[Moved] Constant current source problem

[himanshul]

pls explain me the working of this circuit.The text says that Current source Q1 functions as usual but with collector volt- age held fixed by Q2's emitter...i.e the collector voltage of Q1 is held fixed by action of transistor Q2.But what i didnt digest is that if the temparature changes then the Vbe of Q2 changes resulting in a change of Ic and Ie of Q2 which further change collector voltage of Q1,then why the text mention collector voltage of Q1 to be constant for any temparature variations?

pls refer the attachment?

 

[erikl]

You're right: this cascode circuit is not very temperature (nor supply voltage) stable. But at least Q1's Vc will stay 1*Vbe below Q2's Vb.

The Figure 2.24 text just says the current stability referred to load changes is improved (compared to a simple constant current source).
This is true, because the cascode transistor Q2 (in base mode circuit) has a much higher output impedance than Q1 alone.