What Q1 and Q4 actually do? - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 12:57:58 pm
content: HiCan You told me What Q1 and Q4 actually do? Is it good or bad schematic?

author: amspire
author_rank: Super Contributor
timestamp: October 03, 2019, 01:08:47 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2019, 01:20:46 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2019, 03:27:34 pm
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: October 03, 2019, 03:53:09 pm
content: 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 VBEto Q4 as possible, given the same current and a similar hFE.

author: dom0
author_rank: Super Contributor
timestamp: October 03, 2019, 04:50:47 pm
content: Circuit building blocksD1+D2, R2, Q1, R3 are a constantcurrent source,which supplies constant current to zeners D9/D10 which provide relatively stablevoltage sources. i.e. D9+D10 provide a stable supply voltage to what is connected to them.Q2, Q3, R5 form adifferential amplifier, with the current source D5,D6,R4,Q4 as acollector 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 asource followerdriven from the output of the differential amplifier (which is taken from either of the collectors, depending on intended polarity).

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 05:02:42 pm
content: THANX A LOTBut at least…is it circuit good or bad?

author: thinkfat
author_rank: Unknown
timestamp: October 03, 2019, 05:04:28 pm
content: Do D1 and D2 also serve as temperature compensation for the reference zeners?

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 05:04:43 pm
content: Quote from: dom0 on October 03, 2019, 04:50:47 pmIs it dangerous? Can You post better design idea?

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2019, 05:46:57 pm
content: The original Duncan's Amp Pages schematic ishttp://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 ofIRF830are 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?

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 07:35:32 pm
content: 250-350v and at least 120mA will be ok