I was trying to reproduce part of this circuit in LT Spice and it seems to conflict with the circuit's stated purpose and I don't know which is correct.
The schematic of the circuit in question is here.
As best I can tell, the voltage at the base of Q2 should be just a bit less than the rectified voltage, meaning the voltage at its emitter should be a diode drop below that. So well good bit above 20V.
On the other hand, the voltage at the base of Q1 will be the results of the voltage divider created by VR1 across the 20Vs created by diodes ZD1 and ZD2. So 0-20V, and thus the emitter of Q1 will be a diode drop below that...
But the emitters are connected together, so who wins? LTSpice tells me the voltage is fixed at over 20 while the circuit is supposed to be an adjustable 20V supply.
And as a separate follow-up question, why couldn't you just remove Q1 and run the voltage divider straight to the base of Q2 to get your 0-(20-diode drop)V?
The schematic of the circuit in question is here.
As best I can tell, the voltage at the base of Q2 should be just a bit less than the rectified voltage, meaning the voltage at its emitter should be a diode drop below that. So well good bit above 20V.
On the other hand, the voltage at the base of Q1 will be the results of the voltage divider created by VR1 across the 20Vs created by diodes ZD1 and ZD2. So 0-20V, and thus the emitter of Q1 will be a diode drop below that...
But the emitters are connected together, so who wins? LTSpice tells me the voltage is fixed at over 20 while the circuit is supposed to be an adjustable 20V supply.
And as a separate follow-up question, why couldn't you just remove Q1 and run the voltage divider straight to the base of Q2 to get your 0-(20-diode drop)V?
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