Analyzing the topology of the LM741 , the current mirror (Q8-Q9) that bias the first stage of the op. amp. is not connected as usual (see the red circle on the picture). Curiosly, this way to connect the current mirror is not explained in the reference text books consulted.
The “usual” current mirror topology , use as reference the current on the transistor connected as a diode, and Iout is the current on the second transistor, where If VBC is greater than zero in output transistor Q2, the collector current in Q2 will be somewhat larger than for Q1 due to the Early effect. In other words, the mirror has a finite output (or Norton) resistance given by
r o = (V A + V C E)/IC
where VA is the Early voltage; and VCE, the collector-to-emitter voltage of output transistor.
Then in the LM741 this mirror source is inverted. Any one can explain why?.
I suspect that a good reason is the following: This mirror is not working as active load, only must bias the differential amplifier. As the active load are Q5-Q6, is desiderable that colectors of Q1-Q2 are grounded for signal. As the equivalent resistence of the diode connected transistor is "re", this way is better for AC response. If this assumtion is true, the question is: why there are not a capacitor in pararell?
tks in advance, darío
Dear Dario,
I think the reason is to reduce the voltage drop on the transistor Q8 of left branch of the mirror by making its VCE=0 always since its collector is tied to the base.
Therefore the rest of the power supply voltage will used to bias the other transistors in the stack active with possible larger output voltage swing. Hence larger dynamic range.
Best wishes
Dear Dario,
I think the reason is to reduce the voltage drop on the transistor Q8 of left branch of the mirror by making its VCE=0 always since its collector is tied to the base.
Therefore the rest of the power supply voltage will used to bias the other transistors in the stack active with possible larger output voltage swing. Hence larger dynamic range.
Best wishes
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