Transistor collector current

[walid1]

Hello everyone
We all know that Input Impedance decreased when we choose a high collector current, and that is ok
I want to ask: When one choose high current?
thank you

 

[audioguru2]

A transistor needs to have a high collector current (and a high base current which reduces its input impedance) and a low resistance collector resistor when it is in class-A and it needs to drive a low resistance load.

 

[walid1]

A transistor needs to have a high collector current (and a high base current which reduces its input impedance) and a low resistance collector resistor when it is in class-A and it needs to drive a low resistance load.

Are you mean like emitter follower?
What about a voltage divider configuration preamp. is there a situation here that we want a high IC current?
I stay, think long time about it, choosing small current (IC) give us a high Zin a long batt life and a transistor works in light duty, why not choose a very small current like 0.1 mA or even 10 uA, i know that big resistors will be used but this is no broblem!
thanks alot.

 

[audioguru2]

A class-A transistor has a fairly high output impedance which is its collector resistor.
An emitter-follower has no voltage gain but has a low output impedance. Because the NPN transistor pulls up strongly but the emitter resistor pulls down poorly then it does not drive a low impedance load well.
A push-pull complementary pair of emitter-follower transistors drive a low impedance load very well but also do not have voltage gain.

 

[Kevin Weddle]

Low current biasing seems like a problem also. It only takes 200 mV before a transistor goes into cutoff. I have made simple cirucits that are just DC biased only, and have watched the DC bias drift by 200mV. But my test lab uses 7805 and 7812 voltage regulators and they're used, and so were the components in the circuit. Test labs are general purpose, allowing for a variety of loads.