I tried drawing the graph of \$v\_o\$ vs \$v\_i\$ for a bipolar junction transistor. I used the below setup (common emitter:)

[![enter image description here](https://i.stack.imgur.com/mOEjV.jpg)](https://i.stack.imgur.com/mOEjV.jpg)

By design of how the base is a small part compared to others, \$I\_b << I\_c\$ and so I take \$I\_c \approx I\_e\$.

I’m taking this transistor to be equivalent to two diodes, one forward biased and another backward, and matching the graphs pointwise against \$I\_c\$ to get graph between \$v\_o\$ and \$v\_i\$:

[![enter image description here](https://i.stack.imgur.com/PLayM.jpg)](https://i.stack.imgur.com/PLayM.jpg)

I’m getting a graph different from what’s given in my textbook.

Mine:

[![enter image description here](https://i.stack.imgur.com/sRELi.jpg)](https://i.stack.imgur.com/sRELi.jpg)

Textbook & circuit given:

[![enter image description here](https://i.stack.imgur.com/sw3St.jpg)](https://i.stack.imgur.com/sw3St.jpg)

I’m not sure where I’m going wrong.