L6726
LINEAR INTEGRATED CIRCUIT
APPLICATION INFORMATION
Refer to fig. 6
Gain regulation (R1, R2):
These resistors set the starting point for the gain-and side tone regulation.
Input impedance pin 6 is about 5.5 kΩ±20%.
Below are some typical values for R1 and R2, for some different supplies from telephone stations.
Battery feed
No regulation
R1
R2
R6
Set for short line gain.
All feedings
22kΩ
47~100Ω
Regulation:
48V, 2·200Ω
48V, 2·400Ω
36V, 2·500Ω
∞
∞
∞
∞
47Ω
75Ω
100Ω
180kΩ
Impedance to line (R3, C1, C2):
C1 in series with R3 and those in parallel with C2 give the impedance to the line from the set.
C2 must normally be there for the interference too.
The network is optimized with regard to the return loss.
The time constant R3, C1 cannot be allowed to be too small. If the time constant is to small, distortion at low
frequencies may be generated.
If R3 is changed this will change the DC-characteristic too, which set by the voltage at pin 4. The input current at pin
4 is about 1mA
Microphone selection (M1, R16):
The microphone can be either dynamic, magnetic or electrets. See fig. 12, 14 and 15.
The input impedance between pin 12 and 13 is about 1.7kΩ with typical variation ±20%
Transmitting gain and frequency response (R4, R5, C3, C4):
The network gives the amplification and frequency response for the transmitter. R5 is used when a greater
reduction of the gain is wanted. Input impedance at pin 3 is about 17 kΩ with typical variation ±20%.The sum of the
resistors R4 and R5 must not be less than 35 kΩ because of DC-current to ground. Below is some simple variations
of this network. (See fig. 7).
DC-characteristics (R6):
Sets the DC-characteristic and take care of some of the supplied power. The resistor have also function with the
protection of the circuit, the transmitter, the gain regulation and side tone. Common values are 47 Ω to 100 Ω.
Side tone level, receiving gain and frequency response.(R7,R8,R9,R10,R11,C5):
The network sets the side tone balance. The network in the application is one of many possibilities. R10 and R11
balance together the signals that go two different ways from the transmitter output stage one from pin1 and the other
from pin2. The balance network consist of R8, R9 and C5.
The example is not optimized to any specified line, it is only given to show the principle.
Fig8, 9 and 10 are examples of balance networks. Fig.10 is especially suited for lines with high current supply.
R12, R13, C6, (R10, R11):
The networks gives the gain and frequency response for the receiver.
Input impedance at pin 15 is about 38 kΩ with a typical variation of ±20%.For different possibilities for the design
of the network, see the network for the transmitter (R4, R5, C3)
REC, R14:
R14 generates the output impedance to the magnetic earphone (fig.11). If a dynamic earphone is used it should
be placed between pin 17 and 18.See fig. 16.
UNISONIC TECHNOLOGIES CO., LTD
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