Thanks for the reply!Those are highly suspicious values. The collector and emitter resistor are out by a factor of about 100. Try making R2 1K and R3 5.1K although swapping them around looks more realistic. You will also get better results by center tapping the tuned circuit, in other words, make C3 and C4 about the same value. You may have to adjust the inductor to re-tune back to 27MHz.
Brian.
Okay I have changed the resistor vales R2 and R3 as you suggested but I haven't changed C3 and C4. The circuit seems oscillate (on simulation) to about 27.7 mhz which is okay for me, do I still need to change C3 and C4 for it to oscillate in reality? I got a he folmula from a book which recomended that C3 should be at least 3 to 10 times greater than c4.Those are highly suspicious values. The collector and emitter resistor are out by a factor of about 100. Try making R2 1K and R3 5.1K although swapping them around looks more realistic. You will also get better results by center tapping the tuned circuit, in other words, make C3 and C4 about the same value. You may have to adjust the inductor to re-tune back to 27MHz.
Brian.
Okay I have changed the resistor vales R2 and R3 as you suggested but I haven't changed C3 and C4. The circuit seems oscillate (on simulation) to about 27.7 mhz which is okay for me, do I still need to change C3 and C4 for it to oscillate in reality? I got a he folmula from a book which recomended that C3 should be at least 3 to 10 times greater than c4.
Hi,
Your oscillator is supposed to be a colpitts with common base configuration.
At 27Mhz, C5 and C2 must be seen as shorcuts but 39pF is a too much low value for that (they are acting like 154 ohms resistances; use for example 100nF).
Current in emitter is quite low (about 7uA). This increases the small-signal resistance rbe, and so decresing the amplifier gain (Ao = R2/rbe). If I am not wrong, your gain is lower than 1 !
In this kind of oscillator, Ao must be greater than (1 + C3/C4) = 6 and this is absolutetly not the case.
In conclusion your original circuit should not maintain oscillations. Be carefull with the simulations parameters. How long are you simulating ? Note that oscillations will anyway start because of the LC configuration but they should not be kept oscillating for long time. May be you are watching the starting oscillation phase.
Let us know about your progress on your project !
Efron
So how do I 'correctly' simulate on Multisim? I usually run the circuit and attach the 'tetronix' oscilloscope, which display virtual sine wave, and I simulate for on average 5 minutes. So how do I simulate correctly as you suggest? I got the capacitor values from a certain book which gives folmulas for appropriate capacitors for given frequencies.
Please help as I want this project to work!!I kind of nearly giving up now!
Many thanks.
Dear Zeller,
You are saying that your are building your project on a circuit board (I hope it is not a breadboard as over 1 Mhz it should never be used !).
By selecting betwixt's proposal above (R2 = 1K, R3 = 510K), emmitter current (in DC) is around 0.78mA and rbe ~ 32 ohms, which leads to a gain in open loop of about Ao = R2/rbe ~ 31, which is now much bigger than (1+C3/C4) = 6. Because of this, the circuit must now oscillate, but only if C5 and C2 are seen as shortcuts at 27 Mhz. Please use 100nF values (if not yet done).
Because of the great difference between Ao=31 and (1+C3/C4), the output is supposed to include much distorsion (a lot of harmonics) --> what are you seeing in your simulations ?
Where are you taking the output signal, at Q collector or emitter ?
You may show to us your simulation graphics !
Note that when measuring with real oscilloscope, you are loading the circuit, usually with a 10K resistance in serial with a 20pF capacitor until ground. But for the moment I can not say if this is having a negative effect on your circuit.
Waiting for good news !
Thanks for this, I changed the resistors but did not change the 39pf capacitor, I implemented the circuit on a circuit board but upon checking with a real oscilloscope, it did not oscillate, I will now change the 39pf capacitors to 100nf and see what I get.
We keep you posted.
Hi Zeller,
Any progress ?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?