Semiconductor Devices: Feedback Biasing

In this video we explore a few other forms of DC bias for the BJT. These circuits, while not as stable as the emitter and voltage divider biases, utilize fewer components.
References: Semiconductor Devices: Theory and Application; Chapter 5, section 5.
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[@aerofart]

This (these) videos are tuned perfectly for the student who has a solid grasp of the fundamentals of DC circuit analysis and BJT biasing. Such a pleasure to see how Professor Fiore describes the big picture as well as works out the small details. Thank you, Jim!

[@PauloConstantino167]

Just wanted to say your videos are bar none, the best I have seen on youtube, and your clarity is commendable.

[@ElectronicswithProfessorFiore]

Thanks, and tell your friends!

[@PauloConstantino167]

@@ElectronicswithProfessorFiore if I had friends I would, but I have none. I am a lone genius and I stay alone studying the electronics of UFOs. I need your help with a study, are you available?

[@ElectronicswithProfessorFiore]

@@PauloConstantino167Sorry, but I made it a rule that I am only available for tutoring for my in-person students at the college. Otherwise, I would never get any sleep!

[@PauloConstantino167]

@@ElectronicswithProfessorFiore Hi there professor Fiore. You will have to obey my orders here. I am sorry but you have no choice on this matter. You will obey and that's it

[@lachydunne1512]

@@PauloConstantino167 go for a walk in the park or something more often my friend

[@MrArmandoX]

Hi Mr. Fiore In your first feedback circuit with the resistor RB connected from Base to the Collector. the maximum current cannot be VCC/RC since it assume that Vce=0V. The collector voltage power up the base of the BJT and it need to provide at least 0.7V to the base to keep the circuit in the saturation mode; hence, the saturation current should be (Vcc-0.7V)/RC

[@ElectronicswithProfessorFiore]

Yes, the maximum achievable collector current is (Vcc-0.7)/Rc. This is apparent from the Ic equation (making beta impossibly large and/or making Rb insanely small). To be clear, Vcc/Rc is the Y intercept on the graph. The point being that Ic can never quite get there. That is, Ic approaches that point but never "arrives", hence the circuit cannot go fully into saturation.

[@MrArmandoX]

Thank you Mr. Prof Fiore for answering my question

[@simonyoungglostog]

If we create a voltage divider circuit for the base, connected to the emitter and collector with resistors on those leads (So we have feedback in the emitter and from the collector, I hope that made sense), how would that compare to a straight voltage divider? So, we have a voltage divider but rather than VCC and Ground, we connect the appropriate resistor values to the E and C of the transistor. I might have to hit the simulator. Great work Doc.

[@ElectronicswithProfessorFiore]

You can combine collector and emitter feedback into a single circuit, and it can be quite stable, but with one more resistor you've got a voltage divider which likely will be better (and it only costs you one more resistor). In many cases, you're better off going with a split supply as you have more flexibility. Bias stability isn't the only thing we need to address.

[@simonyoungglostog]

@@ElectronicswithProfessorFiore I just ran a simulation on a few variations and it looks like the voltage divider bias has less distortion than the collector bias. Although I'm new to distortion analysis (I used the same emitter degeneration resistor value).

[@ElectronicswithProfessorFiore]

@@simonyoungglostog For a fair THD comparison, make sure the output signals are the same amplitude. Also, set the gain the same.

[@simonyoungglostog]

@@ElectronicswithProfessorFiore I was struggling to get the variables to similar values. I've just had another go (spooky) and can see that when the output voltage is the same but the gain is less, the distortion is less. Which you indicated. I can't tell if the distortion is better when I link 'R1" of the voltage divider to the collector instead of Vcc (I change the values of R1 and R2 in this configuration to try and match the collector current). It's all fun experimentation and learning. I shall have to have a rummage through my 'Doug Self' small signal amplifier book, he did lots of work on distortion.

[@ElectronicswithProfessorFiore]

@@simonyoungglostog Doug Self's work is well known among amplifier enthusiasts.

[@rudygomez6996]

cool