Now briefly about my 80's "current steering inventions"...

In 1982, I invented a spot LED indicator where red LEDs extinguished a green LED by conecting in parallel to it...

Then I developed this circuit to a linear (fully analog) LED indicator... which operated very much like the sine wiggler - diodes were "stacked" on top of each other... and only the latter was active (lit)...

Of course, the diode string was supplied by a transistor current source.

Finally, in 1983, I transmuted it to the most sophisticated (fully analog) 2-dimensional LED indicator. It consisted of 9 LEDs organized in 3 x 3 matrix; in a given moment, only one of them was lit.

The two input voltages VX and VY were moving this spot horizontally and vertically... like in a XY oscilloscope... So this circuit was serving as a XY zero voltage LED indicator.

By the way, during laboratory exercises on Semiconductor devices (e.g., last week), I introduce my students to this 80's idea... and we conduct interesting experiments with LEDs. Here are some of them:

https://drive.google.com/open?id=1oNkUDQYDZyml3YNN2LLOZ4jsMFgaO3oS

https://drive.google.com/open?id=1ph3UNC_7Ryntu0PZHGGzHoxo_ETO5cns

https://drive.google.com/open?id=1yiQ8V1PMb6UMO-KntL8ap_62LQMHmde3

Oh my God... one month after I have asked this question about one of the greatest circuit ideas from 30's... I can not believe that no one is interested in this subject...

For the time being, I am convinced that in-depth thinking about the essence of things is the most deficient thing in this world of modern technologies... A very pitiable finding... especially in the field of education...

Cyril - per accident I stumbled over this contribution.

I have some problems to understand the task, therefore my questions:

Quote 1: "Supply an emitter follower with +10V and by applying a relevant base voltage, set its emitter voltage to +3V. The collector-emitter junction will behave as a "diode" (voltage stabilizer) with 7V threshold voltage"

Can you explain this assertion? Why do you think there is any "diode effect" resp. a "stabilizer" effect?

In contrary, I think the resistor RE in series with the C-E path will behave as a simple voltage divider - controlled by Vin at the base.

Quote 2: "Finally, join the two emitters. As a result, the 6V "diode" connects in parallel to the 7V "diode"... and completely diverts its current... "

While connecting the "6V diode" in parallel to the "7V diode" both parts (your term: "diodes" ) change their voltages, correct?

So - we are not allowed to further use the terms "6V diode" and "7V diode"?

Hi Lutz,

Thanks for "reviving" this discussion. I need little time to answer you later...

The next day... My idea is to explain the current steering phenomenon by something simpler than a transistor - diode, potentiometer... So, I think, if we know what happens when connecting two diodes with different threshold voltages VF in parallel, we will understand what happens when join the outputs of two emitter followers with different input voltages... My reasoning is as follows:

Imagine an emitter follower supplied by a constant voltage VCC = +10V. If we apply, for example, +3.7V to the transistor base, it will maintain almost constant output voltage of +3V towards the ground (or -7V towards VCC) at the emitter. Since the supply voltage is constant, the complement of this voltage (to VCC) - VCE = 7V, is also constant. So, we can think of the collector-emitter junction as of a "zener diode"with VZ = 7V.

Really "the resistor RE in series with the C-E path will behave as a simple voltage divider - controlled by Vin at the base", but note that its upper "resistor" (the collector-emitter junction) is non-linear and voltage-stabilizing... like a 7V zener diode.

Then imagine another emitter follower supplied by the same VCC = +10V. If we apply, +4.7V to its transistor base, it will maintain almost constant output voltage of +4V towards the ground (or -6V towards VCC) at its emitter. The complement of this voltage (to VCC) - VCE = 6V, will be also constant. Again, we can think of this collector-emitter junction as of a "zener diode"with VZ = 6V.

Now join the two emitters. As a result, the two collector-emitter junctions (7V and 6V "zener diodes") will be connected in parallel. The +4V emitter of the second transistor will "pull" the +3V emitter up to +4V... and thus will cut-off the first transistor (like in an ECL gate). Thus the current will be completely steered through the second transistor... and the output voltage will be determined completely by the second transistor.

If we explain this by means of complementary voltages, we can say that the 6V "zener diode" is connected in parallel to the 7V "zener diode"... and completely diverts its current... and completely determines the common output voltage across the two "zeners"equal to its voltage of 6V (or +4V towards the ground).

Dear Kirill, thank you for a series of your wonderful articles published in the Bulgarian magazines of the 80s of last century!

Thanks, Michael!

Glorious times were then - components were discrete, circuits were realizable... and we believed that we could do something real... Now everything is hidden inside chips... and we can only analyze what others done:)

Best regards,

Cyril

"Now everything is hidden inside chips... and we can only analyze what others done:) "

Cyril - does this mean that your students will never hear something about the working princile of a BJT? I am sure this is not the case!

Lutz, really this is not the case in my laboratory:)

Thanks for the support!

Cyril