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Old 1st May 2012, 03:25 PM   #1
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Default Current Mirror emitter resistors smoking

Hi everyone,

I have built my own class-B audio amplifier and in simulation it looked good. However in testing, when I connected it up and switched the power on, the resistors at the emitters of my current mirror were smoking and I don't understand why.

I tested first with no load and no input, just to see what the dc conditions were like. It is a 100W amplifier and I am using +- 50V for the supply. Does anyone know any reasons why these resistors are getting too hot?

Thanks
Michael
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Old 1st May 2012, 03:54 PM   #2
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Also I have used 2N2369 transistor for the current mirror. Would this be a cause as I believe they are small signal transistors.
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Old 1st May 2012, 03:55 PM   #3
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Without a schematic we cannot offer any kinda help as we have no way of knowing what circuit you used.
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Old 1st May 2012, 04:13 PM   #4
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Sorry, I have attached the circuit diagram now (I hope)
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Old 1st May 2012, 05:17 PM   #5
wg_ski is offline wg_ski  United States
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It would be hard to believe that those resistors are smoking if the DC conditions are right. If the LTP were in avalanche (perhaps the input pair is in backwards?), or if the tail current is WAY high it could, but there would be more smoke than just that. How hot is R7?
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Old 1st May 2012, 07:44 PM   #6
jaycee is offline jaycee  United Kingdom
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2N2369, apart from coming in a TO-18 case, have a Vceo of 40V. Rail to rail, they could see 100V. They have probably failed short.

Consider using 2N5401/2N5551 for your small signal parts. Also your VAS and it's current source transistors Q6 and Q12 need to be of a higher rating. MJE340/350 work OK and are cheap.
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Old 1st May 2012, 08:15 PM   #7
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by jaycee View Post
2N2369, apart from coming in a TO-18 case, have a Vceo of 40V. Rail to rail, they could see 100V. They have probably failed short.
They're diode-connected. Not likely to see Vcc unless something else is badly wrong. The LTP and the current source have to stand off rail voltage. And even then it wouldn't go much more than one Vcc.
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Old 1st May 2012, 09:00 PM   #8
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R7 may well have got hot but after R3 and R4 started smoking I disconnected the supply and only felt if the current mirror was hot (it was warm). I've never used current mirrors before but from books that I've read the transistors I used should be fine, unless there is a fault in one. The circuit drew a low current from the supply with no load or signal, which from my experience normally means the circuit is connected ok. I'm just struggling to see why so much current is going going through these resistors to make them smoke!
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Old 1st May 2012, 09:08 PM   #9
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Something must have gone awfully wrong there in any case. Maybe a dead Q11, or the layout not conforming to the schematic.
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Old 4th May 2012, 01:11 AM   #10
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Thanks for your replies. I replaced Q9 and Q10 with new transistors and it works fine now.
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