U4030B Datasheet, PDF (5/14 Pages) TEMIC Semiconductors – Telephone Speech Circuit

U4030B
Transmit Output Amplifier
The output signal of the microphone and DTMF
preamplifier is internally coupled to a second amplifier
W ^{( TXA) which is used to modulate a controlled current}
source. Assuming a termination of 600 at a line, the
gain from MICO to V_L is typically 15.6 dB.
Receive Amplifier
The receive signal is taken from line via capacitor C_CK.
A resistive attenuator ( –32 dB ) sets the appropriate input
W ^{level for the following output amplifier. The input}
impedance at Pin 4 is typically 80 k .
Voltage gain is:
G_R = 20 log ( V_ear / V_L ) dB
= –32 dB + 20 log [ ( RR1 / RR2 ) + 1 ] dB
= +1.8 dB
The adjustment range for receive gain G_R is typically
–4 dB to + 8 dB.
The built-in ear protection limits the output swing at Pin
7 to 2.4 V_pp ( V_D > 4 V). For high receive gain, the
maximum undistorted output level might not be sufficient
due to clipping by ear protection.
Sidetone
The amplified microphone signal is available at the input
of the sidetone loop, MICO. The loop consists of a
transmit amplifier ( transconductance STX ), the
impedance at line, receive attenuator ( gain) and the side-
tone network ( figure 7 ).
The sidetone cancellation is achieved by comparing a part
of the line signal with the output of the sidetone network
( VST → VR). Assuming a real impedance of the tele-
phone ( R_Tel ) the optimum sidetone network can be
calculated:
a
= ( S_TX R_APP G ) / ( 1 – S_TX R_APP G )
Z_NW = S_TX R G Z_Line
Adjustment to the sensitivities of the handset can be done
independently from the sidetone network because receive
and transmit gain are set outside of the sidetone loop.
Power Down (PD)
The speech circuit is switched low ohmic by selecting a
high level at PD during the pulse dialling. The voltage
drop across the IC will be typically 1.5 V. During this time
the capacitor CVD will not be discharged, because an
internal power down switches off all internal amplifiers.
In order to avoid cracks, it is recommended to activate
power down while sending the dial pulses ( figure 8 ).
R
aR ZNW
MICO
Mic.
+ ^{V ST}
VH
– VR
G = 1/40
VL
STX = 18.8 mS
RTEL
ZL
Figure 7. Schematic of the sidetone
93 7624 e
Rev. A2, 27-Jan-98
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