Transistor Biasing using a current source possible?

[gauthamtechie]

Can i bias the Base-emitter junction of a CE amplifier configuration transistor using a current source?

Since I believe that the base-emitter forward drop of 0.7 volts must be breached, how does a current source work in that case?

 

[audioguru]

The base-emitter voltage and the current gain of a transistor are different for each transistor even if they have the same part number. The voltage and current gain also change when the temperature changes.
Therefore a linear transistor is usually biased from a voltage divider having about 10 times the base current and an emitter resistor is used for some negative feedback and so the base-emitter voltage doesn't matter.

If you are using the transistor as an on-off switch then use a base current that is about 10 times the collector current.

 

[crutschow]

You can use a current source to bias the emitter current (which is sometimes done such as for differential stages), but due to the transistor variations that AG mentioned you shouldn't use it to bias the base.

An ideal current source generates whatever voltage in needs to provide the desired current, so the 0.7 Vbe is readily generated (not breached) by the source.

 

[MikeMl]

Here is an illustration that a current source can be used to bias a transistor used as a CE amplifer. This would provide a much higher input impedance than an average CE stage with a classic four resistor stabilized bias network, but would have inferior thermal stability and tolerance for different transistors..