Simple LC oscillator schematic build I have not seen before, any advantages? - Page 1
Forum section: electronics / rf-microwave

author: William Edwards
author_rank: Contributor
timestamp: June 05, 2017, 05:49:13 pm
content: HelloIm in the build of a very small FM transmitter and have used the very bottom schematic before when I now found this very different one:https://i0.wp.com/makezine.com/wp-content/uploads/2016/06/schematic.pngI have not seen any type of simple oscillator schematics where another transistor acts as a emitter resistor or what it is or to call it, and also no emitter resistor involved and Im also not familiar with the placement of the 12pF? Shouldn't that be placed the same as the other one directly parallell to the coil? Im not familiar how this would work.Are there any advantages or disadvantages of the above compared to this schematic:http://www.talkingelectronics.com/projects/Spy%20Circuits/images/1TrBug.gifThe first is totally different to what Im used to see in an simple LC oscillator.Anyone that perhaps could enlighten me?Thanks!Regards William

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 05, 2017, 06:10:04 pm
content: C4 blocks the power supply from reaching the transistors so it clearly will not work.

author: William Edwards
author_rank: Contributor
timestamp: June 05, 2017, 06:58:04 pm
content: True that way but it would through L1 tough? But its not the right way to do so as I see it but could it work that way instead perhaps?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 05, 2017, 07:13:53 pm
content: Quote from: William Edwards on June 05, 2017, 06:58:04 pmAh yes, you are right of course, my bad. It is drawn in a bit of a weird way, but C4 is parallel to L1.It still seems strange to have the tuned circuit between the base and collector, they will have the same dc voltage.Ok, simulation time... To my surprise it works, just had to increase C5 a little but using different transistors:(Oops, VCC at the wrong side of L1)

author: William Edwards
author_rank: Contributor
timestamp: June 05, 2017, 08:06:06 pm
content: Thanks for that sim!Dont get how it could work though, totally different design Im used to where LC is drawn directly by the collector and Base feedback from the Emitter - Collector capacitor, dont get it. Is Q2 crucial for the function for the circuit in anyway or could it be omitted with a resistor at the emitter of Q1 or something?They are parallell in the original schematics, true, but then VCC is connected directly to the Base and that should just not work as I see it.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 05, 2017, 08:14:37 pm
content: I agree it looks strange, but what can you say if it works?Q2 is only for modulating and can be left out, and I also corrected the VCC connection. It still works...

author: William Edwards
author_rank: Contributor
timestamp: June 05, 2017, 09:41:30 pm
content: Dont get it but Im ok with it.Really cool it works either way, you learn something new everyday.Is it then possible to even modulate it, with say a 2.2µ cap connected to the base, to get FM modulation without Q2?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2017, 11:53:46 pm
content: It works because VCC for the oscillator is 1 * Vbe.Tim

author: William Edwards
author_rank: Contributor
timestamp: June 06, 2017, 11:53:10 am
content: I meant the very way how LC is connected since I have always only seen these connected directly to the collector.But when I think about the connection a little more it could perhaps work and well it obviously do.Thanks for the help guys!Regards William

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2017, 03:56:10 pm
content: It's a variation on Colpitts. The capacitors couple emitter back to base, with the collector as a tap; or some relative grounding variation upon that (you can take a basic oscillator and ground any node you like, supplying bias to the rest with chokes as needed; this generates half the named oscillator configurations right there).The bottom transistor acts as a choke, but a better one (it has a high impedance all the way to DC -- a current sink), but also a worse one (it has a few pF collector output capacitance, which will load the oscillator a bit). This varies the amplitude and/or bias (and therefore frequency) of the oscillator, in a rather inconsistent and nonlinear way, but it will technically do what's claimed.Tim

author: William Edwards
author_rank: Contributor
timestamp: June 06, 2017, 04:13:42 pm
content: Thanks for the explanation, interesting, very appreciated!Regards William