SAN2202
7 Circuit description
7.1 8Ω loudspeaker amplifier
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
Ptot = VCE * (ILine - IIC1)
where:
V
V
CE,Q4 = VBE,Q4+UD2 and
CE,Q6 = VBE,Q6+UD1
VBE(Q4,Q6) ≈ 0.7V, VTH(D2,D1) ≈ 0.52V.
SA2531/2= typ. 3mA (see data sheet)
I
Power dissipation at Iline =100mA:
Ptot = (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
amplifiers´ supply voltage VCC must be limited to <2.44V 1)
.
This is accomplished by
V
CC = VBE(Q6) + VTH(D1)+ VTH(D2)+VBE(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
the C-E saturation voltage of Q3 is assumed >60mV
To maintain full transmit swing (± 2Vpeak VC(Q3) = VCC =(2.5V-60mV) < 2.44V
4.5V ± 2Vpeak(max. sending level),
)
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 VPP must be considered: If VPP
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 VSS, VLI cannot
be shorted to VBE (as in the standard application AN1500 with the collector of Q3 being connected to VSS).
During pulse dialing, VLI will be ≈2.5V, thus increasing the total “ON”-resistance by
2.5V − 0.6V
∆R
=
= 111Ω(20mA)....19.6Ω(100mA)
ON
I
− 3mA
Line
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
sa me s
SAMES [ SAMES ]
