Stuck at darlington pair.

Hey there,

I'm doing a small revision of BJTs - and including the difficult Darlington pair calculations. Oh - I'm not asking for any answers - I did my homework already, but I got stuck into a point. Any assistance will be very great.

Okay, the question asks me to find the current gain. Here's what I come up so far:

V base = 4 V where Vcc * (R1 || R2)

V emitter = 4 - (0.7 * 2) = 2.6V

Overall beta ac is 150 * 100 = 15000

the emitter current IE is 1.733mA from 2.6V/1.5K

total of input resistance (Rin ) is 33k || 22k || 15000*1.5k is 13.192k

and input current Iin is 1V / 13.192K = 75.8 micro amps

Current gain = 1.733mA / 75.8micro = 22.86.

And unfortunately, the answers in the book reveals 8.8. Any which part I'm missing?

that is because the out current u selected is dc emitter current ...IE where actually u need Ie while doing ac analysis.. here u can do it by a formula derived... Ai= [ (B1*B2) (R1 || R2) ] / [ (R1 || R2) + (B1*B2)Re ]

I get .88

The input current is .75uA. Multiplied by 15,000 (current gain) = 1.136363

Then take the reciprocal to get the reverse gain so 1 / 1.136363 = .88