It works... but I don't know why!

[nee]

Hi All,

Been fooling around with bits and pieces of circuits and put this together, BUT... I made a mistake when reading a schematic! The 100K resistor on the +9V should have been only 100 Ohms. Thing is, it sounds good as it is, and if I do put in a 100 Ohm there is no output. There's also no output if there's no resistor there at all. Can some kind soul take a look and tell me why that is?

Many thanks!

[gez]

With the source resistor bypassed, gain is the transconductance of the device multiplied by the drain resistor.  In short, the value of the drain resistor determines the voltage gain.  Make it too small and bugger all happens.  Although increasing the drain resistor to a large value will increase the voltage gain of the circuit, it should be remembered that it will also increase output resistance and it may affect biasing adversely.

Are you sure the drain resistor's value wasn't 1K and not 100R?  The latter would be a pretty weird value to pick.

[gez]

Whoops, didn't see that the drain had two resistors (scratch the above).

Do you have a multimeter?  Out of interest, I'd like to know where the drain biases with the 100R in place (do before and after measurements if you can).  Plus source measurements if you can.

My guess is that the MOSFET is being pushed into saturation with its gate held at V+ (or as good as) and that transconductance drops right off.  Although that doesn't sound too convincing either...are you sure it's wired up right? 

[gez]

Ah, I see.  The 100R was part of an RC filter - the way it's drawn confused me.

My guess is that you have something miswired.

Incidentally, you can increase the value of that 100K from V+ (via the 100R) to the gate of the MOSFET.  Unless high input impedance isn't an issue here?

[JDoyle]

That's weird, because that type of biasing is pretty common, it is a type of voltage feedback, but to be honest:

YOUR schematic makes sense, how you describe the one you 'screwed up', doesn't.

Consider it Mother Nature shining down her love on you!

Regards,

Jay Doyle

[nee]

That's weird, because that type of biasing is pretty common, it is a type of voltage feedback, but to be honest:

YOUR schematic makes sense, how you describe the one you 'screwed up', doesn't.

Consider it Mother Nature shining down her love on you!

Well, I did consider that, Jay! I plugged it in and was immediately impressed by its potential. However, I will take a few readings and post them, if it will be enlightening.