Woohoo! The old negative resistance oscillator. I built one of those as a kid. It was a Radio Shack P-Box Kit, a regenerative shortwave radio. The regeneration control was a 500 ohm pot in the emitter.
The capacitor across the collector-emitter of the transistor turns this into an unstable class-E amplifier. This can be seen by looking at the transistor conduction angle, which is very small. As such, it not only generates a pretty good sine wave, but it can be very efficient to boot. Thanks for going over this! One thing you have to watch out for in this configuration is that you don't get an out-of-spec Vbe. I had to add a few diodes across the base-emitter junction to keep it from getting less than -6 volts...
I usually use the ratio of capacitors to scale the swing on the emitter. The circuit might have 20pF from collector to emitter and a 100pF from emitter to ground. I usually design for about a 2V swing on the emitter. This puts it well into class C
Videos like this are very helpful to me because they remind me that I can just try things. A part of me wants to have a project that has lots of plans and rules and an expected outcome but this reminds me I can do like you do and just TRY SOMETHING... thanks again!
I would have liked to have seen it soldered onto some perf board because of all the weird stuff that goes on with proto boards. And maybe see stability over time and maybe any weird harmonics. You have much better toys than I do!
@kensmith5694 Yes .... but all of these can be derived from a amplifier (inverting/non-inverting) with feedback provided by an LC bandpass filter. The equations for the general case can be specialized to each of these Colpitt/Hartley/Clapp/etc realizations by choosing three L or C reactances. That UNIFIES the understanding of all of these Oscillators within a single analytical framework !😇!
Used to have a J.Bull £5 kit build FM transmitter with an audio pre-amp transistor and a Colpitts with as I remember the centre tap of the inductor feeding back to the emitter via a capacitor. Apologies to my college neighbour who had his bath-time Radio 4 disrupted. Correction: centre of five turn coil was the aerial attachment.
Class C very efficient Class A wastes in resistors heat. This one resonates LC circuit. Good for Conduction trace on old Oscilloscope the Sine wave is required for a good trace in diagnostics. Square waves give a distorted trace. It is good to vary the frequency for device testing using a curve trace.
It would be the series combination of the two capacitors that set the resonant frequency if the transistor wasn't basically shorting the one between its emitter and collector on each cycle. I say that the circuit is Colpitts because it uses a capacitor voltage divider. Here is how I break down oscillators. Others may disagree. Armstrong did two independent windings like a transformer Hartly combined the windings to make a tapped coil as the resonator. Colpitts used tapped capacitor divider to replace having a tapped coil Clapp realized that with 3 capacitors, you can optimize for the transistor and the coil at the same time. Buttler used two active devices making a low impedance and a series tuned resonator
Breadboard adds parasitic capacitance, especially in those critical nodes, and alters math results. More precise results can be reached, soldering it on a pcb with the dead bug method.
Yes or doing an real layout. I made the version of this that got sold by doing a PCB with estimated values and then adjusting the values to make it work exactly on frequency.
Fascinating -- I'm a bit surprised because the amplifier involved is clearly CB. The "tank circuit" would be loaded with about 50ohms (based on the usual r_e estimate and your circuit). It surprises me because all the loading of the tank would result in it decaying quickly (if the transistor wasn't continually kicking it, that is). Thanks as always!
@@IMSAIGuy sorry, my mistake. Anyway, when r_e is reflected back to the tank circuit, the equivalent resistance is equal to r_e x the square of the ratio of the capacitors.
When you remove the top capacitor, there is still capacitance between the collector and the emitter, from the component leads, inherent capacitance in the transistor etc. It's much smaller, probably a few pF - but it's enough to complete the tank and keep it oscillating.
Yes, in real designs it is best to put in a real capacitor bigger than the transistors C-E capacitance so that the circuit repeats better. Also, it is worth noting that his scope probe was also part of the circuit. Its capacitance might be more than the transistors C-E capacitance and be setting the frequency.
It’s almost a slayer exciter circuit from a Tesla coil rig. I’m consistently getting 1.6-1.8 mhz and beautiful sine wave with the gate biased between a 47 ohm resistor (positive rail) 5k potentiometer wiper connected to the gate of an N channel mosfet. And trying to find the origin of the Slayer exciter circuit, I found the diagram of an inductive load test for mosfets on the last pages of most of the mosfet data sheets. So slight changes to components and configuration is what make one circuit, different from another?? Thanks for the video, as always.
Trying to reproduce this on a breadboard and some random components I've laying around.. I've found picking the inductor too small and it does not start osculating. I'm using colour ring inductors as these are breadboardable.
You mentioned, "If you read the literature....", can I ask from which literature you got the inspiration for the circuit? Enjoy your videos, keep them coming!
Its remarkably close to a pure sine wave. I wonder whats giving it that slight slant and the flattened negative peak? Ill have to build my own to play with i think 😊
It also seems to have a bit of Joule Thief dna since it creates a higher voltage than the power supply voltage. But a surprisingly nice sine wave for sure 👍
You can play with these things until the cows come home. The more you learn about it the more you realize you don't know. Lol. I have been studying a single LED for the past 2 years. My latest craze is to count how many photons are emitted as a function of the current through the LED. The current has to be very low and it has to be really dark. So I had to make a dark chamber out of cardboard and black paint. It never ends. Lol.
The flattening is a combination of three things. One is the fact that the transistor tends to saturate AKA bottom on the negative part of the swing flattening the bottom. A second one is that the transistor get slower when there is less than about 2V on it. This makes a rounded bottom effect. The 3rd is the fact that on the negative extreme of the swing, the inductor sees a different capacitance. This makes the thing run at sort of an average of two frequencies. On the positive peak, the resonance is at a higher frequency than on the negative peak.
the electron to photon conversion in an LED is called the internal quantum efficiency. In a GaN led, this number is almost 100%. the problem is getting the photon out to the world. it is in a high index material (n~=3). it likes to bounce around and not come out.
