Transistor As a Switch

[Electroenthusiast]

Transistor a Switch

Should the Load always Be in Collector side?
Why cant Active Region Used for Switching?
Any circuits/applications Please

 

[Ross Craney]

No it doesn't . What needs to be considered is that with the load in the emmitter side the load impedance will impact on the switching characteristics since it will affect Bre. In many cases it is prefferable to have it in the emmitter as not only will C-E current flow through the load but also the B-E current

 

[Sceadwian]

The active region can be used for switching in specialized circumstances, primarily extremely high speed switching as it takes time to draw off the charge when a transistor enters saturation. Sometimes the power loss of working in the active region is offset by the fact that it can only switch at that frequency in it's active region without incurring dramatic switching losses.

 

[BrownOut]

I once had an electric motor that raised the lid of a vacuum chamber. Every once in awhile, the motor would attempt to raise the lid while the chamber was still under vacuum, with disasterous results. So, I designed a switch, more accurately a driver, which used the constant current region of the driver transistor so that the motor woudn't pull itself apart. The only problem was that the driver would go into thermal runaway if the lid was stuck. So, I had to use a second transistor and resistor that sensed the supplied current and to cut off base voltage to the driver transistor under this circumstance.

So, in short, in some applications, the driver needs to supply a current ( active region ) and not a voltage ( saturation region )

 

[Sceadwian]

Keep in mind the situation for Mosfets is much more mixed up. Saturation in a Mosfet will cause it to act as a constant current source where saturation in an NPN transistor it will act as a switch. Their fundamental making is similar however their real voltage/current interactions are very different.