TS34118

12-20

2004/09 rev. B

Microphone Amplifier

Figure 7. 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

most op-amps, the amplifiers have 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 80dB , and the gain bandwidth of each amplifier is ≈1.0MHz (see Figure 16). The maximum p-p

output swing of each amplifier is typically 1.2 volts less than Vcc with an output impedance of <10Ωuntil current limiting is

reached (typically 8.0mA). The output current capability is guaranteed to be a minimum of 5.0mA. The bias current at HTI

is typically 30nA out of the pin.

The connections to the coupling transformer are shown in the Block Diagram (Figure 2). 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 TS34118, from pins FI to

FO is a buffer with a high input impedance (>1.0M

Ω), and a low output impedance (<50Ω). The configuration of the

external components determines whether the circuit is a high-pass filter (as shown in Figure 2), a low-pass filter, or a

band-pass filter.

As a high pass filter, with the components shown in Figure 8, the filter will keep out 60Hz (and 120Hz) hum which can be

picked up by the external telephone lines.

Figure 8. High Pass Filter