U4082B Datasheet, PDF (24/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

8.2.1
8.2.2
8.2.3
Transmit Gain
The transmit gain, from the microphone output (VM) to Tip and Ring, is determined by the output
characteristics of the microphone, and the desired transmit level. For example, a typical electret
microphone will produce approximately 0.35 mVrms under normal speech conditions. To
achieve 100 mVrms at Tip/Ring, an overall gain of 285V/V is necessary. The gain of the transmit
attenuator is fixed at 2.0 (+6.0 dB), and the gain through the hybrid of Figure 1-2 on page 2
(GHT) is nominally 0.4 (–8.0 dB). Therefore, a gain of 357V/V is required of the microphone and
hybrid amplifiers. It is desirable to have the majority of that gain in the microphone amplifier for
three reasons:
1. The low-level signals from the microphone should be amplified as soon as possible to
minimize signal/noise problems;
2. to provide a reasonable signal level to the TLI2 level detector;
3. and to minimize any gain applied to broadband noise generated within the attenuator.
However, to cover the normal voice band, the microphone amplifier's gain should not
exceed 48 dB (Figure 7-7 on page 19). For the circuit in Figure 8-3 on page 23, the gain
of the microphone amplifier was set at 35V/V (31 dB), and the differential gain of the
hybrid amplifiers was set at 10.2V/V (20.1 dB).
Receive Gain
The overall receive gain depends on the incoming signal level and the desired output power at
the speaker. Nominal receive levels (independent of the peaks) at Tip/Ring can be 35 mVrms
(–27 dBm), although on long lines that level can be down to 8.0 mVrms (-40 dBm). The speaker
power is:
P_SPK
=
1----0---^d_---^B_---^m_----^/_-¹_---⁰_----×_{------0---.--6--}
R_S
(1)
where R_S is the speaker impedance, and the dBm term is the incoming signal level increased by
the gain of the receive path. Experience has shown that approximately 30 dB gain is a satisfac-
tory amount for the majority of applications. Using the above numbers and equation 1, it would
appear that the resulting power to the speaker is extremely low. However, equation 1 does not
consider the peaks in normal speech which can be 10 to 15 times the rms value. Considering
the peaks, the overall average power approaches 20 to 30 mW on long lines, and much more on
short lines.
Referring to Figure 1-2 on page 2, the gain from Tip/Ring to the filter input was measured at
0.833V/V (–1.6 dB), the filter's gain is unity, and the receive attenuator's gain is 2.0V/V (+6.0 dB)
at maximum volume. The speaker amplifier's gain is set at 22V/V (26.8 dB) which puts the over-
all gain at approximately 31.2 dB.
Loop Gain
The total loop gain (of Figure 8-3 on page 23) must add up to less than 0 dB to obtain a stable
circuit. This can be expressed as:
G_MA + G_T + G_HA + G_ST + G_FO + G_R + G_SA + G_AC < 0 ( 2 )
Using the typical numbers mentioned above, and knowing that G_T + G_R = –40 dB, the required
acoustic coupling can be determined:
G_AC < –[31 + 20.1 + (–15) + 0 + (–40) + 26.8] = –22.9 dB( 3 )
24 U4082B
4743D–CORD–03/06