Say can I ask what is perhaps a silly question?
(I won't wait for permission :^)
Now, I don't do much design with transistors, (since opamps are so
much easier),
and noisier unless you pay dearly
but I thought that you should choose the collector
voltage to be ~1/2 the supply voltage for maximum swing of the
output. This comes from AoE... which IIRC also states that the
maximum gain from a common E amp is 1/2 the supply voltage divided by
the thermal voltage (25mV). (I know this ignores the CE saturation
voltage.)
That's another "rule of thumb," but it is not gospel. I've
gradually (that means I'm mentally slow) learned that there
are lots of considerations. Anyway, if you "work the
equations fully" and take into account all the important
things too then you will find that the 1/2 supply rule isn't
reality, either. It's decent, that's all. Centering the
maximum non-saturated collector swing involves more things
than that, though. And it's not always the goal, besides.
Since you bring of AofE, take a look at the student manual
for it. They actually walk you though a CE design starting on
page 115. You will see some competing considerations there
and no discussion at all about setting Iq. Which may also be
important. No rule of thumb is gospel. They just help a
little, is all. And it works a lot better when you have a
large magnitude supply rail pair than when you don't. This
guy was working with 5V.
Some, but not all, considerations: The DC operating point
should, for temperature stability, have the emitter resistor
with as much voltage drop as you can afford to have. This is
because of kT/q in the BJT emitter (it's the cause of little
re which depends on Ie and thereby also on Iq). That voltage
(around 26mV at room temp) is HIGHLY temperature dependent.
So dwarfing it with a drop across Re helps make it
irrelevant. AofE's student manual recommends at least 1V
there to make the T-dependent 26mV not so important. The
author of that video didn't say any of this, but the 1.6V he
assigned is not only reasonable it's also a good idea for
temperature stability. This, of course, "steals away" some of
the range then available between Rc and Vce. Also, you should
allow for a continuously reverse biased BC junction, if
possible, as well. So that sets up a minimum Vce of 0.8V-1.0V
that you should NOT put into your calculations of the "center
point" for the collector. Again this steals away some of what
you plug into your calculations. If you have a 15V power
supply or more you won't care. Just center Vc and be done
with it. But if you are stuck designing something for a pair
of 1.5V batteries, then you start thinking more about the
details and struggling just a little differently when
balancing your priorities. There is no bright line rule
anywhere. Your brain cannot ever be fully disengaged and
there is always more to learn, too, I think. No matter what
you think you know, there is something else out there that
you haven't yet experienced and your "rules" will then get
you in trouble if your brain isn't turned fully on.
So assume I have 5V and use AofE's rule of 1V for Re. And
then apply my own 1V for Vce-min. This leaves 3V total (5V
minus 2V) for collector "swing." So I take the 1V for Re, add
1V for Vce-min, then add 1.5V for half of the 3V swing and
wind up with 3.5V for quiescent Vc, right? That's not 2.5V.
It's 3.5V. But it maximizes the swing under my rules of not
allowing BC to forward bias and allowing AofE's rule for
temperature stability.
I don't use the 1/2 V-supply rule unless I'm teaching someone
who knows nothing about BJTs and wants to just get started.
So it seems that Shahriar could have choosen his operating point a bit
better and gotten closer to his gain of 100 in one stage. Say if you
want, even more gain can you run the collector even lower and give up
the maximum voltage swing?
Yes, he doesn't say it but he makes that point indirectly
when he talks about the max gain of A=-64 depending upon
40*Iq*Rc, or 40 times the quiescent voltage drop of Rc.
But as I've said above, you have other considerations as well
that compete with just throwing more voltage drop at Rc.
Jon