U4084B Datasheet, PDF (13/26 Pages) TEMIC Semiconductors – Low-Voltage, Voice-Switched Circuit for Hands-Free Operation

U4084B
V_B
12
R_T
11
C_T
I₁
C_T
2kΩ
I₂
60µA
–
to attenuators
+
Attenuator
control
4
C1 ... C4
Volume control
Dial-tone detector
AGC
12675
Figure 6. C_T attenuator control block circuit
Operation of the C_T Circuitry
–
W m R_T is typ. 120 k and C_T is typ. 5.0 F.
–
To switch to receive mode, I₁ is turned on (I₂ is
off), charging the external capacitor to + 240 mV
above V_B. (An internal clamp prevents further
charging of the capacitor).
–
To switch to transmit mode, I₂ is turned on (I₁ is
off) bringing down the voltage on the capacitor
to – 240 mV with respect to V_B.
–
To switch to idle mode quickly (fast idle), the
current sources are turned off, and the internal
W 2.0-k resistor is switched on, discharging the
W capacitor to V_B with a time constant = 2.0 k
C_T.
–
W ^{To switch to idle slowly (slow idle), the current}
sources are turned off, the switch at the 2.0-k
resistor is open, and the capacitor discharges to
V_B with a time constant = R_T C_T.
Microphone Amplifier, Pins 7, 8 and 9
The non-inverting input of the microphone amplifier
(Pins 7 and 8) is connected to V_B, while the inverting in-
put and the output are pinned out.
Unlike most op amps, the amplifier has an all NPN output
stage which maximizes phase margin and gain band-
width. This feature ensures stability at gains less than
unity, as well with a wide range of reactive loads.
The open loop gain is typically 80 dB (f < 100 Hz), and
the gain-bandwidth is typ. 1.0 MHz (see figure 4). The
W ^{maximum p-p output swing is typ. (VS – 1 V) with an out-}
put impedance of < 10 until current limiting is reached
(typ. 1.5 mA). The input bias current at MIC is typically
– 40 nA.
RMF
RMI
8
from
Mike
MIC
VS
VB
+
–
VS
7
MICO
9
MUTE
90kΩ
75kΩ
12676
Figure 7. Microphone amplifier and mute
TELEFUNKEN Semiconductors
Rev. A1, 31-Jan-97
Preliminary Information
13 (26)