how to analyse a darlington pair?

[arjob]

Is the LEFT circuit of darlington pair, equivallent to the RIGHT transistor circuit?

Is my asumption true?
I replaced the two transistors in darlington pair with one; having beta equal to the product of betas of the two transistors, and the voltage from of 1.4 (0.7+0.7) across Base and Emitter.

Then I am calculating the Ib and hence the Ic.
In practical experiments, I found that my calculations are not very exact, but close enough.

For example, for the circuit to the right I found:
* Ib = 63 nano Amprear
* Ie = 4.5 mA

In measurements, I am finding the Ie about 5.5 mA.

 

[chuckey]

For most applications your approximations are OK. However the collector current of the T2 does not flow through the 50 Ohm resistor, so your approximation could show T2 saturating , when it is not.
Frank

 

[arjob]

Thank you. Yes i realized that now.