Newbie question about bjt biasing in TB MkII q1 Vs q2

[Mcentee2]

Hi,

Please excuse the rather basic question, but it's been puzzling me and I know I am missing something in my beginners/basic understanding of bjt transistor biasing.

In a MkII circuit q1 has a (-)9v supply, through a 10k resistor and manages to bias at about (-)8-9v nearly full rail.

In comparison

The q2 (or the q1 of a fuzz face) has the same supply and a 33k resistor, but seems to bias at a really low (-)0.3-0.5v.

Both emitters are straight to 0v "ground".

Is there something I am missing as to why q2c (or fuzz face q1)  is *so* much lower than q1c ?

Does the difference between the 10k and the 33k really account for that huge proportional drop?

I am assuming it's to do with the q2b/q3e feedback path and probably the quiescent DC currents flowing through each q -  but my brain hasn't yet got it!

I understand a lot of the theory in general - bjt operation, leakage affects etc  but some the basics are missing in my head somewhere

[antonis]

1. Q1 in MKII is solely biased by leakage current..
2. Q2    //      is biased via 100k feedback resistor from Q3 Emitter..
3. Same stands for Q1 in Fuzz Face..

Cases 2 & 3 call for a (more or less) "healthy" V_BE, in the mean of working BJT, hence decent Collector current resulting into significant voltage drop across Collector resistor..

[Mcentee2]

Thanks!

So it's basically the be current in the TB q2 (FF q1) is higher leaving the bjt more "on", this lowers c voltage, so is much nearer saturation?

In the TB q1 there is just the leakage current which just about turns the bjt "on".

Hope that's fairly right now!


Thanks again

[antonis]

the be current

"Voltage", actually..

BJTs are voltage controlled devices (like FETs), despite what is stated in numerous textbooks..
(Base current is an "annoynig" imperfection rather than the basis of operation..)

[Mcentee2]

Yes, that is something I have read a lot and have parked it as "will try and understand later"!

Characteristics curves for bjt do seem to indicate be voltage control of ce current.

[FSFX]

the be current

"Voltage", actually..

BJTs are voltage controlled devices (like FETs), despite what is stated in numerous textbooks..
(Base current is an "annoynig" imperfection rather than the basis of operation..)

Sorry, with respect, that is completely wrong. I thought that you claimed to be an educated EE and, as such, would actually have an understanding of how the physics of semiconductors actually work.
If what you say was correct then devices like the LM3900 'Norton' amplifier and the current mirrors in many op amps would not work. 

[antonis]

I thought that you claimed to be an educated EE

I never did so 'cause I never have received even an elementary EE course..

P.S.
I shouldn't wish to dig further to "prove" who is right and who is wrong for a matter that can be satisfactorily interpreted via both ways..

[FSFX]

I thought that you claimed to be an educated EE

I never did so 'cause I never have received even an elementary EE course..

P.S.
I shouldn't wish to dig further to "prove" who is right and who is wrong for a matter that can be satisfactorily interpreted via both ways..

Thank you for clarifying that.

[Phend]

As Woodrow Wilson said,
"I use not only all the brains I have, but all I can borrow. "

[PRR]

> <i>an understanding of how the physics of semiconductors actually work.</i>

Cherry and Hooper are/were both EEs. They propose the Charge Control model for all amplifying devices.
https://worldradiohistory.com/BOOKSHELF-ARH/Technology/Amplifying-Devices-andLow-Pass-Amplifier%20Design-Cherry-Hooper-1968-RR.pdf

[Phend]

This isn't very long, but interesting, on the application to a simple circuit with a simple resistor and inductor. Watch the blackboard and not the instructor which is where many of us can end up missing the boat. PS, 2.71828 is an interesting number, wiki it (e). Anyway.....you can easily expand this stuff to a simple fuzz circuit.