# [Op-Amp] current for max. output swing

#### krlngc

Joined Dec 9, 2013
11
Hi all,

For the op-amp circuit in attachment i'm asked to find the $$I_3$$ current that renders the output swing maximum and the $$R_E$$ that allows input swing between -1V +1V. The circuit parameters are given as;

$$V_CC=5V$$
$$V_EE=-5V$$
$$I_1 = 50uA$$
$$I_2 = 500uA$$
$$R_L = 1K$$

I think that$$I_3$$ sets the bias point for Q7 and determines the transconductance. In small signal case, the positive half-wave of the input will cause the $$i_c$$ to flow over $$R_L$$ and the negative half-wave will result in a negative current over $$R_L$$ in order to complete $$I_3$$. However I can't go any further. How can I make any comment about the max output swing? What should be my approach on this question?

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#### Jony130

Joined Feb 17, 2009
5,241
What about the situation when Q7 is in cut-off?

#### krlngc

Joined Dec 9, 2013
11
I can't really tell what drives Q7 to cut-off. A significant rise in $$V_E$$ can reverse-bias the BE junction maybe ? Is this my upper limit?

#### Jony130

Joined Feb 17, 2009
5,241
Q6 can cut-off Q7 transistor. And when Q7 is in cut-off, you will get maximum negative voltage swing.
But on the other hand when Q6 is cut-off you will get the maximum positive voltage swing.

#### krlngc

Joined Dec 9, 2013
11
I see now. But how can Q6 cut-off Q7 ? By stealing all the current that I2 provides?

Thank you.

#### Jony130

Joined Feb 17, 2009
5,241
But how can Q6 cut-off Q7 ? By stealing all the current that I2 provides?
Do you see any other way??