ECL logic family, BJT, etc.

[PG1995]

Hi

Could you please help me with this query about ECL logic family? Thanks.

Regards
PG

 

[ronv]

Take a look at this write up.

https://www.onsemi.com/pub_link/Collateral/AN1406-D.PDF

 

[MrAl]

Hello there,

I had a whole book dedicated to the Motorola ECL line but it's too bad i discarded it because other forms of logic were more common and nothing i found for my own personal use used ECL.

The main idea behind ECL is to bias the transistor in a way which allows it to switch as fast as possible. The NPN transistor can be very fast, but there is a catch: the storage time. The storage time occurs because of charge stored in the base, and this charge must be removed before the transistor can turn off. This coupled with the maximum reverse base emitter voltage rating of the base emitter diode means there is a practical limit to just how fast we can remove this charge.

The storage time can delay the switching time by a full microsecond or even longer, which is unacceptable for high speed logic. But the storage time only becomes a problem when the transistor is allowed to enter into saturation. So prevent the transistor from entering into saturation and you've prevented the transistor from exhibiting storage time which will make it switch much much faster.

If you look at the switching spec's for some transistors especially the higher power ones, you'll see the storage time can be as high as 10 microseconds. That makes them a pain to use sometimes unless something is done about preventing them from entering into full saturation. Various ways to prevent or reduce this have come into existence most notably the Baker Clamp. The Baker Clamp uses diodes as feedback to the base of the transistor to keep the transistor collector emitter voltage from going too low and thus prevents saturation. Preventing the transistor from entering saturation of course reduces efficiency from the point of view of the collector emitter voltage, but increases efficiency from the point of view of the switching time.