Friday, April 15, 2011

Mini Audio Amplifier circuit by BC559-BC549

This is very small audio amplifier circuit which can be handled just with 1.5V DC battery. This circuit just need very small power to work.
The maximum output current is IP=VP/RL=62,5mA. If the transistor beta is B=150 then the maximum base current is about 400µA. This is quite much for an amplifier input so a preceding stage is very useful. IC designers normally would even build a 3 or 4 stage amplifier (and e.g. a 20 transistor bias circuit), but our PA should be as simple as possible. For maximum power gain in a direct coupled (best for low component count) amplifier the common emitter configuration is best suited, and a CB or CC stage is much worse. At low voltages a CC also has the disadvantage that the output voltage smaller than the input voltage by VBE. If you want to drive a CE stage from a CC stage you need an input voltage of 2·VBE»1,4V, which is nearly impossible form a single cell supply. Two CE stages are much easier to connect, but this makes feedback more difficult because the total amplifier will be noninverting. A solution will be a differential pair which has nearly the same characteristics like a single CE stage but features an inverting and a noninerting input. A further advantage is that the inherent temparature compensation makes the biasing more stable.


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