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Your circuit is an example of collector feedback. Collector feedback does
not work well with large signal swings and it lowers the input impedance. A
lower input impedance means that the drivers will also need a lower output
impedance. The bottom line is that you will not see this bias method used in
a power output stages. Another option is to use emitter feedback but for the
emitter resistors to be effective, they will drop alot of signal output and
waste power, which explains why those resistors are usually very low values,
They have very little effect unless the emitter current is large. At the DC
bias current level, they don't do a thing.
Local feedback (emitter feedback or collector feedback) both create more
problems then they solve for stabilizing the power output stage bias point.
To keep the output stage bias point stabilized, the use of overall feedback
is the standard practise. A simple typical amplifier might have a
differential input stage, followerd by a voltage amplifier, followed by the
power output stage. The output from the power stage is fed back to the
differential input stage. High open loop gain with large feedback is the key
to better stabilization of the operating points. Fix it with feedback is a
term to remember.
This point is made, again and again, so it must be true! And
if it weren't true, why else would opamps have been such a
successful building block?? So I completely buy the idea.
I am still trying to study each part, though. At some point,
I will raise my head a bit above that level and take a larger
look. But I'm not yet prepared for it, as the pieces
themselves are still too fuzzily understood. I want to
quantify those, in detail, before expanding my view. In
doing so, I hope to have a somewhat better understanding of
opamps, themselves, too. Not just from a large scale view,
but also in understanding problems within them and how to
choose among various approaches when struggling with a
specific application in mind.
I take your point. But it doesn't change, at all, my
interest in seeing what can be reasonably done with at teh
Vbe multiplier level to accomodate variations in current.
That remains interesting in and of its own right.
The typical bias chain using diodes can be made with resistors as well, but
diodes have the advantage of dropping the bias voltage while having a lower
impedance to the signal.
That makes sense.
Sometimes you will see those bypassed with a large
cap if the impedance causes to much signal loss.
I had thought of that, as well. Though, of course, I hadn't
put values to it.
Diodes can also offer temperature compensation.
Their Eg and N would seem to suggest some difficulties with
curve matching, but I agree broadly.
In any case, an output stage will have way more
current flowing in that bias chain than is actually needed as base bias
current.
This seems to argue with something I read John L. saying, but
I didn't accept it (or reject it), yet. I will need to get
there in due course. But I'm still taking pieces one at a
time.
The voltage drops developed in the bias chain will not be greatly
affected by changes in the base emitter junction because the base bias
current is small compared to the current in the bias chain.
I read this sentence a few times to try and make sure I
followed it well. If I do, and I may not, I think I
addressed this when I tried to calculate the R_ac figure.
The numbers I come up with for a 5mA "bias chain" current
with 1mA in the base bias current and 4mA in the collector,
come out as around 15 Ohms, or so. If I'm right about that,
it seems almost certain that there is _some_ response to even
modest variations in current through it. A 500uA change
yields a 7.5mV change. When I LTspice it, I get a simulation
that matches what I calculate, too.
And remember,
that " Fix it with feedback " applies here too.
Yes, the mantra is slowly deepening within me.
So variations in the power
supply have a very reduced effect on the bias point. The feedback signal is
a voltage, and enough feedback will compensate to keep the output voltage
offset at zero. It will not compensate for for excessve collector currents
or power dissaption if the offset voltage remains low.
Good point for me to remember!! Thanks.
That is why temperature compensation is used too.
I begin to see, better. Thanks, again.
In the early years of transistors, it was common to see transistor stages
using many of the techniques used with vacuum tubes. Dc coupled amplifiers
were rare, because any bias shift was amplified in further stages. Feedback
was applied locally, and overall feedback had no effect on the DC operating
points.
I seem to recall that vacuum tube amplifiers even let the
consumer modify the global NFB. But, as you say, since it
wasn't so critical to the design that was probably why it was
allowed in the first place. With BJTs, it seems now to me
that global NFB is _so_ important that such things cannot be
left as "tweeks" by some consumer playing with a knob!
The trend now is to stabilize everything with feedback. It works,
and it works well. Unless you are a purist and have some religious reason to
avoid this technique, there is no sense in reinventing the wheel.
It's not a religious reason, unless _learning_ is a religion,
I suppose. I don't mind being told that "one day when you
are ready, you will use global NFB to take care of this." I
can gather and accept it, of course. But I also cannot
believe an amplifier can be designed with bags of random
bolts tossed together and "fixed with global NFB" in the end.
There are parts in there and they need to perform some
intended function to some reasonable approximation. And I am
still working on understanding each piece as well as some
thoughts about various approaches at that level to improve
the ideas.
For example, it's important to understand not just vaguely,
but quantitatively on various scores, how a diff-amp behaves
and why I may want to have a current mirror on the tails. I
don't want to just hear "put a current mirror there" and
learn nothing then about why. Later on, when I'm looking
globally at an amplifier, I can look backwards and say, "Hmm.
That Wilson mirror is great, but I really don't need it. The
bog standard 2-BJT mirror is fine enough." But I want to say
that from _understanding_ the details, not from others merely
assuring me about it.
See the difference?
Meanwhile, I'm still interested in seeing if my quantitative
analysis was correct (or wrong) and if there are some other
topologies for it, other than the two I mentioned, that may
be interesting to look at.
Thanks,
Jon
