Semiconductor Devices: Class B Power Amplifiers

[@yousifali1737]

This is the first video i see that makes sense and obviously made by someone who really understands the circuit

[@davidluther3955]

EXCELLENT PRESENTATION!I WAS JUST WONDERING HOW DO YOU MATCH TRANSISITORS SO THAT THIS WORKS PROPERLY?

[@ElectronicswithProfessorFiore]

Manufacturer's make complimentary pairs, that is, an NPN and a PNP that have similar operating characteristics (similar betas, Pd, Vce and Ic max, etc.). For small signal transistors, the 2n3904 and 2n3906 are a classic example.

[@rolanportento9704]

greetings from the philippines! this vid helped a lot

[@simonyoungglostog]

Just as good this time around. What would a typical audio amplifier (power amp) voltage compliance be, please? I can see the V2/R squaring up and a couple of extra volts can make a huge power difference.

[@ElectronicswithProfessorFiore]

Easy to figure that one out, backwards. Let's say you have an amp rated for 100 watts into 8 ohms. P=V^2/R, so V = sqrt(P*R). That's sqrt(100 * 8), or about 28 V. That's RMS. Thus, you'd need about 40 Vpeak, meaning supplies of a bit beyond +/- 40 VDC.

[@simonyoungglostog]

@@ElectronicswithProfessorFiore I feel a little foolish. I should have seen that working out. Thank you.

[@diago2805]

Beautifully explained

[@bhu1334]

You explain this beautifully

[@ElectronicswithProfessorFiore]

You're welcome. Plenty more where this came from!

[@phile7420]

Do you have a tutorial for designing a simple amplifier with given input, gain, output power, load, etc... Like how to choose appropriate resistors, reading transistors datasheet, capacitor...

[@ElectronicswithProfessorFiore]

There's an entire series of videos regarding amplifiers. I don't know your starting point but I suggest you go to the beginning of the Semiconductor Devices playlist and work your way through. The videos are listed in the same order as the topics are presented in the book.

[@phile7420]

@@ElectronicswithProfessorFiore I'm currently taking all the videos from your semiconductor devices playlist, really good explanation of the devices, with given exercises and simulations, which come with predefined resistors and capacitors value. But I don't really know why you can choose those value. If you are talking about the op amp series, my university course assignment is to design amplifier with no op amps, kind of bjt only.

[@ElectronicswithProfessorFiore]

@@phile7420 Ah, so you're looking for a video to complete your assignment, then? I suggest that, instead, you look at those circuits and figure out what would happen if you changed some of the values. What happens to the bias? How does that affect the gain or the output compliance? etc. You'll learn a lot more that way. ;-)

[@simonyoungglostog]

Thanks for another great explanation. With respect to the AC signal between the diodes: Is it the case that the small current flowing into the diodes node reduces the current flowing down through the top diode but the diode is always on so the current increase/reduction allows the signal to pass through? This hasn't quite clicked although it makes sense. Thanks in advance. It's the thought that something can go backwards up a one-way street where it's really just slowing down and speeding up but always travelling in the right direction. (That was me thinking out loud, is that right?).

[@ElectronicswithProfessorFiore]

The diodes confuse a lot of people. The thing to remember is that, unlike a rectifier, the diodes are always on (due to the DC bias). Thus, for AC the diodes can be modeled as their AC (AKA dynamic) resistance. You calculate this the same way as r'e. Thus, if the bias current is 10 mA, the diode resistance is less than three ohms. This can be the source of a small amount of distortion, and in larger amplifiers (typically using a Vbe multiplier instead of a string of diodes), this will be bypassed with capacitors, making the AC resistance tiny and reducing any THD.

[@simonyoungglostog]

@@ElectronicswithProfessorFiore Excellent, thank you. It's coming back to me, slowly but surely. You're a great help.

[@AL-kn4yx]

I have a question. When you say that the collector current Ic is in saturation, does that mean that the transistor itself is in saturation region? That kind of confuses me , because every book I've read said that for an amplifier to work properly the transistor must always be in the active region of operation.

[@ElectronicswithProfessorFiore]

Right, the transistor must be in the active region to avoid distortion in a linear amplifier. That means "not in saturation". When the collector current hits saturation, you get clipping which produces gross distortion.

[@MrArmandoX]

Hi Mr. Professor Fiore Are there any possibility of having both transistor (NPN & PNP) conducting at the same time? Thanks

[@ElectronicswithProfessorFiore]

That's called shoot-through and can be an issue in class D amplifiers which superficially look the same as class B. It's generally not an issue in class B unless there is some manner of severe fault.

[@sheheryarkhan6275]

Shouldn't the compliance voltage 7.5Vp ?

[@ElectronicswithProfessorFiore]

This has a dual polarity supply (+ and -15 V). That's equivalent to a single 30 volt supply. Ideally, you get the entire supply voltage for the peak-to-peak output swing, and obviously half of that for peak. Thus the compliance is 15 volts peak, 30 volts peak-to-peak, and a little over 10 volts RMS, ideally.

[@sheheryarkhan6275]

@@ElectronicswithProfessorFiore Thanks for clarifying in details

[@rockpadstudios]

10:58

[@mohamednofal5256]

thank you

[@mburdett260]

You gotta be a ham

[@ElectronicswithProfessorFiore]

Oink?

[@roncaruso931]

It is great to know the theory, but todays electonics are mostly ICs. IC goes bad, replace it.

[@ElectronicswithProfessorFiore]

Sure, but two things to consider. First, there are a lot of older discrete designs out there that might need to be repaired at some point, and second, whether or not a design uses discrete or integrated circuitry, someone (or some group) has to do the initial design and testing. If people only know how to replace ICs and no one knows the theory, there can be no progress. This goes back to the argument of the value of education versus training.

[@roncaruso931]

@ElectronicswithProfessorFiore I have no problem with learning theory. I did. But with all new designs of electronic devices having ICs, CPUs, SOC, and IC audio amps, and perhaps more, discrete designated are becoming a thing of the past. True theory about say repairing an old radio would be helpful.

[@ElectronicswithProfessorFiore]

@@roncaruso931 And if that old radio uses a discrete class B output, what then? ;-) Besides, like I said, who is going to design the ICs that (internally) use a class B stage? Further, that new IC might use a class D output, but again, someone had to design that, even if all the repair tech does is replace said IC. Taking the opposite approach is a little like saying that no one needs to learn how to write code anymore because you can find lots of code already out there. All you have to do is patch it together. (sorry, no.) For that matter, why does anyone need to learn a musical instrument when they can just sample existing music and then edit and rearrange it? I think there is value in starting with the fundamentals in order to build a strong foundation, and working forward from there. Too many people try to skip that in order to get to "the latest and greatest", but the results are usually less than exemplary. If it's not obvious yet, this channel exists to help support people who are trying to learn the material from the bottom up. It is NOT designed for people looking for a video with just enough info on how to fix "X" with minimal knowledge. I think there are a sufficient number of those channels kicking around. And if that's what someone is looking for, I say all the power to them and have at it!

[@roncaruso931]

@ElectronicswithProfessorFiore I agree with you about old radios. But eventually, even old radios will be discarded. Today's young generation are not interested in saving these beautiful old analog radios.