How to make a traditional diode envelope detector radio receiver

[Fluffyboii]

Hello.
As some of you know I recently made a small AM transmitter. It made me really happy even though it has a range about 3 meters due to not optimal antenna and low power of it. I want to improve that transmitter by adding RF amplifier of some kind but I can't figure out a way to amplify a signal about 5-10V peak to peak with simple CE amplifier since high input voltage swing causes BJT to get out of active region and weak RF output of that circuit would probably drop from the input impedance of an RF amplifier and I am not qualified to calculate and compensate for all of that. My small research revealed me that RF transmitters and amplifiers are just an endless rabbit hole so unless I decide to torture myself by getting some RF classes I will not be able to fully understand and come up with stuff myself.
Anyway since I made the transmitter but only radio I have here is one that plugs into mains and hard to move I can not show it to anyone in action. I want to make a simple envelope detector radio receiver to go with it. I actually made something like that at a lab session with op amps but it was very bad since we used silicon diodes for detector and overall gain was very low so it required direct feeding of AM signal into the circuit and didn't had the capability of getting the small signal in the air. I wasn't satisfied at that time:

Book explanation of envelope detector:

I don't know my transmitter bandwidth but from its sound quality I am sure it is not 20Khz but something like 5-10Khz at best. My carrier freq is about 1.2Mhz which would mean for 10K detector resistor I would need a capacitor at least few pF. I don't have 5-10pF capacitors around and I don't think having a value that low can be reliable so maybe I need a lower resistance about 1K without passing to much current over the diode. Anyway I have Germanium diodes with about 0.3V drop and Schottky diodes that have around 0.2V drop for the job.

Input filter can be made with the 30pF-60pF variable capacitor I have and a 50uH inductor but range will be 1.3Mhz-900Khz according to my calculation. I would like to have the whole MW range in there but honestly there are no radio stations here to listen that broadcast MW so it doesn't matter. I can always 3D print a larger variable capacitor housing and cut some metal discs or make a large adjustable inductor if I get a reliable circuit going and want to improve on it.

My real problem is the gain required and the finding antenna specs that are optimal. I saw many old, cheap MW radios that had 9 Transistors branding on them which is impressing to be honest. I honestly don't know how much gain I need but I assume at least 1000 since 100 gain doesn't even get close. I had this BJT amplifier with bootstraping at input to have a high input impedance here that has about 100 gain.

But since this is not a homework why wouldn't I cheat by using a mosfet at the input to have technically infinite input impedance. My only problem is that if I bias the mosfet with an voltage divider I would need to use very large resistance values about few megaohms to not load the weak input signal. I think I need something that biases the base with some kind of feedback network to avoid having a resistor to ground but I am not sure what is optimal here. Than after the mosfet buffer at the input I can have my 100 gain to feed my envelope detector than apply some more gain if needed. I will try to simulate it in LTspice. And at the last output buffer I can have a small toy speaker to hear the results. When I think about it the gate to emitter capacitance of the mosfet perhaps can be an issue. Cascode configuration was able to get rid of that miller capacitor and gave good gain perhaps it is a good idea to use it here.

For some reason whatever I tried I wasn't able to get and gain out of 2n7002 in LTSpice. I tried normal CS amplifier, I tried cascode with different configs, included perfect current sources for max gain but nope nothing got me gain more than 1. Maybe I am unable to see something in front of me since it is 5AM now and I used all of my mental resources. Added the LTSpice file. Unfortunately the one in the image with bootstrapping is lost.

I really can not get any transistor amplifier in LTSpice running now. Check Draft2 for example, there is no gain whatsoever. I don't know why maybe my LTSpice is broken.
I need to choose bias voltages more carefully. I totally forgot everything again.

 

[JimB]

Fluffy, your receiver circuit is very poor.

1 That single tuned circuit offers very little (none) selectivity.
While there may be no other radio stations around 1.2MHz in Turkey, there may be a lot of noise from badly designed electrical equipment.

2 If I read your schematic correctly, an LM741 will not give any gain at 1.2MHz.

At the moment I am not sure what else to suggest.

JimB

 

[Fluffyboii]

Fluffy, your receiver circuit is very poor.

1 That single tuned circuit offers very little (none) selectivity.
While there may be no other radio stations around 1.2MHz in Turkey, there may be a lot of noise from badly designed electrical equipment.

2 If I read your schematic correctly, an LM741 will not give any gain at 1.2MHz.

At the moment I am not sure what else to suggest.

