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SL34118N Datasheet(PDF) 6 Page - System Logic Semiconductor |
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SL34118N Datasheet(HTML) 6 Page - System Logic Semiconductor |
6 / 12 page SL34118 System Logic Semiconductor SLS Figure 4. CT Attenuator Control Block Circuit MICROPHONE AMPLIFIER The microphone amplifier (Pin 10, 11) has the noninverting input internally connected to VB, 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 VCC 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 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 withing the range of ground and VCC. If the mute function is not used, the 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 HTO-) is set by external resistors (gain = - RHF/RHI 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 VCC 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 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. |
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