Modem amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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The circuit is a positive feedback amplifier. It is installed on the telephone line parallel to the modem. Gain 1,5..2 times. Operational amplifier DA1 can be used by anyone, given the purpose of the pins. Used 553UD2 in plastic DIP. 3 +Vcc=9...15V, -Vcc=-9...-15V. Those. you need a bipolar source with an average "earth" (gnd). L1 - any choke at 200..400uH. Used DP1,2-400. R1 - adjusts the gain. VS1, VS2 - 20V symmetrical zener diodes to protect DA1. 2 series-connected KS210B were used. Capacitor C3 is connected correctly: (+) to gnd.
Setting
If there is an oscilloscope, with the power on and the t / f line connected (the handset is up!) The presence of generation at pin 10 of DA1 is checked at the leftmost position of the R1 engine according to the diagram.
By gradually increasing R1 to achieve the disappearance of generation. This position of R1 corresponds to some threshold gain level. For stability, add 5..20% to the current value of R1.
All these operations can be done without an oscilloscope by ear, by connecting a parallel device, but you will need to take into account the additional load in the line. The final position of R2 is at a specific connection.
Publication: cxem.net
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