ts34118 Taiwan Semiconductor Company, Ltd. (TSC), ts34118 Datasheet - Page 12

• Manufacturer Part Number: ts34118
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: Taiwan Semiconductor Company, Ltd. (TSC)

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
(gain=-R
/P
in Figure 2), and its output drives the second amplifier, the gain of which is internally set at –1.0. Unlike
HF
HI
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 <10 until 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:), and a low output impedance (<50:). 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
TS34118
12-20
2004/09 rev. B