Figure 4. CT Attenuator Control Block Circuit

MICROPHONE AMPLIFIER

The microphone amplifier (Pin 10, 11) has the non-

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 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

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

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

= 5.1 K

Ω) 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

pin should be grounded.

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

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

<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 KK34118,

from pins FI to FO is a buffer with a high input

impedance (>1.0 M

Ω), 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

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

KK34118