Troubles with Colpitts Oscillator - Page 1
Forum section: electronics / rf-microwave

author: fnpfar
author_rank: Contributor
timestamp: July 14, 2016, 05:21:08 pm
content: Hello. I´m trying to build a colpitts oscillator to use as a local osc. for various projects.I build this schematic, but when powered, it doesnt work, only the transistor heats up, and the voltage on the base remains near 0v, despite I have added a potentiometer to add bias.What is wrong with it? I tested it with various combinations of coils and capacitors, but It doesnt oscillate.Thank you for your help!!

author: Tom45
author_rank: Frequent Contributor
timestamp: July 14, 2016, 05:38:17 pm
content: Shouldn't you have an inductor where you have the 1K resistor?

author: fnpfar
author_rank: Contributor
timestamp: July 14, 2016, 06:01:47 pm
content: But without this limiting resistor current would flow through the transistor burning it.. i think (?)

author: Tom45
author_rank: Frequent Contributor
timestamp: July 14, 2016, 10:25:34 pm
content: The inductor is more or less a dead short only at DC. Compute the impedance of the inductor at the oscillator's frequency.

author: orbanp
author_rank: Regular Contributor
timestamp: July 25, 2016, 11:41:35 am
content: Hi fnpfar,That is no way to set the bias for a transistor!Add a 1k resistor form the emitter to the ground and bypass it with a 100n cap.Then you can set the bias comfortably!The oscillator in that configuration runs in the simulator with the bias (emitter) current set to about 1.8mA.Good luck, Peter

author: rfbroadband
author_rank: Unknown
timestamp: July 31, 2016, 01:39:59 am
content: download a free spice simulator and start by simulating and verifying the bias. Next do transient sim....

author: uncle_bob
author_rank: Unknown
timestamp: July 31, 2016, 02:46:19 am
content: HiWhere did you get that schematic from and why do you call it a Colpitts?What is your objective (just a sine wave out)? Why not use a crystal? For 20 cents that would solve a lot of issues.To do it correctly:Keep the emitter hooked to the junction of the two caps.Move the ground to the bottom of the inductor (L1/C1 junction).Put a 1K or so from emitter to groundPut a 10K from 9V to the base.Put a 10K from base to ground.Run the collector to 9V through a 330 ohm resistor.Start with C1 = C2Start with the impedance of C1 and C2 in the 10 to 100 j Ohm range at your desired frequency.Start with an inductor with a Q > 100 and an impedance of 2x that of C1 or C2 (it resonates C1 in series with C2).The circuit should come up without the coil in the circuit at:4.5V on the base3.8V on the emitter8V on the collectorConnect the coil and you should see several volts p-p on the 330 ohm resistor. This is with a high impedance probe running into your oscilloscope.Bob

author: fnpfar
author_rank: Contributor
timestamp: August 01, 2016, 11:52:41 am
content: Thanks a lot for your questions. I tried some other configurations (common base and common collector) and they worked well. Now I understand why that circuit didnt work.Thank you!.Fran

author: tatus1969
author_rank: Super Contributor
timestamp: August 01, 2016, 02:04:13 pm
content: don't know if I overlooked it, but what is the value of L?