Transistor saturation

[colin55]

A 12k resistor will deliver about 1mA. A 1k2 will deliver 10mA to the base. The cct is asking for 1mA.

 

[crutschow]

This is totally unrelated to the topic: "The transistor gain (beta or Hfe) value is for small signal linear applications, not switching." And it's rubbish.

If you think that's rubbish then you are quite ignorant about transistor operation and aren't qualified to be making comments about them.

And how is that "totally unrelated" to the original topic "Transistor saturation"? It's exactly related. Do you even understand what saturation is?

 

[colin mac]

A 12k resistor will deliver about 1mA. A 1k2 will deliver 10mA to the base. The cct is asking for 1mA.

Sorry 12K I should say, not 1K2.

 

[colin mac]

If Hfe is 100 for a Ic(sat) of 10mA then Ib(min) = 0.1mA. But you'd supply more base current than that to ensure saturation. Maybe 5 times.
1mA is 10 times but if Vce sat is always spec'd with the base current being 1/10th the collector current it's a logical rule of thumb

 

[audioguru]

Colin has the R1 and R2 numbers mixed up. R2 can be a high value and is not needed if the input goes to near 0V.
R2 turns off the transistor when the input signal goes open or does not go low enough.

 

[audioguru]

This is totally unrelated to the topic: "The transistor gain (beta or Hfe) value is for small signal linear applications, not switching." And it's rubbish.

Colin, please read the datasheet of every transistor where they spec the max saturation voltage when the base current is blasted at 1/10th or 1/20th the collector current.
Sure, a few transistors out of a thousand are better and need a lower base current to saturate. But then you need luck (or measure all of them) to find them.

 

[k7elp60]

Thank you all for your replies, especially audioguru. I looked at the data sheets on the 2N3904 and found the information about saturation. My data sheet has a chart so if I want saturation current different than 10mA its a piece of cake....so to speak. As audioguru pointed out R1 input will be a 0 on occasion as it connects to pin 3 of a 555 timer. The collector of the 2N3904 will be used to pull the gate of a power mosfet near ground as a high side switch.
Thanks again folks.

 

[audioguru]

This is totally unrelated to the topic: "The transistor gain (beta or Hfe) value is for small signal linear applications, not switching." And it's rubbish.

Please do not talk rubbish. The transistor is used as a saturated switch, not as a small signal amplifier so it needs the base current (1/10th its collector current) that is listed in its specs. European transistors (BCxcxx) with a very high gain have their max saturation voltage loss spec'd with a base current that is 1/20th the collector current.