amplifieranalogbjtdesignmosfet
I have a degenerated common source amplifier. I want to know what's the maximum input signal I can apply before the output signal begins to clip.
What I currently do is to increase the input signal until I see in the scope that the output signal begins to clip. It's a trial an error method.
But I would like to know what's the general procedure to compute (by hand) the maximum input signal without clipping.
simulate this circuit – Schematic created using CircuitLab
According to the schematic you included, it is an amplifier with differential output. You have to connect the loudspeaker between pins 5 and 8, not to ground. Both outputs will probably carry a signal half way between 0 and Vcc, with one output going down and the other going up in the rythm of the music. Output 5 and 8 are each others mirror.
As with the speaker, to measure output voltage, you again have to measure between pins 5 and 8, and not with reference to ground. Now with a multimeter, this is pretty easy to accomplish, but with a scope this is probably a different story.
The problem with a scope is that the scope ground may very well be attached to the circuit ground for safety. Either through its own power supply, or the attached music player('s power supply) or any other attached device.
If this is the case, then attaching scope ground to one of the pins6 or 8 will short circuit the amplifier which is ... bad. Most oscilloscopes with at least two channels have the option to add or subtract the two input channels. Use the subtract mode to measure the voltage difference on both outputs of this amp.
That new schematic works fairly nicely. You should be able to click on the circuit below and simulate it. You end up with the output being clipped but with nicely curved corners rather than hard stops at the top and bottom.
simulate this circuit – Schematic created using CircuitLab
Best Answer
Make all resistors 1Meg Ohm. Then go back and increase RG1 to 2meg Ohm. Define Vthreshold as zero volts. Define VDD as 10 volts. You will find Vdrain sitting at 2/3 of VDD or 6.6 volts. Your "linear swing", or swing with some small clipping, will be 6.6volts +- 3.3 volts. Approximately.
This works, approximately, for FETS and bipolars.