# LC Oscillator, how to prevent dampening sine wave

Hello, I'm working on a project where I plan on using an LC oscillator to provide a sine wave signal. I'm new to analog electronics, so I am teaching myself about LC oscillators and am stuck on how to get them to not have their oscillations dampen. I understand that when you quickly apply power to the circuit, the circuit oscillates at it's resonant frequency, and these oscillations dampen quickly to zero due to resistance in the circuit. I was wondering how you can prevent this? I found this video from GreatScott, where he briefly says around 4:27 that this can be achieved by "connecting the output of the tank circuit to the input of an amplifier.... by choosing the amplification factor just right the output produces a steady sine wave." My two questions about this are (1) what type of amplification circuit is he using? It is shown briefly in the video. and also (question 2) how do you choose the amplification factor "just right?"
I realize this is probably a lot I'm asking, but any help would be appriciated. Thanks!

Electronic Basics #17: Oscillators || RC, LC, Crystal - YouTube (talks about LC resonators at 2:55)

the circuit oscillates at it's resonant frequency, and these oscillations dampen quickly to zero due to resistance in the circuit.

You need some amplifier to provide energy to keep the oscillations going.

Now you don't actually have an "oscillator"

Some info HERE:

You will have to learn about the various oscillator circuits available such as Colpitts and
chose one. Some expect the inductor to be tapped, or even to be a transformer.

And to answer your other question, usually you don't set the "amplification factor" "just right" You usually include some non-linear element that limits the output amplitude.

The power supply voltage is normally the limiting factor in practice.

Very low distortion oscillators use tricks to maintain the oscillator gain at unity
to prevent any overload distortion. Self-heating thermistors are one way to do this.

"Self heating thermistors" i.e., non-linear elements. I have seen tungsten bulbs used for this, too.

In your garden variety LC oscillator, they just rely on the gain being a bit more than one at low signal levels, and that the gain goes down with higher signal levels.

The current in the active element is not linear, but the voltage across the tuned LC circuit is very low distortion.

Hi,
What frequency?

Tom...

KeithRB:
"Self heating thermistors" i.e., non-linear elements. I have seen tungsten bulbs used for this, too.

Same thing, actually.

Thank you all!
Two more questions...
I understand the role of the capacitors and inductors in these circuits, but what about the resistors?
The Colpitts Oscillator is a very good explaination of colpitts oscillators, but I don't understand how to calculate what resistor values to use.
Also, how would one frequency modulate a colpitts oscillator, using the colpitts oscillator to make the carrier frequency?
Thank you...

tinkerkid:
Also, how would one frequency modulate a colpitts oscillator, using the colpitts oscillator to make the carrier frequency?

Really, it depends a lot on what the carrier frequency is. What is it (repeating reply #7)?

Around 90 MHZ, and very low power.

Hi, What is your electronics, programming, arduino, hardware experience?

I think you will find you will have stability problems using a LC oscillator at that frequency, capacitive effects and mechanical layout will have to be considered in the design.

You are on an arduino controller forum, consider a direct digital synthesis IC to generate your 90Mhz.

What is the application that needs a FM modulated 90Mhz transmitter?

Tom...

If you really want to frequency modulate an LC oscillator, you need to make some of your "C"s varactors. Or use a wobbulator. 8^)

You need to learn about how to bias a bipolar transistor. The most common and stable type of biasing is voltage divider, emitter feedback.

This link provides information on other types of transistor biasing and the problems with them before it gets to the meat, but it contains a lot of good information.

polymorph:
You need to learn about how to bias a bipolar transistor. The most common and stable type of biasing is voltage divider, emitter feedback.

Voltage Divider Bias of a BJT Transistor

That circuit doesn't work. The transistor is saturated.

Why do people not do the most basic of proof-reading when they put these resources on the web ?