For the newcomers, have a look at the log 2. moving forward of a related project. The comments are great too :-)
So I want to make ECL gates. I want to make them fast and if possible reduce the amplitude and frequency of the PSU's "digital noise". Oh and I would love to have an even lower supply voltage, because about half of the power is burned by the common resistor, that wiggles between 0.8 and 1.2V. If I could win maybe 0.5V there, the PSU would have to supply only 1.5 or 1.6V and save 20 to 25% power. Furthermore I might want to "stack" layers of ECL circuits later...
According to @Julian, reducing the current below 2mA starves the circuit and the speed is reduced, so I'll stick to 2mA for now and try to get the equivalent of the 470 ohms I now have.
What that would bring me :
- The ability to change the settings and working points of the whole circuit with current mirrors spread around the PCB and trim the values with one pot,
- reducing the operating voltage (and power), or increasing the current at will to see the effect on the speed
- reduce power noise and decoupling
There is not much margin though : 0.8V is already low, and a degeneration resistor will already eat 0.1 or 0.2V and the transistor's Vcesat can impose limits. The complexity of the system would be overkill while a simple resistor already works rather well.
But there is something else : the lower the resistor, the lower the swing at the common node, and the lower the sensitivity of the inputs... Maybe a transistor could help there too ?
Looking at https://wiki.analog.com/university/courses/electronics/text/chapter-11, I find this interesting circuit :
but this is a current mirror : this means that to get 2mA in the output (which can be replicated/multiple) there must be another place where 2mA is sunk. Which is not great for power savings. Things start to get complicated from there, if I want to reduce it to 200µA for example...
R2 would be about 50 ohms and drop 0.1V at 2mA. Energy saving means I should get the lowest Vce, so it must be (almost) saturated. Which means : pumping significant current in the base. Which means that the R2 drop would also increase because of that base current and might reduce its effectiveness. The transistor's gain should thus be high to reduce base current and Vce, but all those counter-effects conspire to make the transistor current sink overly complicated.
After all : looking at Motorola's MECL Data Book, no transistor is used in the major old families:
- MECL I : 1.24K
- MECL II : 1.18K
- MECL III : 365
- MECL 10K : 779
The high-side resistors however are lower than my values : 100 (MECL III) to 300 (MECL I & II). I could investigate reducing my values but this comes at the prince of increasing the current...
Do I really need a sink transistor ?