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

Discussion in 'Homework Help' started by krlngc, Nov 8, 2015.

1. ### krlngc Thread Starter New Member

Dec 9, 2013
11
0
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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Last edited: Nov 8, 2015
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,082
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What about the situation when Q7 is in cut-off?

3. ### krlngc Thread Starter New Member

Dec 9, 2013
11
0
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?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,082
1,149
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.

5. ### krlngc Thread Starter New Member

Dec 9, 2013
11
0
I see now. But how can Q6 cut-off Q7 ? By stealing all the current that I2 provides?

Thank you.

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,082
1,149
Do you see any other way??

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