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What Q1 and Q4 actually do?
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001:
Hi


Can You told me What Q1 and Q4 actually do? Is it good or bad schematic?
amspire:
They are acting as constant current sources. Q1 is providing a constant current source for the reference zener.

Q4's current is controlled by the current through D5 and D6. Q4 acts as a very high impedance on Q3's collector giving Q3 a very high voltage gain.
TimFox:
Q1 is a DC current source.  Q4 is a "current mirror".  There is a large literature about current mirrors, which are extremely useful BJT circuits.
T3sl4co1l:
D5 should be changed to a resistor, so that it actually serves as a mirror.  As given, it's more of a current source, which isn't much help to loop gain.

Tim
Zero999:
Ideally D5 or D6 should be a BJT matched with Q4, with the base shorted to the collector and the other diode a resistor, as mentioned above. The BJT should have as closer VBE to Q4 as possible, given the same current and a similar hFE.
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