UNISONIC TECHNOLOGIES CO., LTD

U4082B

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

8 of 13

www.unisonic.com.tw

QW-R108-020.A

FUNCTIONAL DESCRIPTION

INTRODUCTION

Between operation of a speakerphone and a handset, the fundamental difference is that of half-duplex versus

full-duplex. The UTC U4082B includes the necessary level detectors, attenuators, and switching control for a

properly operating speakerphone. The detection sensitivity and timing can be externally controlled. The UTC

U4082B

also provides background noise monitors to make the circuit insensitive to room and line noise, hybrid

amplifier to interface to Tip and Ring, the microphone amplifier, and other associated functions.

ATTENUATORS

The transmit and receive attenuators are complementary in function, and they are never both fully on or both

fully off. For example, when one is at maximum gain: +6dB, the other is at maximum attenuation: -46dB, and vice

versa. But sum of their gains remains constant at -40dB typically. To control the transmit and receive paths to

provide the half-duplex operation required in speakerphone is the attenuators’ purpose. The attenuators are

non-inverting, have a -3dB (from max gain) frequency of ~100kHz, their input impedance of each one is nominally

10kΩ, and to prevent distortion, their input signal should be limited to 350 mVrms(same as 990mvp-p). After the

output current limit (typically 2.5mA) is reached, the output impedance is<10Ω.

96KΩ

4.0kΩ

10kΩ

To Attenuator Input

VB

TXI/RXI

These attenuators can be controlled by the Control Block’s output signal. When the CT pin which measures the

signal is at +240mV with respect to VB, the circuit is in the receive mode and so receive attenuator is at +6dB. When

the CT voltage is equal to VB, the circuit is in an idle mode, and it also causing the attenuators’ gains to be halfway

between their fully on and fully off positions (typical -20 dB each). The most direct method to monitor the circuit’s

mode is monitoring the CT voltage (with respect to VB).

LEVEL DETECTORS

There are two level detectors on the receive side and two level detectors on the transmit side. The terms in

parentheses form one system, and the other terms form the second system. All the four level detectors are high

amplifiers which have back-to-back diodes in the feedback path, and then it is resulting in non-linear gain, which

permits operation over a wide dynamic range of speech levels. TLI1, TLI2, RLI1, and RLI2, the external resistor and

capacitor determine the sensitivity of each level detector. Through a diode each output charges an external capacitor

and limiting resistor, thus providing a DC representation of the input AC signal level. The capacitor and an internal

350Ω resistor provide the outputs’ quick rise time. And an internal current source and the capacitor set a slow decay

time. On the receive side, RLI1 is at the receive input receiving the same signal as at Tip and Ring, and RLI2 is at

the output of the speaker amplifier. On the transmit side, TLI2 is at the microphone amplifier, while TLI1 is at the

hybrid output.