Inductors on transistor circuits

[sonaiko]

I kind of have no problem understand the usual transistor circuits that contain only transistors and resistors.

But I start confusing things once I see inductors connected to the transistors as it is the case with LNAs.

If I have a simple common source MOSFET circuit that has inductor connected at the drain in series of the drain resistor, how would that change things? In DC things wont change for sure, but the AC analysis would change but I am not sure how. In simulation, this would increase the gain of the amplifier but I could not figure out why.

Also how about connecting inductors at the source, or the gate of the transistor.

 

[Prototyp_V1.0]

. . . . In DC things wont change for sure, but the AC analysis would change but I am not sure how. In simulation, this would increase the gain of the amplifier but I could not figure out why.

Because the impedance increases with frequency.

Also how about connecting inductors at the source, or the gate of the transistor.

How? Upload picture 8)

 

[snafflekid]

The impedance increases with frequency. And that is good if you want high gain at a high frequency. In RF you are typicaly operating at a particular frequency and you set the inductor to give you the need gain

In theory you could just use an equivalent resistor except that:

1) it may be totally impractical resistor size to integrate, but the needed inductor size may be easy to integrate.
2) the inductor will be noiseless, but the equivalent resistor will be noisy
3) the small resistor allows you to set a lower dc gain and adding the inductor allows a much higher ac gain at the same time.