This video shows one way that distortion and thermal sensitivity (among other things) is reduced in a single transistor amplifier. Prepare! A RU-vid rant is included.
Excellent demo of a classic circuit design. Clearly shows why the emitter resistor is pretty much mandatory.Rock solid thermal stability, very low distortion, predictable gain, AND almost any transistor of the correct polarity will work about the same.That is the building block circuit that made transistorized equipment comercially possible.
Love the video John cannot understand why on earth they would demonetize any of your videos they are knowledgeable and informative brilliant..................................Thank you........................ Berni
The video you did on how to determine the difference between one tda over another was great,, I found out that I was sold fakes instead of the real thing,,, wondered why the chip didn't work the way it should have..People need to start giving youtube the gears!
Sucks that those videos got de-monetized, I don't bother monetizing mine though, I hate ads and don't want to annoy my viewers with them. I've had plenty of copyright claims though, my last video got one less than an hour after it was uploaded, I'm even still getting new copyright claims on videos I uploaded years ago! I'd better watch what I do from now on. I heard about the trouble Cody is going through with strikes on his channel, I went and subbed to his backup channel but I think he's got things sorted out now.
To delve a little deeper into how the emitter resistor (Re) improves the performance of the stage, it is necessary to understand how the gain of a common emitter stage can be calculated. The simplest way is first to consider that the emitter current (Ie) is equal to the collector current (Ic), which it effectively is when transistor β is 100 or more. At any given Ic, the dynamic base resistance of a transistor (re) is approximately 25mV/Ic at room temperature. So for a collector current of 5mA, the intrinsic dynamic base resistance of the transistor is about 25mV/5mA = 5 ohms (5R). A small change (Vin) to the input voltage applied to the base will produce a change in Ie of about (Vin/5) amps (although Vin will only be a few millivolts ac). But any small change in Ie will produce the same change in Ic, therefore the voltage across the collector resistor (Rc) will change by (Vin x Rc / 5) volts. The ratio of the change in the collector voltage to the change in the input is the gain and that is equal to Rc/5 in this circuit. In general, the gain of the common emitter without an emitter resistor is Rc/re, where re=25mV/Ic. You can look at that as Rc.Ic/25mv, or the voltage across the collector resistor divided by 25mV. That shows that the maximum gain of a common emitter stage depends on the supply voltage _and nothing else!_ However, the instantaneous gain of the common emitter stage without an emitter resistor can be seen to be directly proportional to the instantaneous collector current, which means that the positive part of the output is amplified more than the negative part. No wonder you get 8% distortion even with a couple of volts output. Alternatively you can consider the gain to be Rc/re and note that re depends inversely on the collector current. Non-linear resistances are an immediate source of distortion. It should now be obvious that if we can add to the resistance seen by the emitter, we can "swamp out" the non-linear re so that it becomes (re + Re), where Re >> re. In the circuit shown in the video, Re is 100R, which is 20 times bigger than re (5R). That reduces the gain by a factor of 20 (from ~200 to ~10), but also reduces the distortion by the same factor (from 8% to less than 0.5%). Having that emitter resistor also reduces the dc gain from 200 to 10, so that millivolt changes in Vbe (from temperature changes) become negligible. Similarly, the effect of changes in supply voltage that cause changes in the output bias point are reduced because of the smaller gain. Hopefully that gives some insight to how the emitter resistor improves those performance characteristics and to the magnitude of the improvement. You should be able to see the effect of using a 50R (or 47R) emitter resistor, or of cutting the collector current down to say, 2mA while maintaining a 100R emitter resistor. Finally, some commentators have suggested bypassing the emitter resistor with a relatively large capacitor. That removes the effect of the Re on reducing the distortion. It results in an ac gain of Rc/(re + Xc), where Xc = 1/(2πjf.Ce) (the reactance of the bypass capacitor Ce) at frequencies where Xc
Thanks for your detailed comment on this. I've been contemplating making another video explaining the intrinsic resistance and Early effect with pictorial descriptions. The math can't be entirely avoided but scares many viewers away.
Very nicely done! I was thinking that as a final improvement if you had placed a capacitor across the resistor that connects emitter to ground you would have picked up some of the gain lost by cleaning up the amplifier. I am not 100 % sure why what I suggested works and kind of hoped to see it explained a bit.
