BJT Amplifier Output Votlage Swing

On Wed, 03 Oct 2012 15:53:32 -0700, I wrote:

>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.

By the way, take note that Shahriar designed in 1.6+1.8 or
3.4V for the quiescent collector voltage. Compare that with
my "3.5V" above. Note the similarity? Yet due to different
explanations. Mine considerations point out the "whys" of
getting there, so that you can do your own thinking in
different cases and objectives. His do not. That's the only
complaint I'd have the video. But he was keeping this very
simple (and I think he'd already considered other things
before coming up with his "1/3rd for each" rule for talking
purposes.)

Jon

On Oct 3, 7:15=A0pm, Jon Kirwan <j...@infinitefactors.org> wrote:
> On Wed, 03 Oct 2012 15:53:32 -0700, I wrote:
> >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.
>
> By the way, take note that Shahriar designed in 1.6+1.8 or
> 3.4V for the quiescent collector voltage. Compare that with
> my "3.5V" above. Note the similarity? Yet due to different
> explanations. Mine considerations point out the "whys" of
> getting there, so that you can do your own thinking in
> different cases and objectives. His do not. That's the only
> complaint I'd have the video. But he was keeping this very
> simple (and I think he'd already considered other things
> before coming up with his "1/3rd for each" rule for talking
> purposes.)
>
> Jon
Big Grin!  Thanks Jon. You're much better at discrete design than I
am.
(Which isn't saying much.)
After spouting off I was reading a bit in AoE.
(I use to have the lab manual, but it went to a new home
a few years ago, hopefully guiding the next generation.)

I totally agree about not wanting to have the gain based on the (25mV)
thermal voltage.  But to do that you can't bypass the entire emitter
resistor.  (Maybe that is the next lesson?)   I do like having big
supply voltages, then wasting a volt here or there doesn't cost you
much.

George H.

On Wed, 3 Oct 2012 17:15:16 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Oct 3, 7:15=A0pm, Jon Kirwan <j...@infinitefactors.org> wrote:
>> On Wed, 03 Oct 2012 15:53:32 -0700, I wrote:
>> >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.
>>
>> By the way, take note that Shahriar designed in 1.6+1.8 or
>> 3.4V for the quiescent collector voltage. Compare that with
>> my "3.5V" above. Note the similarity? Yet due to different
>> explanations. Mine considerations point out the "whys" of
>> getting there, so that you can do your own thinking in
>> different cases and objectives. His do not. That's the only
>> complaint I'd have the video. But he was keeping this very
>> simple (and I think he'd already considered other things
>> before coming up with his "1/3rd for each" rule for talking
>> purposes.)
>>
>> Jon
>Big Grin!  Thanks Jon. You're much better at discrete design than I
>am.
>(Which isn't saying much.)
>After spouting off I was reading a bit in AoE.
>(I use to have the lab manual, but it went to a new home
>a few years ago, hopefully guiding the next generation.)
>
>I totally agree about not wanting to have the gain based on the (25mV)
>thermal voltage.  But to do that you can't bypass the entire emitter
>resistor.  (Maybe that is the next lesson?)   I do like having big
>supply voltages, then wasting a volt here or there doesn't cost you
>much.

The capacitor (soon) acquires a voltage equal to the DC
operating point across Re. All it does is short out the AC
part. But the DC operating point (and therefore Iq) at the
tip of the emitter holds solid. (Shahriar, shortly after
about 6 minutes into the video, shows you the Ie equation he
easily derived, but without showing you how.)

The point of Re is to maintain the biasing point so that the
expected useful collector peak to peak remains valid. And it
does. The capacitor doesn't impact that, since it is almost
entirely determined at DC (the AC does wiggle things very
slightly.) So the biasing point, and Iq, remain stable over
temperature.

Temperature impacts kT/q, which impacts A due to the fact
that the value of "40" in the AC gain of 40*Iq*Rc is just
q/kT and is inversely dependent on T. But exact gain usually
isn't the goal in a single BJY stage -- just a rough design
guide for validating assumptions (The room allowed for
collector swing, for example.) If exact gain overall is
needed there are other places to add adjustments (and a LOT
more work on considering sources of time and temp drifts and
their impacts -- or else you stick the circuit into a more
temp stable place like the body cavity of a living human.)

You can improve on the temperature dependence of AC gain,
while keeping Re's DC operating point, by inserting another
resistor (Rx) in series with the emitter bypass capacitor.
Then the gain stops being 40*Iq*Rc and looks a lot more like
Rc/Rx.

And you can then improve the "stiffness" of the base (greatly
reduce its loading on the prior stage or sensor source) by
using a small, simple bootstrap capacitor from the emitter
itself backwards to the bias pair node (needs another
resistor from the bias pair node to the base, too.)

Jon