U4084B Datasheet, PDF (19/26 Pages) TEMIC Semiconductors – Low-Voltage, Voice-Switched Circuit for Hands-Free Operation

U4084B
I) Loop Gain
The total loop gain (of figure 21) must add up to a value
< 0 dB to obtain a stable circuit. This can be expressed as:
G_MA + G_T + G_EXT + G_ST + G_EXR + G_R + G_SA + G_AC < 0
......1
Using the typical numbers mentioned above, and using
the equation G_T + G_R = –40dB, the required acoustic cou-
pling can be determined:
G_AC <–[31 + 20.1 + (–15) + 0 + (–40) + 26.8] + –22.9
......2
An acoustic loss of at least 23 dB is necessary to prevent
instability and oscillations, commonly referred to as
“singing”. However, the following equations show that
greater values of acoustic loss are necessary to obtain
proper level detection and switching.
II) Switching Thresholds
To switch comparator C1, the currents I₁ and I₃ have to
be determined. When a receive signal V_L is applied to
Tip/Ring, a current I₃ flows through R3 into RLI2 (see
+ ƪ ƫ ^{figure 21) according to the following equation:}
I₃
V_L
R₃
G_EXR
G_R
G_SA
2
......3
where the terms in the brackets are in V/V gain terms. The
speaker amplifier gain is divided by two since G_SA is the
differential gain of the amplifier, and V₃ is obtained from
one side of that output. The current I₁, coming from the
microphone circuit, is defined by:
+ I₁
V_M
G_MA
R₁
......4
where V_M is the microphone voltage. Since the switching
threshold occurs when I₁ = I₃, combining the above two
equations yields:
+ V_M
V_L
R₁
R₃
[G_EXR
G_R
G_MA
G_SA]
2
...5
This is the general equation defining the microphone volt-
age necessary to switch comparator C1 when a receive
signal V_L is present. The highest V_M occurs when the
receive attenuator is at maximum gain (+ 6.0 dB). Using
the typical values of equation 5 results in:
V_M = 0.52 V_L
......6
To switch comparator C2, the currents I₂ and I₄ need to
be determined. When sound is applied to the microphone,
a voltage V_M is created by the microphone, resulting in
a current I₂ into TLI1:
+ I₂
V_M
R₂
ƪG_MA
G_T
Æ«GEXT
2
......7
Since G_EXT is the differential gain of the external transmit
amplifiers, it is divided by two to obtain the voltage V₂
applied to R₂. Comparator C2 switches when I₄ = I₂. I₄ is
defined by:
+ I₄
V_L
R₄
[G_EXR ]
......8
Setting I₄ = I₂, and combining the above equations results
in:
+ V_L
V_M
R₄
R₂
[G_MA G_T
G_EXR
G_EXT]
2
...9
This equation defines the line voltage at Tip/Ring neces-
sary to switch comparator C2 in the presence of a
microphone voltage. The highest V_L occurs when the
circuit is in transmit mode (G_T = + 6.0 dB). Using the
typical values for equation 9 yields:
V_L = 840 V_M (or V_M = 0.0019 V_L) . . . . . . 10
At idle mode, where the gain of the two attenuators is –20
dB (0.1 V/V), equations 6 and 10 yield the same result:
V_M = 0.024 V_L . . . . . . 11
Equations 6, 10, and 11 define the thresholds for switch-
ing, and are represented in figure 22.
The “M” terms are the slopes of the lines (0.52, 0.024, and
0.0019) which are the coefficients of the three equations.
The M_R line represents the receive to transmit threshold
in that it defines the microphone signal level necessary to
switch to transmit in the presence of a given receive signal
level. The M_T line represents the transmit to receive
threshold. The M_I line represents the idle condition, and
defines the threshold level on one side (transmit or
receive) necessary to overcome noise on the other.
M_R
V_M
M_I
M_T
12678
V_L
Figure 22. Switching thresholds
TELEFUNKEN Semiconductors
Rev. A1, 31-Jan-97
Preliminary Information
19 (26)