5.5.5 Separation of Stages - Preventing the Oscillation
On of the significant problems that occur at devices that comprise more cascade-linked amplifying stages is the occurrence of the feedback over the conductors that connect those stages with the positive pole of the battery, or the power supply. By the way, the feedback is a phenomenon when part of the signal exiting an amplifier gets on its input. Under certain conditions, this feedback causes the oscillation of the stage, which in devices that have the loudspeaker on output, manifests itself as strong whistling, squeaking and similar.On of the ways to prevent this feedback is given on pic.5.16, where a block-diagram of a radio receiver that has four amplifying stages with active components (transistors or IC’s) that require the battery supply is shown. Separation of stages for the AC current (preventing the feedback) is accomplished by the LF filters with resistors and capacitors. Resistors are from couple of hundreds of Ohms to 1 kOhm. Capacitances of C1 and C2 are from couple of tenths till couple of hundreds of nF, and of C3 from couple of hundreds of nF to about 100 mF. The stage PCBs should be designed in such way to make the contact where right end of the capacitor is soldered as close to the contact where the positive end of the power supply voltage is brought (e.g. on pic.5.9, the right contact for C6 should be as close as possible to the contact where pin 8 of NE612 is soldered).In the devices supplied from the battery, the C5 capacitor, which has capacitance of couple of hundreds of micro Farads, serves to take the role of the battery when it gets emptied a little bit, and strong tones have to be reproduced at the loudspeaker (in simple terms, C5 acts as a small accumulator that helps the worn-out battery to give enough power to the power amplifier, when necessary. When its help isn’t needed, the capacitor is refilled). This capacitor is not needed when the receiver is supplied from the adaptor that already has an electrolytic capacitor on its output, and when the wires that connect the adaptor to the receiver are not longer than about 15 cm.