# Painfully Yet Steadily Growing Oscillator Output

I have worked out an LC-based Colpitts Oscillator (sharing the schematics) that produces output which grows painfully slow from 10 pV to 500 mV in more than to 3 minutes and settles there. Despite banging my head loads, I have been unable to solve the issue. Any help shall be greatly appreciated. By the way, the oscillations have frequency of 3.5 MHz.

• A picture of the output might be nice Commented Dec 1, 2022 at 14:49
• Try decreasing R3 to increase Q1's transconductance. Commented Dec 1, 2022 at 14:53
• 3 minutes wall clock time, or 180 seconds on the simulation t axis? Does it spend most of that time at pV level and then finally get big, or does it rise linearly or exponentially? Try giving one of the capacitors an initial condition of voltage, the .ic directive, to give it an initial kick. Try altering a component that defines the loop gain, like almost all of them, and see if that variation stops it oscillating at all or gets it to start more quickly. Commented Dec 1, 2022 at 14:53
• Yes sir it's literally clock time Commented Dec 1, 2022 at 15:16
• No sir, it spends almost equal tone in pV and nV ranges and eventually settles around 500 mV Commented Dec 1, 2022 at 15:20

Your schematic shows a direct connection of 12 Volts to the base of the transistor through L2. There should be a capacitor in there somewhere.

• O! Damn! How did I miss that one. Thanks. Any other fault sir? Commented Dec 1, 2022 at 15:17

I think you need to recheck your circuit as per the above notes.

• Ah! Thank you sir. I must be a dumb doing that. I'll shift it in BE loop n see if it helps Commented Dec 1, 2022 at 15:19

Besides what others have already said, C4 makes L1 pointless.

L1 is an RF choke that presents a high impedance at RF at the collector. C4 is a relatively high value capacitor that will be a low impedance at RF.

Since the gain is the roughly the ratio of the impedance in the collector circuit to the impedance in the emitter circuit, lowering the impedance at the collector will lower the gain, basically, C4 shorts out your RF signal.

What we would expect to see here is either no capacitor, or a small value capacitor(pico or nano Farads) that is resonant with L1 at the frequency of interest.

• Greatly appreciated. Thanks. I'm in learning phase so I thank you all. Commented Dec 1, 2022 at 16:13