Semiconductor Devices: Class B Enhancements

[@thomaslockwood2950]

Love these videos

[@chrisw.3077]

I am working on a Hadley 622C currently and came across this video. This is a very early solid state amplifier featuring a quasi-complementary output stage in a bridged configuration, almost exactly as you described at the end of this video and now I understand it much better, thank you.

[@ElectronicswithProfessorFiore]

Cool! That's reaching back a ways.

[@davidluther3955]

EXCELLENT TUTORIAL!HOW DO YOU GET THE TRANSISTORS TO BE MATCHED IF THEIR BETAS AND POTENTIAL HILL BARRIERS VERY?

[@ElectronicswithProfessorFiore]

Manufacturers sell complementary pairs. These are NPN/PNP models that have similar betas, max voltage, current, power, etc. A classic small signal pair is the 2n3905 and 2n3906. The matching doesn't have to be perfect. We try to design circuits that can tolerate device-to-device variation.

[@rudygomez6996]

Do you have video with example of these enhancements? I understand better when i see the actual numbers at each step of the process. Thank you

[@ElectronicswithProfessorFiore]

In that case, download my free textbook, Semiconductor Devices (link in the description). You will find more info on this topic.

[@rudygomez6996]

@@ElectronicswithProfessorFiorethank you!

[@rudygomez6996]

6:04 sorry for so many questions. Why is it that second transistor in darlington need to be bigger power tansistor? And by bigger power transistor do you mean it has a larger beta?

[@ElectronicswithProfessorFiore]

The 2nd transistor of the Darlington handles a much larger current than the first one, thus its power dissipation is much greater. By bigger I mean larger Pd.

[@yousifali1737]

nice video can I ask what reference book do you use?

[@ElectronicswithProfessorFiore]

Mine. It's free for the download. It's called "Semiconductor Devices: Theory & Application". Check out my websites for PDF and ODT downloads, or for a very low cost print version, head to Amazon.

[@rudygomez6996]

6:57 “the drive stage doesnt have to be as powerful”. Is it because without darlington configuration, the direct coupled drive transistor needs to have a larger beta to increase input impedance?

[@ElectronicswithProfessorFiore]

In a nutshell, yes.

[@rudygomez6996]

@@ElectronicswithProfessorFiore thank you

[@davidluther3955]

WOULD YOU DO A PRESENTATION ON A CLASS C AMPLIFIER?

[@ElectronicswithProfessorFiore]

I'll look into it. Class C is rather specialized, though. Primarily used for relatively narrow band high frequency communication (e.g., radio broadcast). Not much use in the audio world.

[@lesleypaulvj_TVPM]

Please do the other transistor amplifier classes also. Thanks

[@ElectronicswithProfessorFiore]

Already did. Check the Semiconductor Devices playlist for class A and class D.

[@lesleypaulvj_TVPM]

@@ElectronicswithProfessorFiore Thank you. I was looking for class AB, which is suited for distortionless audio amplifier and class C for radio power amplification for ham radio. Your teaching method is good to understand.

[@ElectronicswithProfessorFiore]

@@lesleypaulvj_TVPM Class AB is a minor variation on class B. In fact, when I talk about reducing notch/crossover distortion by producing a slight forward bias, that's class AB. Class AB is not "distortionless" (no amplifier is) but it does offer a dramatic improvement. Class C is rather specialized, primarily used for narrow band radio transmission as you mentioned. The basic idea is to bias with a conduction angle less than 180 degrees. You can visualize this as just getting the tips of the waveform. The remainder of the wave is filled out using a resonant circuit which is tuned to the broadcast carrier frequency. The advantage is increased efficiency but it only works for relatively narrow band frequency ranges (like a radio broadcast channel). Not much use for something like audio.

[@lesleypaulvj_TVPM]

@@ElectronicswithProfessorFiore Ok, thank you very much. That did explain in brief what those classes are.

[@cooliocrib4409]

What are the frequency limitations of this circuit? How can you increase frequency?

[@ElectronicswithProfessorFiore]

Could you be more specific? Are you trying to change the upper or lower cutoff frequency? In general, I would refer you to the frequency analysis of small signal class A amplifiers. The issues are similar.

[@cooliocrib4409]

Thanks for the reply. So I want to use this topology to design a poweramplifier with a frequency range from 400KHz to 2.5MHz. I want to use +-15v psu rail and I want the output to be +-12v. What would limit the upper frequency limit?

[@ElectronicswithProfessorFiore]

That's a bit more involved than I can answer in a reply. I suggest that you start with chapter 6 of the Semiconductor Devices text for the general ideas and then look at exercise 21 in the accompanying lab manual which directly addresses some of these issues. In short, you're going to have reduce the input and output sections into equivalent lead and lag networks. There will be interactions between the resistors, coupling capacitors and the junction capacitances of the BJT.

[@rudygomez6996]

2:16 could you please exlpain this part in a different way, i dont understand 😭 🙏

[@ElectronicswithProfessorFiore]

It's just a direct coupled stage. A simpler version would have a collector R above the first transistor, and this would be capacitively coupled to the class B stage. If you set the collector current of the first stage equal to the bias current of the second stage, you don't need all of that extra stuff because you use the same current path for both.

[@rudygomez6996]

@@ElectronicswithProfessorFiore I will ponder this thank you