basics(differential amplifier and darlington)

[quaternion]

basics of differential amplifier

I have two basic questions

1 - for an active loaded differential amplifier why we take the output from the transistor that is not diode conected (to be specific active loading I talk about is achieved by 2 transistors that form a currnt mirror).

2 - I doubt about the darlington pair (for its benifit in magnifing β); can a single transistor made by appropriate design have β that is equal to the darlington pair β or the issue is in the fabrication technology or what (am I miss a concept)!?
[I am speaking about darlington pair as a three terminal device]

 

[ieropsaltic]

For the first question :
We take the output fron the transistor that is not diode connected ,because the diode connected transistor is equivalent at the small signal analysis to a resistance of value 1/gm which is usually a small value .The gain at the output of the unloaded amplifer is equal to Gm*Ro where Gm is the amplifier's transconductance (=Ii/Io when Vo=0) and Ro is the output resistance of the amplifier .If the output is taken from the diode connected side, the gain will be small (Ro=1/gm) ,if taken from the other side ,Ro=ro which is much larger value .

 

[quaternion]

Thanks ieropsaltic , I get it, I didn't analyize it well .

What about the Darlington pair (I feel that the question isn't clear)

I mean for the purpose of having a high β three terminal device is it possibole to design 1 transistor to have this high β (in place of 2 transistors that are connected darlington pair ).Please if I miss some points clarify it to me.

 

[ieropsaltic]

Hello,
Concerning Darlington pair, the BJT's β relies mainly on 2 factors :
1-the width of the base region .
2-The relative doping of the base and the emitter regions (Na/Nd)
Thus, to obtain high β, the base ahould be very thin and Na/Nd should be very small(emitter heavily doped) .Even if the process allows smaller base width and higher doping for emitter, these would result into more significant eraly effect (decreasing ro greatly).Also, this would lead to a lower breaking point for the transistor (when the whole base becomes completely depleted) .So, darlington pair is a much better solution in many cases when high β is required .