In the RegenRX2 RegenRX Yahpoo group, an overflow group for RegenRX, WU2D posted RegensWU2D (a PDF file), showing two two transistor regenerative receivers.
The first one, “Bipolar Two”, shows the schematic for a two transistor receiver using a NTE108 for the regen front end and a 2N3904 for the audio amplifier. The NTE108 is a UHF transistor with three times the fT of a 2N3904, or 600MHz minimum. Substitutes could be the MPS3563 or 2N918. But I really don’t understand why the author used such a high frequency transistor on a 3.5 MHz receiver. I could understand using this transistor for a two meter or higher receiver but for the HF bands the 2N3904 should work just as well.
Between the two stages the author used a pi network to filter out the RF, consisting of two .001 uF capacitors and a 2.5 mH choke. The choke seems overkill to me, I think the two capacitors with a 1k resistor would give just as good a performance with a lot less cost and the resistor would be a lot easier to obtain.
The second schematic shows a two transistor regen receiver with a FET for the regen front end. The FET is a MPF102, all of which have a wide spread of Idss and often need to have the bias adjusted to the right current for proper operation. If this circuit doesn’t want to work properly, it may be necessary to adjust the 3.3k source resistor. Oddly, the audio is tapped off of this source resistor.
I also noticed that two 1N4001 rectifier diodes are being used for adjusting the regeneration. I find that odd because in this case they are being used as varicaps to adjust the amount of capacitance on the drain. One should also make sure that the 2.5 mH choke has a resonant frequency that is above the band it is receiving, in this case 7.5 MHz.
One other nit-pick I have is wasted battery current. In this case the 470 ohm resistor and 6.8V zener diode make a shunt regulator, which is wasteful of power. A less wasteful way of regulating the voltage should be tried, such as a low current three terminal regulator chip.
Both of these receivers are for voice communications, and may benefit from rolling off the low audio frequencies at 300 Hz. To do this requires smaller coupling capacitors and bypass capacitors. I would reduce the value of the coupling capacitors after the volume controls and the value of the 22 uF emitter bypass capacitors.
There is a standard warning that I add to any device that is connected to an antenna, and has static sensitive parts on the front end. If a thunder storm passes nearby, it will most likely burn out a FET or even a transistor if it is used at the antenna input. In this case, the two separate windings give the best isolation for the transistors, but Nature always sides with the hidden flaw, and the lightning’s strong electromagnetic pulses can still pass through the coil and do damage. I tell people to put the transistor in a socket and tape a bag of spare transistors inside the case of the radio – you’ll probably need them.
Back to experimenting…