JimB

Yes I know. It was past lab assigment. You can see my note under it saying it doesn't get the gain irl it supposes to get. I will not use it. I was looking for a multistage bjt design with more layers of filtering.

 

[Nigel Goodwin]

Yes I know. It was past lab assigment. I will not use it. I was looking for a multistage bjt design with more layers of filtering.

You're wanting to build a radio - then google radio circuits - this is nothing like a radio.

By the way, the standard tuning capacitor for MW/LW was 365pF to give full MW/LW coverage (with two switched aerial coils on the same ferrite rod).

There are some simple reflex circuits here:

Reflex Receivers

The second example, with two transistors, is a fairly standard reflex design. It's easier because it onle needs an RF choke, and not a transformer. Notice the use of a 365pF tuning capacitor throughout the designs.

 

[Fluffyboii]

I checked few two transistor designs and there had no diode to demodulate but only a LC tank circuit to grab the signal which did not made sense. The type of receiver I wanted to build had a name and there was one site explaining it but I couldn't find it. This one seems similar or what I wanted to do. It did not occur to me that I could just use the diode at the input, connected to an antenna and filter directly with some DC biasing. Instead of using a transformer to impedence match I would rather use an extra emitter folower stage. I will give it a try.

 

[Nigel Goodwin]

I checked few two transistor designs and there had no diode to demodulate but only a LC tank circuit to grab the signal which did not made sense. The type of receiver I wanted to build had a name and there was one site explaining it but I couldn't find it. This one seems similar or what I wanted to do. It did not occur to me that I could just use the diode at the input, connected to an antenna and filter directly with some DC biasing. Instead of using a transformer to impedence match I would rather use an extra emitter folower stage. I will give it a try.

That's basically a crystal set (google it), the poorest type of radio possible - the reflex designs I linked to above are vastly superior, and work really well. It's a clever trick using a single transistor twice.

 

[Fluffyboii]

Nigel Goodwin l I made the circuit from there.

It worked fine and as it claimed it was able to capture signal from different sources. While I was testing it, it suddenly shut up. I checked my transmitter which was working just fine. I couldn't find a reason. Now it only works if I directly connect it to transmitters output. It's gain dramatically dropped permanently for no reason. I removed and test all of the transistors with my tester and all of them were fine. I tried different diodes, nothing changed. So I am at the same situation again. Something working fine suddenly decided to degrade just like my ring modulator I build in the past that worked excellently with red LEDs but later got very bad even though everything was same and there was no scientific reason for it to act different.

You may notice that I don't have a transformer for impedance matching. Instead I used a small speaker that has higher resistance. It is not the problem. I tried and inspected everything and couldn't find the reason behind the sudden lack of gain.
I will try reflex one when I get my hands on some ferrite rods.
I used 2N3904 and 2N3906 for this.

 

[Fluffyboii]

Checking with oscilloscope revealed that I was losing gain at PNP transistor which led to the realization of that it was in reverse. I turned it around and it somewhat functions better. What I am failing to understand is that when I touch the 2nd transistors base, 1Meg resistor radio works much better. It gets to clear audio from nothing when I touch it. Similarly touching 100K pot increases volume somewhat. Touching anywhere between first stage and last stage increases sound quality but start of the 1Meg resistor gives best performance. I wonder if I am being an antenna or my transistors have low beta and my static lets base get some current.

Apparently attaching an alligator cable to 1M resistor does the same thing. It doesn't make sense to me though. I can record a video of this.

 

[danadak]

What are the Vce of each stage on the transistors ?


Regards, Dana.

 

[Fluffyboii]

What are the Vce of each stage on the transistors ?


Regards, Dana.

Grounding of the oscilloscope kills the radio. Should I try my multi meter.
My multi meter isn't reading voltages accurately right now so I can't measure DC voltages. But I am sure this happened after I made the circuit. Something happened and made it as the way it is right now. And since I remember I can say it wasn't anything major. I went over all solder joints so I am out of ideas. Will get new AAA batts for my multi meter and try to measure voltages.

 

[danadak]

Actually ground the input to radio to take the measurements. We are trying to see if
Vce of the stages properly biased..

Meter or scope fine.


Regards, Dana.

 

[rjenkinsgb]

Grounding of the oscilloscope kills the radio.

Do you have your oscilloscope probe set for "x10"? It should always be in that setting, otherwise whatever the probe touches will be loaded down to some extent.

