A simple BJT can be a transmitter-no RLC needed

[GaffarEEE]

hi friends,
we have experimented these following circuit in lab and found 108Mhz without any LC resonance we thougth the wires that we experimented on breadboard would make any LC resonance but a great surprise to us that when we have made a pcb board and we have found the same result: 108Mhz and if we change the biasing resistance frequency vary.Finally we have used a old fm radio to test it.It is a FM wave coming from this BJT and the capacitor connected to its two point collector to emittor.Now we thing it would be useful to make my thesis about this amazing characteristics of BJT but we cant find anything inside BJT 2n2222.

i think someone has already encountered about this amazing nature of BJT.
please advise me.

 

[flatulent]

You have made a broad band amplifier. Without the LC network on the output you will also generate and radiate harmonics.

 

[GaffarEEE]

is there any theory that help me to find out what happened................

 

[RF-OM]

Capacitor actually is kind of complex network. There are not only capacitance, but also some resistance, other capacitance and inductance. Chip capacitor may have self-resonance frequency close to 108 MHz. Therefore, in your schematic caps must be replaced with resonant tanks. When you do so you will see that now you have an amplifier (common collector) with high current gain. This amplifier may oscillate and therefore work as transmitter. This transistor has high gain on 108 MHz. If you want to eliminate such effect chose right components.

Unfortunately many students and engineers often do not care about circuit element's and layout parasitics and this may lead to poor designs.

Best regards,
RF-OM

 

[vfone]

There is a mistake on your schematic. You pick-up the output signal direct from the 10V DC supply line. In this way (if there is a good decoupling capacitor on the supply) you are not going to see any signal at the output.
If is no decoupling cap and the wire from the collector to the 10V supply is long, the parasitic inductance becomes the collector choke.

 

[RF-OM]

Yes. This is the major point: parasitic are responsible for this phenomenon.

BR,
RF-OM

 

[GaffarEEE]

There is a mistake on your schematic. You pick-up the output signal direct from the 10V DC supply line. In this way (if there is a good decoupling capacitor on the supply) you are not going to see any signal at the output.
If is no decoupling cap and the wire from the collector to the 10V supply is long, the parasitic inductance becomes the collector choke.



your point has been considered firsrt but it seems the bjt only can generate this frequency.please take the bjt in consideration.

 

[RF-OM]

I doubt that BJT itself without external components can generate signal, unless you made the major discovery. In this case you may be the next Nobel prize man!

BR,
RF-OM

 

[FvM]

Oscillations at 100 to 150 MHz are a well known effect with small signal BJT that have a low impedance base connection. They are operating as Colpitts oscillator, based mainly on circuit parasitics, as said.

As the effect is normally unwanted, a low inductance circuit ground can be used to avoid it. Alternatively, a 50 to 100 ohms base series resistance can stop the oscillations. GHz RF transistor can even oscillate (at higher frequency) on bond wires inductance only.

It's actually surprizing, that you seem to have hit the same frequency in breadboard and PCB design (if the frequency determining resonance isn't from an external component, e. g. a cable). Also your 1 nF capacitors may have a resonance in the said range.

When designing a transmitter, topics as antenna impedance matching or (as said) control of harmonics come into play. At this point, you can't avoid to determine the circuit parameters (including parasitics) to calculate the design.

 

[Borber]

The results of making experiments on breadboard are often unpredictible. Connection wires with 30mm length have inductance of about 20nH. This is not a simple wire but must be considered as inductor.
What was the reason or goal of your experiment at all. Schematics make no sense by connections and elements values.
The simplest answer to question why this circuit oscillate is that conditions for oscillations are fullfiled. And why they are fullfield we can only guess knowing the oscillator topologies.