PBL3852 データシートの表示（PDF） - Ericsson

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PBL3852

Universal Transmission Circuit

Ericsson  

PBL 3852

microphone amplifier a similar RC

combination ought to be connected from

the other input, pin 13, to ground.

An electret microphone with a built in

FET amplifier is to be seen from outside

as a high impedance constant current

generator and is normally specified with a

load resistance of ≈ 2k. This is to be

considered as max. value and by using it

will render the max. gain from the micro-

phone. This level of input signal that is

unnecessary high will result in clipping in

the microphone amplifier and in mute

condition permeate through the input to

Strong

cc gen.

the circuits reference and this way to all

functions, resulting among other things in

a bad mute. Hence it is better regarding

noise perfomance and mute to rather use

the gain of the microphone amplifier than

the gain of the microphone itself (in case

of electret) flat out. A more suitable level

of gain from the microphone is achieved

by using a load resistance of 200 - 470Ω.

Gain setting to the line is done at the

input of the transmitter.

It is possible to use the microphone

amplifier as a limiter ( added to the limiter

in the transmitter output stage ) of the

DC ( ref. ≈ 1.16V )

ref. minus a diode ≈ 0.5V

transmitted signal (See fig. 9). The

positive output swing is then limited by the

peak output current of the microphone

amplifier. The negative swing is limited by

the saturation voltage of the output

amplifier. The output of the amplifier is

DC-vice at internal reference level

(1.16V). The lowest negative level for the

signal is reference minus one diode and

sat. transistor drop (1.16-0.6-0.1 = 0.46V).

The correct clipping level is found by

determining the composite AC- and DC-

load that gives a maximum symmetrical

unclipped signal at the output. This signal

is then fed into the transmitter amplifier at

a level that renders a symmetrical signal

clipping on the line. (adjust with ratio R4,

R5) The total transmitter gain when an

electret microphone is used can then be

adjusted with the load resistor of the

electret microphones buffer amplifier.

11

DC-

AC-

load

load

DC-load = R4+R5

AC-load = R4+R5// (R6+ZDTMF) //ZTI

ZDTMF = DTMF generator impedance

Figure 9. Microphone amplifier output clipping.

(a)

PBL 3852

11

13

M

12 +

Dynamic

microphone

(b)

PBL 3852

(c)

11

13

M

12 +

Magnetic

microphone

R

C+

(h)

11

4

PBL 3852

13

M

12 +

Balanced electret microphone.

An additional RC filterlink is

recommended if pin 4 is used

as a supply.

(g)

PBL 3852

11

Im

13

M

12 +

Unbalanced electret

microphone

Rx

Ry

DC1

Mic. ampl. supplies the mic.

current Im, set by Rx and Ry.

Figure 10. Microphone solutions.

PBL 3852

(d)

11

13

M

12 +

4 PBL 3852

11

13

M

12 +

2 For dynamic mic.with

reduced gain

regulation

Unbalanced electret

mic. with balanced

signal, DC-supply from

pin 4.

DC1

(f)

PBL 3852

11

13

M

12 +

Balanced electret

microphone with reduced

gain regulation

DC1

(e)

11

PBL 3852

13

M

12 +

Balanced electret

microphone

7

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