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Old 6th April 2014, 06:56 PM   #1
ammel68 is offline ammel68  United States
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Default Transistors in Buffer Circuit Running Too Hot

I built the following buffer circuit using BC550B/BC560B devices. After powering the circuit up for about 30 seconds, the transistors are all too hot.
Is this because they aren't biased correctly or am I using the wrong transistors all together?
The circuit doesn't state which devices to use.

Thank you...
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Old 6th April 2014, 06:59 PM   #2
Mooly is online now Mooly  United Kingdom
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Q3 should be NPN, and the emitter and collector are drawn reversed.
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Old 6th April 2014, 07:01 PM   #3
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Q3 is shown to be a NPN in the drawing and that's what I used. So, I need to reverse the emitter and collector connections?
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Old 6th April 2014, 07:17 PM   #4
Mooly is online now Mooly  United Kingdom
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Yes, the emitter should go to the 47 ohm. I wouldn't like to guess how well it will work though, its a bit of an unlikely looking circuit at first glance. Also your transistor/s may have suffered as a result of the problem... just try it though, it will either work or it won't
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Old 6th April 2014, 07:17 PM   #5
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No, Q3 in your drawing is a PNP (Emitter arrow pointing toward the Base), you need a NPN (Emitter arrow pointing away from base), and yes, as the output stage is an emitter follower, the emitter for Q3 should connect to R4. If you have stability (oscillation) problems after you make those changes, try putting a 1 kOhm resistor between the input and the bases of Q1 and Q2.

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Old 6th April 2014, 07:19 PM   #6
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by ammel68 View Post
Q3 is shown to be a NPN in the drawing and that's what I used. So, I need to reverse the emitter and collector connections?
Q3 is drawn as an PNP. Q1 is drawn correctly as an NPN. Look at the way the arrow points on the emitter. It points out for an NPN.
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Old 6th April 2014, 07:38 PM   #7
ammel68 is offline ammel68  United States
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Sorry, I wasn't actually looking at the symbol, but at NPN on the diagram.

Ok, I switched the emitter and collector on Q3 and the circuit does work as far as audio going through it.

Problem now is I can't get the DC offset below .170V with adjusting the trimmer. I didn't have a 10K trimmer, so I took a 5K and added 6.8K ohm resistors in series on the end pins connecting to the power supply rails.
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Old 6th April 2014, 07:42 PM   #8
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Quote:
Originally Posted by Michael Bean View Post
No, Q3 in your drawing is a PNP (Emitter arrow pointing toward the Base), you need a NPN (Emitter arrow pointing away from base), and yes, as the output stage is an emitter follower, the emitter for Q3 should connect to R4. If you have stability (oscillation) problems after you make those changes, try putting a 1 kOhm resistor between the input and the bases of Q1 and Q2.

Mike
Thanks Mike for the suggestion. I went ahead and put a 1K ohm resistor in series with Q1 and Q2.
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Old 6th April 2014, 08:31 PM   #9
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That circuit is just old National Semiconductor LH0002.
http://www.ti.com/lit/an/snoa725a/snoa725a.pdf
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Old 6th April 2014, 09:12 PM   #10
ammel68 is offline ammel68  United States
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That old National circuit does look very similar.
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