Differential Amplifier, the Basics 

That's actually the first really helpful video about differential amplifiers. No math needed at first, but a fundamental understanding of what it does. Great job...

The best explanation! Never though about differential amplifier as two A class amps together. My life is complete now. Thank you!

@THEtechknight You can control the sound quality with the second input.

You always present great videos on RU-vid. I look foward to your videos. Thanks

The beauty of a Differential Amp is that it only amplifies the signal Difference between the two inputs i.e. anything else, signal (noise) between an input rail and B+ or B- (or ground on a single supply typology) will not be amplified, thus a useful low-noise tool is born. When we modify the circuit by sharing a common emitter resistor (and capacitor, in some cases), the "Long Tailed Pair" that is commonly used for Phase Inversion in a Class AB Amp is created. This structure is used for the input of every Op Amp. You can also see this method applied to many Push-Pull Audio Tube Amps ( look at the schematic for Fender Twin Reverb, as an example). Prior to this, it was common to use just a single tube or transistor for Phase Inversion; the signal would be applied to the base of a transistor or the grid of a tube, equal resistor values would be applied for the Plate and Cathode of a tube (say 56k), or the Collector and the Emitter of a Transistor (say 10k), thus making a somewhat balanced output signal for the positive and negative rail outputs. The problem with this typology, is that the positive signal is referenced to the B+ and the negative signal is referenced to the B- (or ground), so noise from those two sources creep their way into the signal chain; not so with the Differential Amp. Also worth noting... the Long Tailed Pair makes a much more pleasing music distortion, whereas the Cathode Style Phase Inverter does not. You have to also remember that tubes came before transistors, and the basic gain typologies have remained the same.

Amazing video! One of the clearest I've seen, thusfar.

One of the best explanation. Thank you.

Thank you for the very instructive video. Besides the valuable content the professionalism in the way it is explained deserves an A+

Excellent tutorial Rick! Clear and well explained as usual. As long as you focus so well on basics principles, basement knowledge becomes strong enough to support more on top! Thank you very much for your dedication preparing this learning material. Best Regards. Pablo

great explanation, greetings from Venezuela! :)

Rick - outstanding video. Just finished the chapter on op amps with my students, where we talked about diff amps. Doug'

Thanks DigiSDR The load resistors are always in the circuit when the signals are the same or different. And so are the emitter resistors, but their for biasing the bases. The most signal will be across the load resistors.

Thanks twirmd I don't know of any reason why MOSFETs could not be used in a differential amplifier. I bought some more MOSFETs for experimenting with making a flip-flop circuit, hopefully easier to build. I'll try a differential amplifier to, sounds like a good idea.

@CassetteMaster Sounds like it will work. Anything that is the same at both outputs is a null between the outputs. It should be greatly reduced or removed. This video is a simple example, there are chips that can do a much better job. I would Google the information.

Very good job, Rick. Clear as glass. If I can understand it, anybody can. Regards, John

Great video and top explanation, thank you

I wish I could put thousands of likes you touched a very powerful subject and the way you explained it it truely elevated my understanding and you deserve a big thank you.

Many thanks for the video. it helped me understanding the differential amplifier so much. Many many thanks

@AndyDaviesByTheSea Hey Andy, I don't remember if I have or not, it was an exercise and lesson in one of my Heath Kit courses. And yes with analog recordings this process is difficult. Today I would used digital recordings. Using my digital sound applications, I have sync-ed two sound sources, recorded by two different digital devices, of the same thing for over 10 minutes and they remained in perfect sync. I was amazed because I remember how difficult it was, now it is nothing. Regards Rick

Thanks! Awesome video.

great explanation, much thanks,

Great explanation, thanks for posting!

@hmerrett If you have just the sound you want to remove recorded, it is possible to remove it, one channel at a time. I think there are sound applications that can do this all in the computer today.

