What Q1 and Q4 actually do?

[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 V_BE to Q4 as possible, given the same current and a similar h_FE.

[dom0]

Circuit building blocks

D1+D2, R2, Q1, R3 are a constant current source,
which supplies constant current to zeners D9/D10 which provide relatively stable voltage sources. i.e. D9+D10 provide a stable supply voltage to what is connected to them.

Q2, Q3, R5 form a differential amplifier, with the current source D5,D6,R4,Q4 as a collector load, which isn't quite a good solution as others have pointed out. The DC operating point of the differential amplifier is set by the voltage over D10, thus the output voltage will be approximately U(D10) * (R9/R10+1), i.e. 200 V * 1.25 = 250 V.

M1 is a source follower driven from the output of the differential amplifier (which is taken from either of the collectors, depending on intended polarity).

[001]

THANX A LOT

But at least
…is it circuit good or bad?

[thinkfat]

Do D1 and D2 also serve as temperature compensation for the reference zeners?

[001]

Q2, Q3, R5 form a differential amplifier, with the current source D5,D6,R4,Q4 as a collector load, which isn't quite a good solution as others have pointed out. The DC operating point of the differential amplifier is set by the voltage over D10, thus the output voltage will be approximately U(D10) * (R9/R10+1), i.e. 200 V * 1.25 = 250 V.

Is it dangerous? Can You post better design idea?

[floobydust]

The original Duncan's Amp Pages schematic is http://www.duncanamps.com/technical/mosfet.html
(you can use google translate with the URL, it might help)

Rated 325-500VDC input and 250V output. No mention of current rating.
SOA limits of IRF830 are only pulse rated- no DC curves. Q1 will run hot. I would use a 15V zener for D12.
I would say the circuit is fine, but I would like to see some current limiting and a better mosfet.

What do you need for voltage and current?

[001]

250-350v and at least 120mA will be ok