Question

Calculate the VI – VO for the level shifter shown in the figure (Assume identical silicon transistor and very large value of β). Transistor QA and QB form current mirror.

(a) 5.56V

(b) 6.00v

(c) 7.98v

(d) 6.65v

I got this question in examination.

My question is based upon Operational Amplifier Internal Circuit in section Operational Amplifier of Linear Integrated Circuits

Answer 1

Correct answer is (d) 6.65v

Expand on the concept with: Since the transistor QA and QB form current mirror, ICA= ICB = I.

=> I = (VCC – VBE) / R0 = (15v-0.7)/12k Ω (for β>>1, output current =input current)

=> I= 1.19mA.

The shift in level is given as VI – VO = VBE + I×R1 =0.07v+1.19mA×5kΩ =6.65v.