Bias question

[SmileT]

Hi everybody!
I am working on an common emitter bjt stage that is part of a phase shift oscillator and i have decided, at first, to bias it with two resitances (base and colletor resistances) and an emitter resistance (feedback resistance)...
Now i realized that this kind of biasing is quite "beta dependent" and a common 4 resistances network should work better but i want to understand if my considerations are right...
I have found the equation below between beta and Ie:

Ie=(Vcc-Vbe)/(Re+Rb/(β+1))

and if we want an Ie lightly beta dependent we have to realize Re>>Rb/(B+1); whith three resistances this could be done only increasing the value of Re and cosequently the voltage drop on Re (if we want Ie to be the same) reducing the dynamic range of the output, is it right?

 

[crutschow]

You can add a capacitor across the emitter resistance which will help maintain dynamic range. The capacitor size should be such that the 1/(2Pi*ReC) rolloff is below your lowest frequency of interest.

 

[SmileT]

You can add a capacitor across the emitter resistance which will help maintain dynamic range. The capacitor size should be such that the 1/(2Pi*ReC) rolloff is below your lowest frequency of interest.

I am talking about the DC analysis...there is a bypass capacitor across Re because it is a common emitter stage...
Anyway thank you for the answer!

 

[crutschow]

I am talking about the DC analysis...there is a bypass capacitor across Re because it is a common emitter stage...
Anyway thank you for the answer!

Since you mentioned dynamic range and it's an AC amp, I assumed you were concerned about the AC dynamic range. When bypassed, the emitter resistor value only affects the DC dynamic range. Why are concerned about that in an AC amp?

 

[SmileT]

Since you mentioned dynamic range and it's an AC amp, I assumed you were concerned about the AC dynamic range. When bypassed, the emitter resistor value only affects the DC dynamic range. Why are concerned about that in an AC amp?

Ok this is a part of a phase shift oscillator and for example if Vcc=5v and the Vdrop across Re is 2v the oscillation will be between about 5 and 2v so the range is near to 3v while if i choose an Re Vdrop of 0.5v i could reach a d range near to 4.5v.

My question is with a 3 resistances biasing network is possible to reach a stable Ie (in other words to make the bias point indipendent from beta) maintaining the same ampliture of the output oscillation?

 

[crutschow]

You are correct. My thought processes were off. The decoupling capacitor will help the dynamic range but it will still be limited by the DC emitter voltage. There is no way around that limit.

And a three resistor bias is dependent upon beta. Don't think there's any way around that. Just go to a four resistor bias. Resistors are cheap.

 

[SmileT]

You are correct. My thought processes were off. The decoupling capacitor will help the dynamic range but it will still be limited by the DC emitter voltage. There is no way around that limit.

And a three resistor bias is dependent upon beta. Don't think there's any way around that. Just go to a four resistor bias. Resistors are cheap.

Ok thank you very much and sorry for my poor english!