That would immediately re-introduce the distortion. There is no "free lunch" with transistor amplifiers because of their non-linear characteristics and you always have to trade off gain for reduction in distortion. I'll make a separate comment as to how this works.
GadgetUK164 found that his tags/keywords were at fault for his demonitisation. It was either "NES" or "fix" that did it for him if I remember right, and when he cleared the tags, monetisation went right back to normal immediately. Wonder what could be so objectionable or marketing averse about repairing 30 year old entertainment computers. Or maybe people who know electronics repair are simply not effective marketing targets?
John I love your channel. Do you have some interest in current conveyor amp circuits? If so, I would love to see a video like this about a simple current conveyor. Audio-GD uses them in their amps, and they are phenomenal.
Let's have some more of this single transistor stuff. Can we have information about the effect of lower collector currents by changing the 1K to 10 K? Also ,biasing with 1 resistor directly from collector to base. As well as distortion and gain,the effect on input impedance resulting from the 100 resistor.
I want to see this self-biasing stuff too. I've got some thoughts regarding high-gain low-noise radio amplifiers for loop antennas, but not quite enough understanding to put them into practice.
hey, John, you can add a bypass capacitor across the emitter resistor that greatly improves the gain if you add let say 1000uf the gain would be the same as the original, thanks this video was very helpful!!!
Only if you don't mind having the 8% distortion return as well. A 1,000μF capacitor would result in a gain of about 200 with the 5mA at 14V supply, but only for frequencies above about 32Hz, so not quite the usual full audio range.
Another great learing video keep up the good work wondering how big of a amp u have ever built that and wondering if u could build me one that i could use just a positive and negative single voltage with the amp u build could draw all the way up to 130a if u wanted and voltage i can go up to 15.3v i modified a dell poweredge server power supply to run a amp i think lowest i can go is 25a at 12v
Because the emitter V is floating across that resistor, the potential from base to emitter is likewise changing. This change is 180 degrees out of phase from the input resulting in degenerative local feedback, lowering distortion at the price of reduced gain.
Thanks for the reply, So as the voltage at the base rises, the collector current (and therefore the emitter current) swings low, this pulls Vbe down, effectively 'reigning in' the input voltage as our form of neg. feedback? How come as the voltage potential drops at the emitter does that not make the base 'more positive' in reference to it?
No, if voltage increases at the base, base-emitter current rises and so does collector to emitter current, multiplied by the beta. This bigger current through the emitter resistor results in more voltage dropped across the emitter resistor. This is the countering signal, the feedback. So the base-emitter current isn't referenced directly to ground but to the voltage at the top of the emitter resistor. 15:40
It bypasses the signals so gain increases, but at the cost of more distortion. Sometimes there are two series resistors with the cap bypassing one of them to get the benefit of some AC feedback.
A large enough cap there would bypass and eliminate negative feedback for AC, but not for DC. So you get the DC bias stability, but none of the AC effects on gain and distortion. Impedance here is much lower than on the input, so you need a larger cap.
the problem is when you study from the books (where electronics is like a perfect world) and then you try to make a project and you realise that the perfect world doesn't exist :( BTW. I tried to amplify a signal with MOSFET and I get a lot of ugly distortions :/ I think I wanted too much gain at once and I forgot about extra resistors at the source also. I didn't know that I can almost boil my electrolitic capacitor as well 😂 Any way you remind me about some ideas and how to resolve the problem of distortions . Thanks!
The whole demonetization thing is hitting all RU-vid channels. I've heard it all before. I don't think people bitching about it in random videos is going to change things either. I know if I was a RU-vid exec spending my ill gotten loot I certainly wouldn't care.
Just ad a 100uf capacitor across the 100ohm Resistor.... Help to prevent feedback & gain losses.... For ac signal... It passes... So it can't produce a voltage dropp
You should put your videos on rumble. It would be nice if there were good electronics channels there. I rarely get on RU-vid any more because they’ve gone insane about people who don’t agree with all their woke trash. I just buy books and read now instead of watching videos. I miss the good electronics channels. I canceled any membership to Google and RU-vid as well. I refuse to let any company dictate what MY opinions should be.
you can get back a lot of the gain on the AC signal, by bypassing the emitter resistor with a capacitor in parallel with the resistor! So why didn't you show that?
