Hi, I must design a 500mW (RMS) AB amplifier to drive an 8 ohm load. So I ended up with this:
I used BCP54 and BCP51 as output transistors, Q3 to drive them both, Q4 for voltage gain, and Q5 for current gain which also adds a lot to input impedance. Input impedance has to be 50k ohm, and voltage gain of 100. I used BCPs current gain to be 60 and BC847Bs current gain to be 100. First thing I figured out is that Vpk of 8 Ohm load must be 2.8V, because: V=sqroot(500mW*8)*sqroot(2), and Ipk = 0.35A. So I decided that voltage swings on R3 resistor must be from 9V to 3.4V, and the Q3 Ic must be from 3.4mA to 9mA. Q3 is biased from Q4, and Q4 is biased from Q5. The problem is that my design doesn't work the way it should if I monitor voltage on the 8 ohm resistor with an oscilloscope.
This is what I get if I use 50mV peak source 1kHz at the input. Sine wave is nice, but there is no enough voltage gain at all (I use A channel).
With 100mV and above peak1kHz, I get this. Again, not enough gain plus saturation. I tried to align it a little bit with the R5 potentiometer (according to the calculations, 100 ohm resistor should be there), but have no luck. So what have I done wrong? Any clues?
Thanks.
I used BCP54 and BCP51 as output transistors, Q3 to drive them both, Q4 for voltage gain, and Q5 for current gain which also adds a lot to input impedance. Input impedance has to be 50k ohm, and voltage gain of 100. I used BCPs current gain to be 60 and BC847Bs current gain to be 100. First thing I figured out is that Vpk of 8 Ohm load must be 2.8V, because: V=sqroot(500mW*8)*sqroot(2), and Ipk = 0.35A. So I decided that voltage swings on R3 resistor must be from 9V to 3.4V, and the Q3 Ic must be from 3.4mA to 9mA. Q3 is biased from Q4, and Q4 is biased from Q5. The problem is that my design doesn't work the way it should if I monitor voltage on the 8 ohm resistor with an oscilloscope.
This is what I get if I use 50mV peak source 1kHz at the input. Sine wave is nice, but there is no enough voltage gain at all (I use A channel).
With 100mV and above peak1kHz, I get this. Again, not enough gain plus saturation. I tried to align it a little bit with the R5 potentiometer (according to the calculations, 100 ohm resistor should be there), but have no luck. So what have I done wrong? Any clues?
Thanks.
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