FET Amplifier Output Distortion

Question

I am new to electronics and still in learning phase. I have made an FET pre-amplifier (using n-Channel JFET) for electret microphone. The output distorts when input signal crosses 200 mV. The top of the positive half-cycle is cut abruptly giving flat top and negative half-cycle shifts upward (towards the axis and then cuts off like positive half-cycle. I'm including the schematic of the amplifier. Please, advise what did I do wrong? Thank you EDITED: As per the demand of Mr. S. Jonathan, I am attaching simulation oscilloscope graphs of both normal and distorted outputs. The violet graph is of output and the cyan is of input.

Answer 1

Please, advise what did I do wrong?

I remember doing this, in my 3rd year of college when I was being taught all about small-signal circuit design.

What you did was to fail to understand just how small a "small signal" is. All three-terminal amplifying elements (tubes, junction transistors, FETs) are nonlinear devices. Some tubes (low-mu triodes in particular) are much better, but given that you can fit a pair of high-quality earbuds inside a 12AX7, we can ignore them for this discussion.

What happens -- with a FET or a BJT -- is that if you try to amplify anything but a really small signal in the common source (or common emitter) configuration is that when the p-p gate-source voltage gets to be more than 10mV or so (or the p-p base-emitter voltage exceeds 1 or 2 mV), you start seeing significant distortion in the drain (or collector) current. (With the appropriate triode you can actually make a pretty good preamp circuit like yours -- but, obsolete, etc., etc.)

It's a really good exercise to use a large signal model of your transistor to see this - or just build your circuit in a simulator and look at it's output.

What you need to do is to use feedback to linearize your circuit. This will rob you of gain, so then you need to use more stages.

You'll find that designing audio circuits is as much of an art as a science. I strongly suggest that if you want early success you dig around the web for schematics (or even find some books on audio circuit design) and shamelessly copy the circuits that you find. Experiment with them to see if you can tweak them into something better, or use your preferred components, or whatever. Then study circuit design and apply what you've learned to making your audio circuits better.

Answer 2

my worries start if I add a second stage in cascade to first

The 2nd stage should not be a FET stage, but something else that won’t have problems with larger signals. Although, since the voltage gain of the 1st stage is relatively tiny, this wouldn’t be a problem.

The 1st FET stage doesn’t have much voltage gain - about 2x maybe. But it does provide a much lower impedance output. The 2nd stage can be an op-amp, or a BJT.