LC in a Transmitter Circuit - Page 1
Forum section: electronics / beginners

author: skinny
author_rank: Contributor
timestamp: November 30, 2014, 04:27:55 am
content: Recently I made a transmitter circuit and wanted to break down how everything actually works. The transmitter circuit works. I understand that the LC tank circuit above the transistor sets the frequency of the device (see attached image). I also understand there's a formula that gets you close, but what I don't understand is exactly how it works.In an effort to understand what is happening, I took the inductor and capacitor, separated them from the circuit, and made the second circuit attached to this post. Next, I placed my o-scope probe on every single location on the circuit trying to look for a generated sine wave and found nothing.My questions: I thought the inductor and capacitor in parallel generated the frequency. Why can't I see it? Must the transistor be present to produce the wave? I thought is was just to amplify the wave. Am I thinking about this in the wrong way, or is my separated test circuit wrong?Thanks for your help!

author: German_EE
author_rank: Super Contributor
timestamp: November 30, 2014, 10:32:49 am
content: You're close but there are a few errors in your guesswork, don't worry, we all learn by experimenting.The frequency of your oscillator is determined by the LC circuit, vary one of these and your frequency will alter up or down. However, the inductor and capacitor are passive devices and as such cannot form an oscillator on their own.Think of it like a childrens swing, the length of the rope determines how fast or how slow the swing moves BUT something needs to give a gentle push on a regular basis to make the swing actually move. That's what the transistor and the other elements in the circuit do, they work with the LC network and supply a regular impulse to keep the circuit going.Your second circuit with the LC circuit, the resistor and the battery does work but not in the way you might imagine. The moment you apply power there will be a very quick burst of signal but this will quickly decay to zero. Connect a fast oscilloscope to the network and you may see something like this:http://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/Damped_sinewave.svg/1024px-Damped_sinewave.svg.png

author: Paul Price
author_rank: Super Contributor
timestamp: November 30, 2014, 12:23:09 pm
content: You don't see anything on your scope in your 2N3904 circuit because its not oscillating. An oscillator requires enough positive feedback.Move C4 from emitter to ground to instead from collector to emitter and it will likely oscillate. Else, remove the 10pf and make a 2-in tightly twisted pair of #24 wires(ends in air unconnected) will make a feedback control, or else use a trimmer cap with a range of 1-10pF. The twisted pair is a cheap way to implement a trimmer cap. and would be a practical way to optimize the amount of feedback. This feedback cap will also become part of the frequency setting circuit. To adj. feedback, gradually untwist the pair wires.You second passive approach to creating oscillation could be called a 'one-shot exponentially decaying microvolt oscillator with extremely short burst output" circcuit.

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2014, 01:43:52 pm
content: Both of the previous posters are correct.The inductor and capacitor act as a resonator and without an amplifier will result in a damped oscillation, like a pendulum which has been push once and allowed to swing until it stops. In order keep oscillating, something needs to keep pushing it to compensate of the losses in friction. In the case of a pendulum this is achieved in a grandfather clock. As for your LC oscillator this is achieved by the transistor which is an amplifier.An oscillator is an amplifier with positive feedback at a known frequency, in the same manner an audio amplifier will oscillate when the microphone is placed too close to the speaker, causing an audible howling.In the case of the circuit above, it's unlikely it will oscillate as configured. Ideally that 10pF capacitor should be connected between the transistor's collector and emitter, to act as a feedback path, as mentioned previously.The transistor is configured as a common base amplifier, with the input being the emitter and collector the output and the collector resistor (RC) replaced with the resonant circuit forming a tuned amplifier, which lots of gain at the resonant frequency. To oscillate the output needs be fed back to the input, which is done with a low value capacitor. A common base amplifier is non-inverting so it will oscillate that the resonant frequency which is when a parallel tuned circuit has a phase shift of zero.

author: skinny
author_rank: Contributor
timestamp: November 30, 2014, 03:53:22 pm
content: Thanks for all the help everyone. I see now that the wave form is decaying too fast for me to see it in the LC circuit. The interesting thing is that the first circuit I attached to the previous post actually works without a cap going from the emitter to the collector. I assume it's working because of some stray capacitance between the two points. This is probably a by-product of me using a breadboard to prototype everything. I've already ordered and received a circuit board based on this design. I attached it below. I guess I'll see what happens when the parts come in tomorrow. Thanks again for the help.

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2014, 05:05:25 pm
content: Quote from: skinny on November 30, 2014, 03:53:22 pmYes that's probably true. A bread board can easily have enough capacitance for it to work. At much higher frequencies (above 300MHz or so) the capacitor across the transistor is often unnecessary as the parasitic emitter-collector capacitance inside the transistor is enough to provide sufficient feedback.QuoteYou should get much better results using a smaller board. In general as frequencies increase the physical size of the circuit needs to be reduced because it there is a limit to how fast the electrical signals can propagate through the wires,.