 

[Fluffyboii]

Do you have your oscilloscope probe set for "x10"? It should always be in that setting, otherwise whatever the probe touches will be loaded down to some extent.

I was wondering what that was for. Will try measuring with that selected. But I think I already did out of curiosity and nothing changed. The first transistor current is just to low I think.
What I am not understanding is that 10K resistor that is there to create ground path for the diode. Wouldn't all current flow from it to ground instead of going into base. While testing it should I ground the antenna or diode directly? I was able to confirm its operation when antenna was directly connected to transmitter. I still don't know why touching it makes it work better though.

 

[JimB]

I was wondering what that was for.

Have a look at this link:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwj2pPWIs-z7AhUQgv0HHXu2AKYQFnoECA0QAQ&url=https%3A%2F%2Fdownload.tek.com%2Fdocument%2F02_ABCs-of-Probes-Primer.pdf&usg=AOvVaw3j0kSxtSr7-gBTQR6_4p2H

Everything that you ever wanted to know about 'scope probes.

I still don't know why touching it makes it work better though.

Be aware that a radio antenna is a "system" of two parts...
The wire of the obvious antenna, and a "ground" which may not be obvious.

The antenna and ground cause a small current to flow in the input circuit of the receiver.
If the ground connection is not there, there may not be enough current in the receiver input circuit for the receiver to detect the signal correctly.
When you touch the ground (0v) of the circuit, the capacitance of your body enables more signal current from the antenna, hence a better received signal.

JimB

 

[Nigel Goodwin]

I was wondering what that was for. Will try measuring with that selected. But I think I already did out of curiosity and nothing changed. The first transistor current is just to low I think.
What I am not understanding is that 10K resistor that is there to create ground path for the diode. Wouldn't all current flow from it to ground instead of going into base. While testing it should I ground the antenna or diode directly? I was able to confirm its operation when antenna was directly connected to transmitter. I still don't know why touching it makes it work better though.

The scope probe setting will make sod all difference in this case, as it's just a (crappy) audio amplifier - the only RF part is before the diode, and far too low for a scope anyway.

 

[Fluffyboii]

The scope probe setting will make sod all difference in this case, as it's just a (crappy) audio amplifier - the only RF part is before the diode, and far too low for a scope anyway.

I should just give up and use LM386 instead I guess. I need some ferrite sticks :/

 

[Fluffyboii]

The scope probe setting will make sod all difference in this case, as it's just a (crappy) audio amplifier - the only RF part is before the diode, and far too low for a scope anyway.

If I directly feed the AM signal into the diode and check it's other side I can see demodulation happening. I don't know the total gain of this circuit but it is very high so it becomes a garbage square wave after some point. I assume the reflex radio you first send will work much better. There are multiple designs in that page. Should I go for a loopstick antenna or wind a loop of wire and hope it has right impedance.

 

[Fluffyboii]

The antenna and ground cause a small current to flow in the input circuit of the receiver.
If the ground connection is not there, there may not be enough current in the receiver input circuit for the receiver to detect the signal correctly.
When you touch the ground (0v) of the circuit, the capacitance of your body enables more signal current from the antenna, hence a better received signal.

JimB

Actually when I touch the ground output volume gets attenuated. It only increases if I touch the base of second transistor and that area.

 

[Nigel Goodwin]

I should just give up and use LM386 instead I guess. I need some ferrite sticks :/

You would probably need an audio preamp as well, one transistor might be enough - but as it's a really useless circuit, why not just build something that works properly?.

Should I go for a loopstick antenna or wind a loop of wire and hope it has right impedance.

Personally I'd use a ferrite rod - and as you're probably taking one out of an old radio, then use the coil that's already on it, along with the correct tuning capacitor.

If not, then wind the simple loop.

 

[Fluffyboii]

You would probably need an audio preamp as well, one transistor might be enough - but as it's a really useless circuit, why not just build something that works properly?.



Personally I'd use a ferrite rod - and as you're probably taking one out of an old radio, then use the coil that's already on it, along with the correct tuning capacitor.

If not, then wind the simple loop.

Yes it looks and act like a stupid circuit. It was described as something really versatile and fun to play with so I stopped thinking and build it. I know there are old radio ferrtie rod antennas in lab, I will ask teacher for one. I found TDA2040 and LM317. I can use LM317 to make a small class A amplifier for using with small speakers and TDA2040 to make something better later. Unfortunately no one were in the lab so still no ferrite. Maybe I wisit the antique bazaar and search for a busted old radio.