Why can't I get any output when a speaker is connected to my common-source amplifier?

Question

I'm trying to build a simple amplifier for my guitar. I created the exact same circuit as a common-source amplifier, I biased it on the necessary Q-point, and AC signals are decoupled.

It worked nicely; I get the amplified guitar output on my osilloscope. The output signal was really satisfying, I get 2 to 10 V V_max, depending on which guitar string is oscillating.

But the problem is that the output signal disappears when I connect a speaker to the circuit. By "disappear" I mean that the signal gets very low and the speaker isn't being driven. When I put my ear on the speaker and play something on the guitar I get a distorted guitar sound which is pretty low-volume, and I can barely hear it.

I connected the speaker right after the coupling capacitor on the drain point. I also tried to connect a 1 MΩ resistor in series with the capacitor and then connected the speaker in parallel with the resistor, but still could't get any results.

What should I do to get my speaker to be driven?

My circuit is like this (when I add a source resistor, the signal also disappears. It doesn't matter if I add the capacitor between source and ground):

Answer 1

What you want is an audio power amplifier, but what you have built is more like a preamp. Let's look at the current path for the positive peaks of a 400 Hz audio waveform. It starts at +9 V and ends at GND. The path is through R3, C4, and the speaker.

R3 = 1500 ohms

At 400 Hz, C4 has an impedance of almost exactly 400 ohms

The speaker is 8 ohms

This is a voltage divider, with R3 and C4 as the series leg and the speaker as the shunt leg. If Vin is 9 V, then

Vout = Vin x 8 / (1500 + 400 + 8) = 9 x 8 / 1908 = 72 / 1908 = 0.0042 V.

That is an output voltage of 4.2 millivolts. That is why the volume is so low.

There are other things going on, such as phase shift caused by C4, but this is a rough guess at how much signal is lost when the load impedance is way lower than the source impedance.

There is such a thing as a 1-transistor power amplifier that can drive a speaker, called a class A amp. In fact that approach is the favorite of a small community of audiophiles. If you really want one, then R3 must be much lower and M1 will need a significant heatsink. A search for

class a amplifier circuit

or

class a amplifier schematic

will bring up tons of schematics and project pages.

Answer 2

You aren't providing any current to the speaker with this design. The current has to go through a 1k5 resistor. The most current that can pass through that resistor at 9.8V is about 6.5 milliamps and you'll actually be getting less than that. You need a low impedance output stage.