BJT, Beta question

[D.M.]

Is there any quick explanation as how to develop a common emitter amplifier using the BJT so that the circuit is not so sensitive to changes in BETA?

 

[Electronman]

What about using a resistive voltage divider for the base connection?

 

[D.M.]

We do have a voltage divider at the base and the equivalent parallel resistance is low, but is there anything else we could do?

 

[D.M.]

What im thinking is;
If i make the voltage divider equivalent resistance large enough, then current will be directed into the base of the BJT. When drawing the small signal model, the drop across the internal resistance (what we call r_pi) is what sets the gain so by decreasing the voltage divider equivalent resistance the base current is decreases. By decreasing the base current the voltage across the small signal r_pi is less. The drop across r_pi is one parameter that controls gain and the less the drop across r_pi is, the greater the gain...

is this clear? does this make sense?

 

[MikeMl]

The usual way is to add a resistance between emitter and ground. Since the emitter resistor reduces gain, it is usually bypassed with a capacitor to restore the gain. This adds a pole to the transfer function of the amplifier stage, so you have to choose the capacitor so as not to constrain the bandwidth at frequencies of interest.

 

[D.M.]

thank you, excellent input. much appreciated!