[Op-Amp] current for max. output swing

Thread Starter

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?

Thank you for your help.
 

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Last edited:

Thread Starter

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
5,022
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.
 

Thread Starter

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.
 
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