# Colpits Oscillator

I am trying to generate sine wave via Colpits oscillator method as described above. But I am unable to generate any. The output is at constant 5V. The condition for starting the oscillation is $$g_m = 0.5, r_o=14k\Omega, g_mr_o > 1$$. Which is met.

Still I could not get the sine wave as expected. Can someone be kind enough to drop couple of clues here, I am breaking my head over this for last one day! I am a newbie.

____________________________

• Why are the two circuits different. My eyes are old and fading but even I can see they are different. Voting to close. Feb 13 '17 at 8:06
• removed the first circuit. Feb 13 '17 at 8:11
• You still have a link to one circuit and you should make it clear to Russell that you are altering the question to remove redundant information and that this may invalidate his answer. Accuracy is one of the major disciplines to learn when involving yourself in EE. Feb 13 '17 at 8:18
• @Andyaka Agree. Comment only: The change did not just remove redundant information but altered the actual circuit and one component type and several component values. Feb 13 '17 at 10:54
• @Andyaka Your vote to close is in violation of site guidelines. FWIW. Feb 13 '17 at 11:03

R2 / R5 divider values are very very very low so base is very "stiff" wrt feedback. Are those REALLY the values used? -> 250 Ohms and 88 Ohms? -
If so increase them VERY substantially say 33k & 10k - about the same DC bias but gives feedback a chance given L & C values used.

______________________________

You edited the circuit and the edited version has two substantial differences.

Were you using both circuits or only the second?

As shown the output CANNOT be at 5V if the transistor is working so you must not be doing what you show here.

Vout is either 0V AC (due to the capacitor)
OR the collector shows an oscillation
OR the collector is at about 4.4V DC
BECAUSE Vb ~= 1.25VDC so Ve ~= 0.65 Vdc So Vr1 ~= Vr4 so Vc must be droped by Ic = Ie in R4
SO you are not doing what you say or saying what is really happening.

R4 should feed the collector end of L1 although the difference should be small. This is what your prior diagram showed so why has it changed?
Did you use an RFC or only a resistor for R4?
Did you change the base bias R's as shown?

Please advise re the inconsistencies mentioned above if you want more input.

_______________________________

There are 107 Stack Exchange Colpitts Oscillator questions here - a look at a selection of these may help.

• The second one, I am using. "Did you use an RFC or only a resistor for R4?" - I tried both, but now the resistor only. Feb 13 '17 at 7:00
• Have you moved R4 to other end of L1? What is Vc? What is Ve? Have you tried this in real life and/or simulation? Feb 13 '17 at 7:15
• Yes I tried in real life, not working either in hardware or simulation. Vc = 4.366V, Vb=1.3V, Ve=610mV, Ic = 6.338mA. Feb 13 '17 at 7:16
• @niki_t1 C5 should NOT be polarised / electrolytic. C1 probably not. | Please answer ALL questions per pass if feasible. Did you move R4 to other end of L?. || What expected freq. Wht so bu=ig values for C5 & C1 ? || AHA maybe - see next comment. -> Feb 13 '17 at 7:35
• In hardware C5 and C1 are polarised, but it should not matter in simulation though. Yes I moved R4 to the other end of L1. Expected frequency is 5Mhz. Feb 13 '17 at 7:38