Bypass 'capacitors' on single transistor amplifier (?)

[audioguru]

I made an Eico FM radio kit about 52 years ago. It used vacuum tubes and was very sensitive, had good selectivity for all the FM radio stations around me and had low distortion. When FM stereo was invented a year later then I made a stereo multiplex adapter kit for the radio that also worked well.

About 50 years ago I used a tunnel diode in an FM transmitter then I used it in a regen FM radio. The regen radio worked very poorly.

 

[Willen]

Time you crossed is so much valuable I feel. These earlier devices made me hobbyiest. A question- Why germanium diode is not necesssary here? Because of FM or because of forward junction of audio amp transistor?

 

[unclejed613]

this is actually an AM detector. it works as an FM detector by what's called "slope detection" where the receiver is tuned offset from the center frequency, and the slope of the frequency response of the detector translates the frequency variations of thr FM signal to amplitude variations. slope detected audio from FM is usually somewhat distorted.

 

[Willen]

Hi, I simulated the circuit. Others are good (I think) but I didn't see good audio sine wave form by 64 ohms speaker. I used 2K and it was very good sine. Please run this file once! Won't it work with general sensitive earpiece like 64 ohms?

Warn me if other arrangements are also not good please!

 

[unclejed613]

Hi, I simulated the circuit. Others are good (I think) but I didn't see good audio sine wave form by 64 ohms speaker. I used 2K and it was very good sine. Please run this file once! Won't it work with general sensitive earpiece like 64 ohms?

Warn me if other arrangements are also not good please!

i doubt you have any 2k speakers.... 2k headphones are very rare and expensive these days unless you can find some military surplus "sound powered telephones"....

 

[Willen]

i have no 2k headphones! I just simulated with 2k resistor. I have general and sensitive earpiece of cell phone which is playing VERY clear and enough sound on a Crystal radio made by me with VERY large antenna. I hope this earpiece will also work on this regenerative Rx.

I want to request to audioguru to run this file (attached on previous post) please!

I with 64 ohms speaker, audio waveform is not good. I added 220 ohms series resistor with 64 ohms speaker then It has very good 100mV peak to peak audio output. Please test once.

 

[audioguru]

I want to request to audioguru to run this file (attached on previous post) please!

The circuit has no audio modulation, it is simply an RF oscillator.
When I simulate it I get a very low level sine-wave at 100MHz, at the collector of the BC557B.

The BC557B has an emitter resistor value that is much too high for it to have a collector load of only 64 ohms.
Also with a collector load of 64 ohms then the base current must be much higher.

 

[Willen]

I added 220 ohms resistor series with 64 ohms speaker and got 100mV sine. I don't know what it is.

Can we modify simply and make it workable little? Or should I have to leave the project?

 

[audioguru]

Why don't you use a little audio amplifier IC to drive the 64 ohms speaker or headphones? An amplifier IC does not wrongly put DC into the speaker like the single BC557B transistor does.

 

[unclejed613]

The circuit has no audio modulation, it is simply an RF oscillator.
When I simulate it I get a very low level sine-wave at 100MHz, at the collector of the BC557B.

The BC557B has an emitter resistor value that is much too high for it to have a collector load of only 64 ohms.
Also with a collector load of 64 ohms then the base current must be much higher.

it's a super-regen receiver shown without the antenna connection, which makes it just an oscillator..... the base of the oscillator should have an antenna coupled by a capacitor, but whoever drew the original schematic forgot to include that little detail.... which makes it either an "easter egg" to force you to buy the kit, or an obvious "oops" that should have been corrected. the OP should also keep in mind that super-regen receivers are banned in many places, because it bleeds RF back through the antenna, and becomes an "unintentional radiator"......

 

[Willen]

To AG- I wanted to make smaller device so didn't tried additional Amp. But if it is necessary, I will add.

To unclejed- I am little confused here, you said 'add an antenna on base using coupling capacitor' But I think RF of base has been grounded with 10n cap, isn't it? If antenna signal is grouned, how it can receive FM? (I guessed 10n won't ground 100MHz, will it?)

 

[audioguru]

The antenna for a super-regen "radio" is usually connected to the emitter but sometimes to the collector since the base is grounded by a capacitor.
The simulation will not show audio since the RF is not modulated.

 

[Willen]

The antenna for a super-regen "radio" is usually connected to the emitter but sometimes to the collector since the base is grounded by a capacitor.
The simulation will not show audio since the RF is not modulated.

Hi, don't feel bore please because going to ask same-
If I added almost 220 or 330 ohms resistor series with 64 ohms speaker, will it able to work little and can play audio better than 64 ohms load? Simulation says yes (may be).
(multimeter shows the DC resistance of my earpiece is 33 ohms which is working very well on my simplest Crystal radio with extremely huge antenna)

 

[Nigel Goodwin]

If I added almost 220 or 330 ohms resistor series with 64 ohms speaker, will it able to work little and can play audio better than 64 ohms load?

Apply ohms law and work out how much power will get to the speaker, and how much will be wasted in the resistor.

 

[audioguru]

Few of us have made a "Mickey Mouse" super-regen radio so we do not know its output level. We made REAL (super-heterodyne) radios instead.

About 50 years ago I made a crystal radio with a sensitive audio amplifier and heard airline communications (about 127MHz) at the airport. Some of the airplanes were fairly far away. On an airplane I was told to turn it off so it won't cause interference (but it had no oscillator).

 

[Willen]

Can I hear airplane communication above 108MHz FM band in all countries? Or has different band in different country? Then can I modify general FM Rx radio by reducing its few turns coil and few pf capacitance (from tuning LC) to receive airplane?

 

[audioguru]

Look in Google to see the airband communication frequencies used in most countries.

FM has a "capture effect" where the strongest signal over-rides a weaker signal then you cannot hear the weaker signal.
Then a weak signal from an airplane in trouble will not be heard if a stronger signal by the air traffic controller or from a closer airplane is made.
So airplane communications are made with AM then all signals will be heard. A normal FM radio cannot receive AM.

A regen radio detects AM or FM so it can be used to hear airplane communications but not on an airplane because a regen radio causes interference.

 

[Willen]

I made a regen-FM radio today. Yes as you said the selectivity is extremly poor and stability is extremly BAD. I guessed it's a toy!

On such radio receiver, I found S9018 or BF199 or BF240 transistor mostly used. These all has almost same spec and has 50mA Ic and VERY HIGH 1.1 GHz fT. Also I have various cheap Chinese radios which has S9018 (may be as a RF amplifier).

- Is it better to use 1.1 GHz transistor (9018) than 300MHz transistor (like 3904) as a 100 MHz RF receive purpose? Why?

- In these old radios I found this S9018 transistors has been damaged. May be received RF amplifier is not much hard type of amplifier but how can they damage? Are they (S9018, BF199) more sensitive (weak) due to such high fT (1.1GHz) ? And has high possibility to damage?