I'm starting to learn and understand more about amplifiers every day in your Channel. thank you so much.

thanks alot for this video! great explanation like Vadim Harenco says.. I also never thought about two class A amps together!

I like the way you explain this movie! Every single word I understood despite the fact that I know pronunciation of English words not very well. And this example with the music and the speech at the end - is just geniously! Thank you!

Very nice explanation of how a differential amplifier works with very practical examples and experimental way of explaining how and what you can do out of it.thanks for the very tutorial lesson; I have learnt quite a lot.

@Serpico261 Yes. The output is not high. I ran the output from the transformer to an amplifier. At this point the output is low because it is the difference between the two transistor outputs. It is useful and it is easy to amplify. And the video is not fudged. what you see is what happened. The transistors are 2N2222.

Men may come and men may go, but your legacy will life forever. Thank you.

alot better than my teacher ever explained it to me. Very informative and clear! thanks!

Great tutorial!

Excellent! thanks

Hi Rick, nice clean demo. Have you ever stripped a sound off a recording like that? I can see the possibility but I could imagine that you’d get some heterodyning unless you could maintain perfect in phase sync. Kind Regards ... Andy

Super video as always Rick

i hadnt seen this one,very well done.

Great as usual. Please do some video about audio transformers, impedance transformes, etc. THANKS.

Very well explained ! Thank you !

Clear explanation. Thanks for making this video...

@AndyDaviesByTheSea Hey Andy, I don't know how many Heath Kit courses I have done, well over fourteen. About the last one was building an helium neon laser in the Laser course. About the last lesson in this course was using the laser to make a sound interferometer. I could hear myself walking and cars passing by the house. I made a video on the interferometer using a laser pointer. In another video using the laser pointer I audio modulated it and the sound quality is very good. Regards, Rick

Thanks CassetteMaster That would be interesting to try, to remove white noise. It is in the background. That just might work.

This was very Educational I enjoyed it . Thanks////// I look forward to your videos. Thanks again.

Thank you , that really helped clear things up for me.

You explained it better than my lecturer! Thank you very much for the video! =)

gud to hv practical application with theory...luv the way u taught👍👌

Good video,great that you demonstrate it....:D

If you had two radio stations and if one of the stations bled over on to the other, in thery you could remove that station with a diff amp buy taking a clean copy of one of the stations signal and the copy that had both, run them through a diff amp and you could get rid of the unwanted audio and just get the audio you wanted. what do you think.

Very nice video. To follow along and get the same results you MUST have a matched pair of transistors and preferably even matched resistors, etc..

Thanks jenko701 Sounds like it will work. Anything that is the same at both outputs is a null between the outputs. It should be greatly reduced or removed. This video is a simple example, there are chips that can do a much better job. I would Google the information.

I wonder if this could cancel out DC bias's background noise while playing back.

HELL YEAH! Now am gonna change majors. haha

thanks helped a lot !!!

super !! tks a lot for this perfect explaination

Well that clears up a lot of confusion, thanks!

Dear AllAmericanFive Radio I really appreciate your effort in putting this video with explanation. If you dont mind can you please provide the transistor type(model) used so that I can understand why those specific resistor and cap values used.

Wow amazing explanation. Thank you so much sir.

@dmottern1952 Thanks Doug, They are very interesting and useful too. Hope your class finds op and diff amps interesting. From the response I'm getting, a few RU-vidrs find them interesting also. Rick

Thanks falcon02012 Yes, it seems like magic too.

Very useful video about differential amplifiers. Thanks

Very good thx!

amazing explanation and demonstration. thank you

does this cancellation have to do with the phase of the 2 identical signals or am i overseeing sth very simple here..?

VERY CLEAR! THANK YOU!

