I made an amp with a transformer on the output which works well but I thought I would build one without the transformer too. The problem is the common emitter low Z amp requires more current than I care to draw from my battery. The best choice would be a common collector circuit. It would look something like this:
When a circuit like this was asked about on the radio board the board experts declared it was a power hungry waste of time and money. To test the theory I will display the circuit current draw and power gains. First the instructor said,"set the DC circuit values and then add the AC components."
I established a current drain of about 1.25ma. I could turn the generator off and see the DC but it's good enough.
In this shot the green trace is the emitter current (the 820 ohm resistor) and the red is the ear bud. Without C2 the voltage drop across the emitter resistor would cancel the input and we would see little gain.
The circuit is producing about 9nw which would drive my earbud to a good volume.
I set the generator to 300hz which is my low frequency audio.
The green is the generator and the red is the earbud current. You can see we have no voltage gain but quite high current gain. Since power is I squared * R, a current gain of 2 would equal a power gain of 4, a current gain of 3 would equal a power gain of 9, etc. This should clearly show the DC and AC should be addressed separately and the circuit can function as a low Z amp without drawing the battery quickly.
The cascode amp using this stage as its output and a voltage amp as its input would be the next step in the progression. maybe next time?
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