Transistors

[mneary]

How did you conclude that the base current will be 5 to 10A?

Very nice numbers which ignore the base resistance. I'm surprised you forgot this important circuit component. Remember nobody said it was good practice.

But I actually went to the bench and found that a real TIP41C actually drew only 1.5A from a 1.5V supply when connected B-E.

A few pages ago somebody said vizier87 wasn't being truthful when he said a 1.5V battery from E-B worked. I believe the criticism was unfair.

Nobody said it was good practice.

 

[Wond3rboy]

This is like the most asked question in electronics isnt it?

 

[unclejed613]

I believe that this is not a big problem for BJTs. The on/off characteristics are much stronger with FETs where you have great G-S and D-G capacitance and impedance.

With BJT this is despresible up to certain frequencies (MHz range).

actually, it's a big problem with BJT's as well. especially in class AB amplifiers. when you get up to higher audio and ultrasonic frequencies, you have to find ways to dump the base charge. and it's not just the Ccb charge, but the Miller capacitance, which is Ccb * Hfe. so if the raw Ccb is 100pf, and the transistor beta is 30, that's a Miller capacitance of 3nf. if you don't dump the charge effectively, the output devices go into common mode conduction and draw huge amounts of current at high frequencies. this is because when one half of the output stage should be turned off, and the other half is turning on, the half that should be turned off is still conducting, and current passes through both halves of the output stage from one rail to the other rather than through the load.

 

[Hayato]

Thanks for the explanation.