Thursday, 26 February 2015

bjt - Power amplifier input source current path


Can anyone draw (or write in text) the small-signal path from signal source XFG1 at input of Q1 base to amplifier load for positive and negative input signal swing?


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Answer



I will try to show you two extreme cases (when clipping).


For the Positive half of a sinusoidal swing:


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And for Negative half of a sinusoidal swing:


enter image description here


This two cases should help you understand what is going on in the amplifier.


Because in the normal operation we have "slightly" similar situation.


For example:



For the Positive half, $Q_1$ reduce his $I_{C1}$ current (just a bit), hence $Q_2$ must increase his $I_{C2}$ current.


But this reduction in $Q_1$ current decreases the $Q_5$ base current also.


And all this means that the $I_{C5}$ collector current is decreasing also.


Hence, the voltage at $Q_{12}$ collector increases. The larger part of a $Q_{12}$ collector current ($I_{C12}$)can now flow into the $Q_7$ base. So, $Q_7$ and $Q_9$ conduct more current. And the load current increases.


I hope this helps you. Because my English vocabulary is limited.


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