I'm getting tired of saying "input resistor", "input transistor", etc.
Component designators were invented for a good reason.  I'm looking at your
diagram, and am going to assign names to the left half.  R1 is the resistor
between the input and the base of the "input transistor", which is Q1.  R2
is between the emitter of Q1 and the base of Q2.  Q3 is the high side switch
on the left side.

Bob Blick wrote:
> And it's very predictable! When Vlogic <= Vmotor, there is almost no
> drop in the input resistor.

When the gain of Q1 is large.  Let's assume that for now.

> When Vlogic > Vmotor, voltage drop across
> the input resistor is close to Vlogic - Vmotor.

As I tried to point out before, this circuit stops working (at least as a
usable H bridge) when Vlogic gets near Vmotor and above.  The reason is that
current can no longer flow out of the base of Q3, which means the top side
drivers stay off when Vlogic >= Vmotor.  Let's therefore only analize the
circuit when Vlogic is at least a little less than Vmotor.  Vlogic must be
no higher than Vmotor minus the B-E drop of Q3 minus the saturated C-E drop
of Q1.

Let's say the gain of Q1 is infinite.  This means the current thru the base
of Q2 and Q3 are the same.  This current is Vlogic minus B-E drop of Q1
minus B-E drop of Q2 divided by R2.  Let's say that B-E drops are 700mV:

  Iq2b = Iq3b = (Vlogic - 1.4) / R2

Since both Vlogic and R2 can be considered predictable, the Q2/Q3 base
currents are predictable.  However, this is where it ends.  The maximum
motor current then becomes this base current times the lesser of the gain or
Q2 or Q3.  This is my point.  The load current limit is directly
proportional to a transistor gain.  Since the maximum transistor gain is
usually not specified, there is no upper bound on the load current.  In
other words, there is no current limiting.  Even if the maximum gain is
specified, there will be a large variation between min and max transistor
gain.  Ratios of 5-10 are easily possible.  This is not what I would call
"predictable" in the current context.


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