J
Jon Kirwan
Okay. So I've been considering current mirror design and, in
particular, the idea of applying resistors on either or both
of the emitters. (I'm ignoring the Wilson mirror, for now. So
just the basic 2-bjt current mirror.) For now, I'm just
trying to see if I can simulate in mathematics form what a
spice program would achieve given a model and a situation. I
think I've got it nailed down but I'd like to expose some of
my thinking for a reality check.
One of the first things that showed it's ugly head is the
ohmic Re' of a bjt. The ohmic Rb' was something I'd already
thought about before (seemed more obvious to me at the time.)
But what I hadn't recognized right away was that Re' is
multipled by (beta+1) when reflected to the base and even
small values of Re' then look a lot bigger than Rb' and will
dominate the calcs. (This is BEFORE adding in an actual Re as
a separate circuit element.)
This didn't become an issue when using a simple mirror,
because both bjts had about the same currents in them. So the
reflected values created the same voltage drops. But it IS an
issue when I decide to use an external Re in one emitter.
Then, there are significantly different currents and I have
to take this into account to gain any useful prediction.
One interesting note is that some of the bjt models I have
don't even specify Re' and LTspice defaults it to 0 if it
isn't given. For example, the 2N4403 in the builtin LTspice
models doesn't supply Re'. But it does supply Rb'. However,
the 2N2907 model does supply both. Hmm. Can't trust anything.
So for example, with Rb'=10 (commonly slapped onto models)
and Re'=.2 (say, the 2N2907 just mentioned), and beta=250 for
that device, I get a reflected value of about 50 ohms, which
is 5 times larger than Rb'. So it can't be ignored in some
situations (as where I may use one emitter resistor.)
The other thing is that when operating a mirror with
different currents, heating will be very different in the
two. Even on a common substrate, let alone with discrete
devices. Most especially if the output bjt is supplying a lot
more current and with a collector that may be causing a large
Vce drop, besides. And at -2mv/C for Vbe, and 10X Ic for
every 60mV change, this makes it almost impossible to
consider if there isn't a closed loop where the actual Ic is
monitored and used to control the current via feedback. Which
reduces its attractiveness.
I find I'm able to manually (with a calculator) replicate
LTspice results to about 3-5 parts per thousand over a very
wide range of circuits and bjt devices. It's iterative, in
the sense that I have to repeatedly do an estimate and plug
that back in. (I couldn't develop a closed equation.) So I at
least think I've got that part nailed down.
But is the above thinking about right? Or is there something
else I've missed about this topology that I failed to mention
above? Seems that a Wilson mirror helps a little bit with the
different heating issue (by returning the Vce on the output
side bjt to something much smaller) but at the expense of
more headroom required and another device.
Anyway, interested in thoughts, points, etc. I can see why
these are much more useful in ICs than discrete. Most
especially if there is any differential currents involved.
My thoughts here are about driving an LED matrix. I an
considering the idea of using an external mirror (but with
gain) so that the 5916 can drive it with smaller controlled
currents. The problem seems to be that I get nothing from the
exercise, as different heating will unravel the whole idea
and force me to figure out a feedback scheme, which then adds
further discrete device complexity and adds still more
headroom I don't want to waste.
Jon
particular, the idea of applying resistors on either or both
of the emitters. (I'm ignoring the Wilson mirror, for now. So
just the basic 2-bjt current mirror.) For now, I'm just
trying to see if I can simulate in mathematics form what a
spice program would achieve given a model and a situation. I
think I've got it nailed down but I'd like to expose some of
my thinking for a reality check.
One of the first things that showed it's ugly head is the
ohmic Re' of a bjt. The ohmic Rb' was something I'd already
thought about before (seemed more obvious to me at the time.)
But what I hadn't recognized right away was that Re' is
multipled by (beta+1) when reflected to the base and even
small values of Re' then look a lot bigger than Rb' and will
dominate the calcs. (This is BEFORE adding in an actual Re as
a separate circuit element.)
This didn't become an issue when using a simple mirror,
because both bjts had about the same currents in them. So the
reflected values created the same voltage drops. But it IS an
issue when I decide to use an external Re in one emitter.
Then, there are significantly different currents and I have
to take this into account to gain any useful prediction.
One interesting note is that some of the bjt models I have
don't even specify Re' and LTspice defaults it to 0 if it
isn't given. For example, the 2N4403 in the builtin LTspice
models doesn't supply Re'. But it does supply Rb'. However,
the 2N2907 model does supply both. Hmm. Can't trust anything.
So for example, with Rb'=10 (commonly slapped onto models)
and Re'=.2 (say, the 2N2907 just mentioned), and beta=250 for
that device, I get a reflected value of about 50 ohms, which
is 5 times larger than Rb'. So it can't be ignored in some
situations (as where I may use one emitter resistor.)
The other thing is that when operating a mirror with
different currents, heating will be very different in the
two. Even on a common substrate, let alone with discrete
devices. Most especially if the output bjt is supplying a lot
more current and with a collector that may be causing a large
Vce drop, besides. And at -2mv/C for Vbe, and 10X Ic for
every 60mV change, this makes it almost impossible to
consider if there isn't a closed loop where the actual Ic is
monitored and used to control the current via feedback. Which
reduces its attractiveness.
I find I'm able to manually (with a calculator) replicate
LTspice results to about 3-5 parts per thousand over a very
wide range of circuits and bjt devices. It's iterative, in
the sense that I have to repeatedly do an estimate and plug
that back in. (I couldn't develop a closed equation.) So I at
least think I've got that part nailed down.
But is the above thinking about right? Or is there something
else I've missed about this topology that I failed to mention
above? Seems that a Wilson mirror helps a little bit with the
different heating issue (by returning the Vce on the output
side bjt to something much smaller) but at the expense of
more headroom required and another device.
Anyway, interested in thoughts, points, etc. I can see why
these are much more useful in ICs than discrete. Most
especially if there is any differential currents involved.
My thoughts here are about driving an LED matrix. I an
considering the idea of using an external mirror (but with
gain) so that the 5916 can drive it with smaller controlled
currents. The problem seems to be that I get nothing from the
exercise, as different heating will unravel the whole idea
and force me to figure out a feedback scheme, which then adds
further discrete device complexity and adds still more
headroom I don't want to waste.
Jon