# How to analyze BJT amp circuit with multiple gains

Im currently trying to solve this problem for total gain, though I'm stuck right off the bat with DC analysis. I was told the emitter current for Q1 and Q2 will be 0.9 mA though I have no idea why.

Why is the emitter current for Q1 and 2 0.9mA?

Would Ic at Q1 be Ic1=(9.3--10)/22k = 0.872mA?

edit: I understand the AC portion so don't bother with that, just need help with DC.

• How you get AC portion whitout DC? – Martin Petrei Dec 10 '15 at 19:47
• @MartinPetrei phrasing, I meant I understand how to go about an AC analysis, but I need to do the DC first. – Hatandboots Dec 10 '15 at 19:53
• You know that $V_{BE}=0.7V$ so what is the current mirror current? SUPERHINT: you only need the left vertical of this schematic for this answer. – Daniel Dec 10 '15 at 20:09
• Q1 is a diode, assume Vbe = 0.7 V: (20V - 0.7) = 19.4 V / 22k = 881uA. Q1 and Q2 form a mirror so assuming Vin does not force Q2 in saturation, Ic(Q2) is also 881uA. Now calculate volatage across 10k resistor, -0.7 V gives voltage across emitter resistor of Q4 etc etc – Bimpelrekkie Dec 10 '15 at 20:23
• @FakeMoustache Oh you're right. I didn't see -10V down at the bottom... 877uA. – Daniel Dec 10 '15 at 20:38