KK34118
Figure 4. CT Attenuator Control Block Circuit
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.
MICROPHONE AMPLIFIER
The microphone amplifier (Pin 10, 11) has the non-
inverting 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 current 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
FILTER
The operation of the filter circuit is determined by
the external components. The circuit within the
KK34118,
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 within 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.
6
KODENSHI [ KODENSHI KOREA CORP. ]
