

10 µF

15 µF

7

+12 V +12 V

R3

15 k

+12 V

Q2

2N3904

220 2.2 k

R2

Q1

2N3906

Vr

Rx D1

33 k

4.7 k 150 k

Cx

6.8 k

R1

100 k

Output

MOTOROLA ANALOG IC DEVICE DATA

7

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