Prevention of LF feedback in FM receivers - uses opposite polarity common supply rails for LF and HF stages

摘要

Feedback from the loudspeaker in an FM receiver affects the previous stages resulting in changes of frequency response and loss of sensitivity. This problem is resolved by using a supply rail for the L.F. stages of opposite polarity to that for medium and H.F. stages. In an FM receiver the high and medium frequency stages (11-15) are subjected to feedback (arrowed) from the low frequency stage (16) and loudspeaker (18) which returns to this stage through the commong power rail. This loop is broken by supplying the high and medium frequency stages from a power rail (A) of opposite polarity to that (B) of the low frequency stage. Decoupling is provided by series resistor (23) and a capacitor (24).