That circuit is trying to be a common base colpitts oscillator. However you have 2 10k resistor there, acting as some kind of bias, so it isn't a common base circuit. It's a lot easier to make an oscillator than a stable amplifier. I think what you have is one of those and it just resembles a colpitts oscillator. Here is link to another colpitts oscillator video. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-wC_uKxu_3AA.htmlsi=2alzfxx3BSVOx0Bh
The frequency didn’t double at 3:45. Trap for old players. The oscilloscope must have been counting that extra ripple and looking like it doubled. But the time base didn’t change and the peaks were at the same place.
I would add a note that the temperature stability of these kinds of oscillators are pretty poor. I've been trying to learn rf design and started it by trying to make a simple ham radio for 14.3 Mhz. I my first choice for an oscillator was something similar to this with a variable cap. I found that the temperature would vary the frequency wildly. I ended up having to use a crystal, but now I can't manually vary the frequency as much. Maybe 20khz. Good enough for a first radio I guess. I'm just happy it works at all.
To make it frequency stable, you need a high Q and also to make it so that you are massively away from matched to the properties of the transistor. You want the impedance of the feedback signal to be either much greater or much less than the impedance of the active device. Basically you want either voltage or current to remain constant with variations in the temperature of the transistor. I have done one that holds within a few percent from -35C to +60C. The divider ratio of the feedback to the emitter is over 10:1 making the impedance driving the transistor quite low. The HFE independent bias was intentionally made to go to a slightly higher current at +60C.
@@kensmith5694crystal's Q is huge. I know the transistor's Vt is 26mV at room temperature. How does it vary with temperature. It affects small signal amplifiers too.
@@EEE-iw3fk 26mV is a constant times the degrees Kelvin. Adding your own resistor to make the design more predictable can be a good thing to do. Otherwise, you want to make the impedance mostly a thing you control.
I realise this is a simple circuit and I may have missed something in the explanation, but I'm still unsure how trans transistor is is being switched, or is it? seems to be lacking a bit more detailed explanation for newbies like myself.
Presume the collector is wizzing up and down. Look at what the AC voltage is at the emitter of the transistor. The two 120pF capacitors work as a voltage divider. If the transistor wasn't loading the junction of the two 120pFs the voltage there would have 1/2 the swing of the collector. This is more than enough to turn the transistor on and off. It is a common base amplifier that is being driven into clipping. This makes it a lot like a switch.
first learn how this works: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-tAK8Q5IZjng.htmlsi=nALAxuLUW7wXfcxs then add inductor capacitor and some feedback
The feedback is to the emitter. Think of the inductor and two capacitors as having a huge current sloshing back and forth in them at resonance. Remember a tuned circuit can have Q times as much current in it as the transistor is applying to it. The 120pF from the emitter to ground, thus ends up with an RF voltage on it that drives the transistor as a common base amplifier.
@@kensmith5694when I had the capacitor on the base it did shut off and clipped on the bottom of the waveform. by letting the base float a little it seems to self regulate, hence the sine wave.
@@IMSAIGuy The tuned circuit has a lot of Q so it filters out the harmonics. With the base open, the amplitude is likely less but still the transistor will be tuning off but not bottoming. As the transistor goes into class C, the effective gm of the transistor starts to decrease, The amplitude will limit by the gain decrease instead of bottoming. You can prove this with one scope channel on the emitter and one on the base.
@@IMSAIGuy oh, I thought th 0:40 e top cap is 100 uF. Then total is half. Anyway, the bottom cap should be greater. You have too much gain. That's why the transistor is saturated when you short the base by adding a cap
Colpitts oscillators are NOT simple. Yes, they have just a few components. But you do not have any control of: amplitude, frequency and distortions. Your output signal is not a pure sine, even without base capacitor. It looks like sawtooth. It is very hard to design a Colpitts oscillator with desired parameters, and with clean sine output.
@@andymouseI hope my comment is not mean - I just wanted to emphasize that the topic is very complicated. But it should not prevent you to have some fun with building it! Even more interesting is to find, what should be done to tune the oscillator to produce pure sine!
No not at all mean !! it is complicated but I'm a bit fussy because I am playing with Bulb Stabilized Wien Bridge oscillators and when they are done right you get fantastic sinusoids but are also tricky to make but have an interesting history as well ! :)@@olelek
Amplitude can be controlled by bottoming and the supply voltage. Frequency can be controlled by making your L and C values a long way from the transistor's characteristics. Distortions are controlled but large. A design like this can repeat well enough for production.
With high Q of the band pass filter (tank circuit), the signal can a pure and stable sine wave. The circuit indeed is extremely complicated. Tons of math is involved. Transistor is operating in large signal mode/non-linear. Band pass filter take care of the non-linearity and stabilize the amplitude and purify and frequency. It performs better than the light bulb in wien oscillator. I'll post a video in a few weeks about it in great details with equations and design limitations.
You really should derive the Amplifier gain + filter band-pass relationships and DEMONSTRATE how the frequency of oscillation and minimum required amplifier gain are determined by the feedback provided by the values of L, C1 and C2. This will increase the level of understanding by viewers. As it is, the video teaches nothing !
I did a survey "Math is sometimes part of doing electronics. your relation with mathematics is:" I love it 39% it is interesting but difficult for me 25% don't love it, don't hate it, can do it just fine 29% my brain does not do well with it 7%
