SAN2202
7 Circuit description
7.1 8Ω loudspeaker amplifier
Ω
The amplifier is a simple Class-A type amplifier. Gain is set by R6, the bandwidth is imited by CC4 (to avoid
howling). Q4 and Q6 should be matched in gain (complementary types).
Nearly all the line current will flow through these transistors, independent of the signal output amplitude. The
maximum power dissipation of Q4 and Q6 is
P
tot
= VCE * (I
Line
- I
IC1
)
where:
V
CE,Q4
= V
BE,Q4
+U
D2
and
V
CE,Q6
= V
BE,Q6
+U
D1
V
BE
(Q4,Q6)
≈
0.7V, V
TH
(D2,D1)
≈
0.52V.
I
SA2531/2
= typ. 3mA (see data sheet)
Power dissipation at I
line
=100mA:
P
tot
= (0.7+0.52) * (0.1-0.003) = 118mW
In order to maintain no interference of the circuit to AC-impedance, DC-mask, maximum sending level, etc..., the
1)
amplifiers´ supply voltage V
CC
must be limited to <2.44V .
This is accomplished by
V
CC
= V
BE
(Q6) + V
TH
(D1)+ V
TH
(D2)+V
BE
(Q4) .
V
CC
= 0.68V + 0.5V + 0.5V + 0.68V =
2.36V
1)
Critical for SAN2202a configuration, not critical for SAN2202b configuration.
VLI can range between 2.5...6.5V
⇒
4.5V
±
2V
peak
(max. sending level),
the C-E saturation voltage of Q3 is assumed >60mV
To maintain full transmit swing (± 2V
peak
)
⇒
VC(Q3) = VCC =(2.5V-60mV) <
2.44V
Transistor Q1,R1,R2 and R3 forms an impedance matching circuit to provide better matching between the receive
output of the IC and the output amplifier stage.
7.2 Amplifier supply in shunt transistor path
This configuration requires no extra components. However, the maximum voltage of V
PP
must be considered: If V
PP
rises >2.44V, the maximum transmit level will be slightly distorted at the negative half-wave.
Additionally, the “Make”-resistance in pulse dialing will be higher, because with CS being pulled to V
SS
, V
LI
cannot
be shorted to V
BE
(as in the standard application AN1500 with the collector of Q3 being connected to V
SS
).
During pulse dialing, VLI will be
≈2.5V,
thus increasing the total “ON”-resistance by
∆
R
ON
=
2.5
V
−
0.6
V
=
111
Ω
( 20
mA
)....19.6
Ω
(100
mA
)
I
Line
−
3
mA
7.3 Amplifier supply by power extraction circuit SAN3020
This power extraction can be used to supply any external load by the available line current without affecting the
performance of the single chip telephone. A detailed description of this circuit is given in application note SAN3020.
“ON”-resistance in pulse dialing (compared to the standard application AN1500) is only increased by
≈10Ω
(see.
Fig. 6).
5/9
sames
SAMES [ SAMES ]
