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| Simple Audio Amplifier |
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| braddrew0:
Thank you, these are exactly the kind of things I wanted to learn from this :) Can you recommend a suitable SMD transistor instead of the MMBTA05/55? I found these through a Digikey search for ~60V (to give a buffer on the +/- 20V), minimum 500mA and SMD (most were SOT-23). Should I search for something that can handle higher current? |
| Kleinstein:
For Q9, Q10 I would consider a slightly larger form factor, like SOT223 or SOT89. So something like BCP56/53 or similar. These transistors see some 6 mA from R15 and maybe up to 20 mA from the base current. This may be still just inside the MMBTA05 specs, but at the limit the transistor would be really hot inside and this has a negative effect on thermal stability. The limiting value is also valid for quite some copper area. The limiting factor is not the current rating, but more power rating, possibly even the SOA. 60 V voltage rating is good, though other transistors (except Q6,Q8,Q9,Q10) could get away with a lower voltage rating. |
| braddrew0:
OK, I've updated R4 to 1k (from 100R), R14 to 470R (from 100R), R13 to 470R (from 100R) and the output transistors to 0R47 (from 0R22). Back to one LED as well. Q9 is now BCP56 and Q10 BCP53. Is there any benefit to running BCP56/53 on the other transistors in the audio path (Q1, Q5, Q8) or even for just all of them except Q11/Q12? Thanks again :) |
| Kleinstein:
Using larger transistors where not needed also has a downside: they have more internal capacitance and thus tend to be slower, especially when used at a relatively low current. So it's better to have the small transistors for Q1-Q8 (Q7 is not critical). There can be a slight advantage with large transistors as the noise might be lower, though often not specified. Noise would be an important factor for Q1 and Q5. For Q7 the larger transistor has a chance to get a lower U_BE and thus more temperature effect, which can be a good thing - it's preferable to have the standing current to go down with temperature and not up. Chances are the difference is small. |
| braddrew0:
Thanks, that's a great explanation! I'll check back in when it's made :) |
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