some electronics questions

[ahmedragia21]

Hi, first why we use current mirrors ?
and why the need to short the Base and the emitter in it ? can i use a normal diode ?

2. there's no gate current in mosfets , but how we apply a voltage at the gate ? it should be there current right ?

3.if i've a device rated as 5v,500ma , if i put 5v , 1A why the device wont be burnt and why it can be burnt if i put 9V on it ?

 

[dknguyen]

1. If it needs a current mirror you can't use a normal diode. We use current mirrors because current sources are useful. You can't just use a diode- you are missing something about the operation of a current mirror which makes it seem like a diode to you (a current mirror takes TWO BJTs, not just one. THe one that's "wired like a diode" has it's base connected to the base of the other BJT- not somethign you can do with a diode since it has no gate. Diodes also don't have beta values. RUn through the operation of the current mirror again, carefully.

**broken link removed**

2. The MOSFET gate is basically a capacitor, and it has to be a certain voltage for the MOSFET to turn on. So when you apply voltage to the MOSFET it takes an initial current pulse to charge up that capacitor, and after that it takes no more current to maintain that voltage (except for capacitor leakage currents).

3. It depends on the device and it's physical structure. SOme devices can handle extra current well since they can take extra heat while other can't do that so well. Others must not have the voltage exceeded because the dielectric strength of the materials inside the device can't withstand that voltage so even if there is no current flowing and overvoltage is applied, the device will still be burned because the electrons will start piercing the barriers inside the device (like MOSFET gates).

 

[ahmedragia21]

3. then i dont care about the current the device which can handle or what ?
you mean i just care about the voltage ?

also are these questions stupid or what or im not good at electronics

thanks for the answer

 

[dknguyen]

3. You do care. It depends on the device you are talking about. A lot of devices don't care about overcurrent as long as you can keep the device cool enough (which explains why you might be able to run it at 1A...at least for a while. It would probably fail if you ran it for too long at that current or have reduced lifetime).

Other devices, especially semiconductors are very sensitive to overvoltage because the electrons start being torn out of the semiconductors in the material damaging them, while others, like inductors and resistors don't really care about overvoltage.

 

[ahmedragia21]

So i concloud that overing current depends on the device , and also overing voltage is based on the device ( bjt , mosfet ..etc ) ?

 

[dknguyen]

YOu conclude right. THere's no set rule since every device is different.