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SL34118

Voice Switched Speakerphone Circuit

System Logic Semiconductor 

SL34118

Figure 4. CT Attenuator Control Block Circuit

MICROPHONE AMPLIFIER

The microphone amplifier (Pin 10, 11) has the

noninverting input internally connected to VB, while

the inverting input and the output are pinned out.

Unlike most op-amps, the amplifier has an all-NPN

output stage, which maximizes phase margin and

gain-bandwidth. This feature ensures stability at

gains less than unity, as well as with a wide range of

reactive loads. The open loop gain is typically 80 dB

(f<100 Hz), and the gain-bandwidth is typically

1.0 MHz. The maximum p-p output swing is typically

1.0 volt less than VCC with an output impedance of

<10 Ω until curent limiting is reached (typically

1.5 mA). Input bias current at MCI is typically 40 nA

out of the pin.

Figure 5. Microphone Amplifier and MUTE

HYBRID AMPLIFIERS

The two hybrid amplifiers (at HTO+, HTO-, and

HTI), in conjunction with an external transformer,

provide the two-to-four wire converter for interfacing

to the telephone line. The gain of the first amplifier

(HTI to HTO-) is set by external resistors (gain = -

RHF/RHI in Figure 8), and its output drives the second

amplifier, the gain of which is internally set at -1.0.

Unlike most op-amps, the amplifiers have an all-NPN

output stage, which maximizes phase margin and

gain-bandwidth. This feature ensures stability at

gains less than unity, as well as with a wide range of

reactive loads. The open loop gain of the first

amplifier is typically 80 dB, and the gain bandwidth of

each amplifier is ≈1.0 MHz. The maximum p-p output

swing of each amplifier is typically 1.2 volts less than

VCC with an output impedance of <10 Ω until current

limiting is reached (typically 8.0 mA). The output

current capability is guaranteed to be a minimum of

5.0 mA. The bias current at HTI is typically 30 nA out

of the pin.

The connections to the coupling transformer are

shown in the Expanded Logic Diagram (Figure 8). The

block labeled Zbal is the balancing network necessary

to match the line impedance.

FILTER

The operation of the filter circuit is determined by

the external components. The circuit within the

IL34118, from pins FI to FO is a buffer with a high

input impedance (>1.0 MΩ), and a low output

impedance (<50 Ω). The configuration of the external

components determines whether the circuit is a high-

pass filter (as shown in Figure 8), a low-pass filter, or

a band-pass filter.

Figure 6. High Pass Filter

The muting function (Pin 12), when activated, will

reduce the gain of the amplifier to = -39 dB (will RMI

= 5.1 KΩ) by shorting the output to the inverting

input (see Figure 5). The mute input has a threshold

of 1.5 volt, and the voltage at this pin must be kept

withing the range of ground and VCC. If the mute

function is not used, the pin should be grounded.

As a high pass filter, with the components shown in

Figure 6 the filter will keep out 60 Hz (and 120 Hz) hum

which can be picked up by the external telephone

lines.

SLS

System Logic

Semiconductor

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