| Electronics > Beginners |
| (Most) Simple "Regen" AM receiver - confused about the feedback circuit |
| (1/1) |
| AxelGreen:
Hi everyone ! I'm sorry to soil this forum with such a question ! I'm majoring in math (first year at Uni in France), but happened to love electronics :-X I can't fathom why (in many regen receivers and particularly this one, see attachment) the signal reenters the LC filter through the middle of the coil. I intuitively understand that it cannot but increase the quality factor but I'm a bit at loss on how to handle the analysis of this circuit. Couldn't this parallel LC filter be replaced with a series LC filter, filtering directly the signal from the emitter of T1 ? Is there any induction from one part of the coil to another ? (this is one single coil, the wire from T1 being placed at the fifth turn from the gnd). Or is it just an electrical way to get the signal back into the filter ? Thanks a lot and sorry for my terrible English ! :palm: |
| Paul Rose:
Your English is good, and regen radios are fun. It would help if you posted a link to (or copy of) an example schematic. You are right that taking the feedback from a tap is about keeping the Q of the overall tank circuit high ( loading it less ). There are other schemes ( often seen in older vacuum tube designs ) where they get the feedback from a separate coil with fewer windings, but wound on the same form. Yes, there is mutual coupling. You can model either case ( tapped coil or separate coild ) in something like LtSpice. Place two inductors with appropriate inductance. To model a tapped coil, connect the "dotted" end of one to the plain end of the other. You may have to enable something in the inductor's properties to show phase dots. Then set the coupling factor between the inductors to something high ( like 1.0 ). Use a spice "K" directive to set the coupling ( example "K La Lb 1" ). You might get more responses in the "RF, Microwave, Ham Radio" section of this forum. |
| Paul Rose:
OK, found this schematic, which is reasonably easy to understand. The collector-emitter voltage drives the top half of the coil. Note that the emitter is AC coupled to the center tap. The feedback into the base of the transistor is taken from the bottom half of the coil. When the top of coil swings positive with respect to the center tap, the bottom of the coil swings negative ( again with respect to the center tap ). This is due to the coupling between the top half and bottom half of the coil. But the collector-emitter voltage swings opposite to the base drive ( more base current = more collector current, so the collector-emitter voltage is lower ). So the net effect is positive feedback. The circuit "wants" to oscillate. But you cut the gain by adjusting R2 so that it doesn't quite oscillate. |
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