To see this working, head to your live site.

Sensitive TRF circuit

I posted a very sensitive TRF circuit on Pinterest: https://www.pinterest.com/pin/1057712662468715434/

On the AM broadcast band you will surely pick up many stations with it. As you climb the frequency range it becomes a bit less sensitive, nevertheless okay.

You can use it with a Q multipler like the "noisy regen" circuit for extra sensitivity.

9 comments

9 Comments

Sean O'Connor

Nov 13

L1 and L2 could be IF cans and you could probably play around with impedance matching with the various connections on IF cans. That should give you a very sensitive radio.

Sean O'Connor

Nov 14

Replying to

If you have a tame audio amplifier circuit you can give me the link. I don't know how many times I have to learn my lesson about the LM386!

If you build the TRF circuit let me know how well it does.

You can put an LED in series with R2 as a useful signal strength indicator.

LarryDee

Nov 14

Replying to

LM386 Is a mess I developed this amp from others. The layout is using Manhattan Style. A 3 to 3.2-volt supply should be very loud in any headsets. Higher voltage makes the transistors run hot and draws too much current. This also works great to drive any line-in on any amp. It will not cause regens to go crazy. It is very well-behaved. It's just amazing, actually. Draws about 5 milliamps, maybe less. Costs pennies to make. I use Google Drawing because it is the best, most simple, logical drawing program out there. Want to publish more when I can.

Sean O'Connor

Nov 15

Replying to

Thanks, I will definitely build the audio amplifier circuit. And try to stop reaching for an LM386 every time.

LarryDee

Nov 09

Any more info, like what transistors or the value of L1 etc ? Looks cool.

Sean O'Connor

Nov 10

Replying to

L1 is an RFC and its self-resonant frequency should be above the top operating frequency of the radio. In fact you could add a variable capacitor to L1 and set it to resonate at the receive frequency to get a double tuned TRF. However then you are getting into some tricky territory.

One reason it is so sensitive is you are getting combined non-linear behavior from both Q3 and Q2 giving improved AM detection via asymmetric signal polatity response.

You can omit Q4 if you like and just have the collector of Q3 go to L1 but then you get lower input impedance.