[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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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
4,974
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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