2 Transistor El Cheapo Radio

millwood

I have been building radios for years (decades actually) and mostly reflex and super regen radios with many parts. They usually worked well but the sound quality was never good - then sound quality isn't what you build AM radios for.

Over the last year or so, I have been trying to build radios without germanium diodes as they are hard to find in the US.

taking inspiration from this page, http://www.techlib.com/electronics/crystal.html, I decided to build a no-diode and simple radio. Here is the result of my experiment so far.

millwood

the design goals are:

1) no diode, and definitely no germanium diode.
2) cheap, really cheapo.
3) good sound quality.

and I am happy to report and it scored well on all three fronts.

millwood

the radio is very simple.

L1/L2 are the loop coil for MW. Mine is a small 3 inch long coil. VC1 is the tunning capacitor, against very small.

the tuned RF signal comes in, gets amplified and "rectified" by Q1 and gets fed to Q2 for further amplification.

Each amplification stage consists of an input capacitor (C1 for Q1 and C2 for Q2). Those are small metalized film capacitors. I used two 110nf capacitors but have used smaller ones from 1nf up, without any ill effect. C1 and C2, however, should never been electrolytic capacitors: using electrolytic capacitors here will greatly reduce the gain of the radio.

Q1/Q2 are biased by two resistors. The biasing is such that Q1/Q2's collector is at roughly 1/2 of their power supply.

each stage idles at about 0.5ma and  provides a gain of roughly 100x.

R5 and D1/D2 form the power supply for the radio. I used two LEDs but it is not critical: the radio works just fine with one LED, or any other LEDs. R5 provides the bias for the LEDs and you may chose their value to make your LEDs happy.

the 4.2nf output by-pass capacitor, C?, is optional. the radio works without it just fine. I have experimented between not having it and using a value as high as 110nf. At 110nf, the high frequencies are attenuated but the sound is very warm / smooth. without it, the amp has a higher gain, but a little bit of hissing. I used a 4.2nf for compromise.

the audio goes out to a pair of Harman Kardon Duet active speaker.

What really impressed me about this radio is its sound quality: smooth and near FM. It actually reminded me of tube radios from good old days.

millwood

further experimentation:

I have a few RF transistors (mps5179 for example) and it made no noticeable effect. I plan on trying a jfet to see if it makes a difference. pretty much any small signal npn transistor should work in place of Q1/Q2.

R1/R3's value is not critical: anything north of 100K should work. a lower value will cause the idle current to go up and minimum the output swing.

R2/R4's value is not critical either. use whatever resistors you have, just make sure that Q1/Q2's collector at about 1/2 Vcc (2v if Vcc=4v).

millwood

I also played with a version for SW reception. if you put in another stage after Q2, and drop in an air coil in place of L1/L2, you get SW reception - but for SW listening, the radio's selectivity isn't that great so stations are group'd together.

millwood

you can also play with one with regen, like this one below.

the one below should work but I have yet to experiment with it.

millwood

this radio is so simple it is almost a crime if you don't try it,

enjoy.

翌阳

楼主能看到中文，但他打不了中文，只好打英文了。
其实网上有网页的输入法，可以输入中文然后贴过来。

夏某

除了可以从图看出来这是一个收音机以外，其他的我一个字没看懂。

e3po

Gracias....

呀美喋

楼主确实是外国人，，，喜欢怀旧的外国人  建议大家用谷歌简单的翻译一下 能看懂

呀美喋

原帖由 e3po 于 2010-8-15 15:54 发表
Gracias....

亚克西。。。。。。。

millwood

another item that's hard to come by and expensive to buy here is a varicap. air varicaps are typically $20 + each.

I have thought about a variety of means to replace that, from using reverse biased LEDs to rectifiers / zeners.

I was just reading a datasheet today and it came to me suddenly that mosfet's gate capacitance can be used as a varicap.

here is the basic concept: the output capacitance of a mosfet varies with its Vds. so if we could change the Vds on a mosfet with a pot, we have got us a varicap.

the next trick is to find the mosfet with the right range of Coss. usually small current TO220 have about 100pf Ciss and 200pf Coss. I picked FQP1N50 since I have many of them. But you can pick similar devices, like IRF510.

larger mosfets, like IRF540, do not work here as their Coss is too big.

so here it is. I will try to play with it later to see if it works.

millwood

I am happy to report that this approach absolutely works.

I wired mine up, powered by a 16v switch mode power supply and I was tuning radio stations to radio stations from a little trimmer!

give it a try and you may just like it.

also, to filter ripple, you may want to put a 4.7uf cap from the wiper of the pot to the ground. Mine worked without it, though.