BL34018 Просмотр технического описания (PDF) - Shanghai Belling Co., Ltd.

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BL34018

BL34018

Shanghai Belling Co., Ltd. 

BL34018

amplifier to be biased slightly higher than normal. This amplifier then becomes the preferred one

in the system operation. Resistor values from 500 k to 10 M ohms are recommended for this

purpose.

SPEAKER AMPLIFIER

The speaker amplifier has a fixed gain of 34 dB (50V/V), and is noninverting. The input

impedance is nominally 22 kΩ as long as the output signal is below that required to activate the

Peak Limiter. Since the output current capability is 100 mA, the output swing is limited to 5.0VPP

while load is 25 ohms. The output impedance depends on the output signal level and is relatively

low as long as the signal level is not near the maximum limits. At 3 VPP, it is < 0.5 ohms, and at

4.5 VPP, it is < 3 ohms. The output is short circuit protected at approximately 300 mA.

When the amplifier is overdriven, the peak limiter causes a portion of the input signal to be

shunted to ground, in order to maintain a constant output level. The effect is that of a gain

reduction caused by a reduction of the input impedance (at SKI) to a value not less than 2.0 kΩ.

The capacitor at Pin 17 (AGC) determines the response time of the peak limiter circuit.

When a large input signal is applied to SKI, the voltage at AGC will drop quickly as a current

source is applied to the external capacitor. When the large input signal is reduced, the current

source is turned off, and an internal 110 kΩ resister discharges the capacitor so the voltage at

AGC can return to its normal value (1.9 Vdc). The capacitor additionally stabilizes the peak

limiting feedback loop.

If there is a need to mute the speaker amplifier without disabling the rest of the circuit, this

may be accomplished by connecting a resistor from the AGC pin to ground. A 100 kΩ resister

will reduce the gain by 34 dB (0 dB from SKI to SKO), and a 10 kΩ resister will reduce the

gain by almost 50 dB.

TRANSMIT DETECTOR CIRCUIT

The transmit detector circuit, also known as the background noise monitor, distinguishes

speech (which consists of bursts) from the background noise ( a relatively constant signal). It does

this by storing a voltage level, representative of the average background noise, in the capacitor at

CP1( Pin 11). The resistor and capacitor at this pin have a time constant of approximately 5

seconds(in Figure 1). The voltage at Pin 11 is applied to the inverting input of the Transmit

Detector Comparator. In the absence of speech signals, the noninverting input receives the same

voltage level minus an offset of 36 mV. In this condition, the output of the comparator will be low,

the output transistor turned off, and the voltage at XDC (Pin 23) will be at ground. If the Tx-Rx

comparator is in the transmit position, the attenuators will be in the idle mode ( ∆Vacf = 75 mV).

When speech is presented to the microphone, the signal burst appearing at XDI reaches the

noninverting input of the transmit detector comparator before the voltage at the inverting input

can change, causing the output to switch high, driving the voltage at XDC up to approximately 4

V. This high level causes the attenuator control block to switch the attenuators from the idle mode

to the transmit mode (assuming the Tx-Rx comparator is in the transmit mode).

The series resistor and capacitor at XDI (Pin 13) determine the sensitivity of the transmit

detector circuit. Increasing the resistor, or lowering the capacitor, will reduce the response at CP2

and CP1.

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-9-

Total 16 Pages

8/15/2006

Wrote by 2006

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