IL34118 Datasheet, PDF (6/12 Pages) Integral Corp. – Voice Switched Speakerphone Circuit

IL34118
Figure 4. CT Attenuator Control Block Circuit
MICROPHONE AMPLIFIER
The microphone amplifier (Pin 10, 11) has the
noninverting input internally connected to V_B, 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 V_CC 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 = -
R_HF/R_HI 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 V_CC 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 V_CC. 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.
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