@AllAmericanFiveRadio Hi Rick, Yes, I guess if you can keep count of things it’s got to make it easier, never thought of that. You say ‘... in one of my Heath Kit courses...’ I don’t know that they did course but I imagine that they must have been hugely informative; I’ll have to look into that a bit more it sounds very interesting. Ps did you see my ‘Soldering tip’ video? Kind Regards ... Andy

It is very interesting... i want to make like this... can i know how or what kind of signal did you put in..and how? Is it alright? Ad please teach me how to put an audio transformer?

thank you!

Fabulous!

Thank you for the video. I would like to understand how this circuit works. How the volts are placed on the elements. Is it possible to show how it works ?

Excellent explanation. Thank you very much.

Wow! That was very interesting , thank you.

Nice video, keep it up, thanks for sharing it:)

Great video thanks

Can this drive use to give the same sine wave to mosfet power inverter

Can't believe what I just saw. That's quite an amazing experience.

this is amazing!!!

Excellent demonstration.

Absolutely brilliantly explained and I was suffering to understand other videos here thank you sir

I have a question, why does the red led turns on with imput 1 and nor the green one, sorry if it is a dumb question

Hey is this having a suond? Or just only type of sound?

can we use C828 Transistors for this..?

Nice and good information!

This is a good demonstration

very good example,thank you

Out of curiosity, why are BJTs more commonly used in differential amplifiers instead of MOSFETs? At least I'm assuming they do, because I can find plenty of resources that cover only amplifiers with BJTs but hardly anything is out there on MOSFET differential amplifiers.

Okay, so if you get a chance to respond to this comment, tell me something: I've been studying how tube amps work, lately, and am now curious about solid state amps. Say you were to put a phase inverter somewhere in the signal path on one side of that differential amp, and hook the outputs up to an audio output transformer, as you did, then short the inputs together in order to feed them the same input signal. Would that give you a double-ended, "push-pull" amplifier that would essentially double the amplitude of the output, rather than having the two output signals cancel eachother out? This is how it works in tube amps that have two output tubes, but I don't know if the same technique is used in solid state amps.

Thanks skylon25

@palotitoe Thanks Pablo, The basics are super important. Everything depends on that knowledge. Regards Rick

The transistor type is NPN. The transistor are two 2N2222.

How interesting. People said that there was a mistake in the movie Atomic Blonde, that when Loraine Broughton recorded David Percival and Aleksander Bremovych separately, and pieced the recordings together to play for MI6 that the recording would have had different background sounds because of parts of the conversations being recorded in bars and clubs playing music and having lots of background noises. However, if she wore a wire recording the conversations, music, and background noises, and had a wire in her coat recording just the music and background noises, that got hung up out of earshot each time she went to the venues, she could have taken the music and background noises recording and passed it through a differential amplifier with the recording of the conversations etc, and removed the music and background noises leaving just the conversations. So technically it wasn't a mistake because her spliced recording had no background sounds, just the conversations, and Eric Gray and Chief C were so intrigued with her story, and thinking Percival was Satchel, that it didn't occur to them that it was strange that the recording had no background sounds. Cool.

Have link 2 output to 1 or individual 2 ? Thanks.

thanks !!!

very good tutorial

I am so appreciated

Killer explanation.

I would expect the signal cancelling if the outputs had inverse phase (out 2 180 deg from out 1, for instance) Since here the circuit is the same at both sides, I guess there is no phase inversion, so, why they cancel eachother? I would expect both signals summing, not cancelling

Thanks!

Best explanation

best explanation sir.

Hi, a 1982 mosfet amplifier has a differential input stage ( pair of PNP's) and a current mirror on top ( a single PNP transistor with 24V Zener in parallel) . The two differential PNP's has their collectors connected to the base of two NPN drivers, where the first NPN driver has it's collector connected to the base on the upper PNP driver. ( 4 mosfet as output stage).....You see, the mosfet schematic had an error that showed the differential input transistors as NPN's (they should have been PNP's)......Could you show when to use PNP's and NPN's......and ...why not not use NPN's as input transistors? thanks for all your fine and useful videos...best regards Kim

super sir very nice explanation

Thank you.