U4082B Datasheet, PDF (11/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

U4082B
3.12
Hybrid Amplifiers
The two hybrid amplifiers (at HTO+, HTO–, and HTI), in conjunction with an external trans-
former, 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 1-2 on
page 2), and its output drives the second amplifier, the gain of which is internally set at –1.0.
Unlike most operational amplifiers, these amplifiers have an all-NPN output stage, which maxi-
mizes 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 approximately 1.0 MHz (see Figure 7-6 on
page 19). The maximum output swing (peak to peak) of each amplifier is typically 1.2V less than
V_S with an output impedance of < 10Ω 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.
The connections to the coupling transformer are shown in Figure 1-1 on page 1. Balancing the
network is necessary to match the line impedance.
3.13 Filter
The operation of the filter circuit is determined by the external components. The circuit within the
U4082B from pins FI to FO is a buffer with a high input impedance (> 1 MΩ) and a low output
impedance (< 50Ω). The configuration of the external components determines whether the cir-
cuit is a high-pass filter (as shown in Figure 1-2 on page 2), a low-pass filter, or a band-pass
filter.
As a high-pass filter, with the components shown in Figure 3-6 on page 12, the filter will keep out
the 60Hz (and 120Hz) hum which can be picked up by the external telephone lines.
As a low-pass filter (Figure 3-7 on page 12), it can be used to roll off the high-end frequencies in
the receive circuit, which aids in protecting against acoustic feedback problems.
With an appropriate choice of an input coupling capacitor to the low-pass filter, a band-pass filter
is formed.
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