All EEs check in please

[Jassi]

Alright, here is my problem. I was making an on-chip RLC filter (2nd order only) but my prof. wants me to take out the resistor and measure the output accross the LC portion of the circuit. I know there is some internal resistance so my LC circuit is not a perfect LC circuit but my textbook says (and my common sense says) that such a circuit will be a band-stop filter.

He then mentioned something about resonance (@ 13.56 MHz - our chosen frequency) and I lost him there. So, my question is, what sort of application can I use a LC circuit for? Other than a band-stop filter. Any and all legit ideas are welcome 🙂

 

[RaynorWolfcastle]

LC filters have many applications, the most common of which is to act as a "tank" which is essentially a bandpass filter. These are often used as tuned loads in an amplifier; if you wanted to make a radio receiver for example, a tuned load is something you want because it means that your amplifier will only work for your target bands while other frequencies won't get amplified.

Remember, you can make a second order low-pass, high-pass, band-pass, or band-stop with two elements. The kinds of filter you actually end up with depends on the topology you choose for them. In the case of a band-pass filter, your important parameters are the resonant frequency of your tank and its quality factor (look up Q-factor in your book if you're interested in more details). If you use ideal L and C models, you'll end up with an infinite Q factor meaning that you have no resistive losses in the filter. The truth of the matter is that for most real situations this is wishful thinking as inductors, by the way they are constructed, will usually have a dominant parasitic in the form of a series resistor.

To make things even more complicated, creating good on-chip LC tanks is usually quite challenging, because on-chip inductors quality is low and there's not that much you can do about it. Specifically, on-chip inductors are usually just a square or octogonally wire winding, so as you can imagine that doesn't make for an excellent inductor. On top of that, you have all kinds of parasitic effects to worry about in the real world, and these become increasingly complex as you look at higher frequencies.

I hope that this explanation helps you out a bit 🙂.

 

[Jassi]

I'll need some time to absorb all of that but my literature research in the field of on chip inductors looks very promising. We have managed to design a 0.789 micro henri inductor will good properties and low internal resistance. Since we are in the MHz range, the frequency response is pretty good. I have been told that in the GHz range is where inductors act funny 🙂

Thanks a lot for your help, 1 final question - Give me an example of a 2 element band-pass filter. All the ones I have seen are 3 element 🙂

 

[RaynorWolfcastle]

789 nH is HUGE for an on-chip inductor, I'd be curious to know the dimensions and Q-factor of the inductor as well as what process you used. From circuits I've seen, you almost never see anything above above 10 nH. My studies are more focused on RF and microwave circuits (GHz and higher) though, so it may be different at low frequency.

Anyhow, you can use one of four L networks to generate a bandpass response (two each L-L circuits and two C-C circuits). The reason the circuits you find are three element circuits is that with two elements you can't select the bandwidth of the filter, so you add in a third element to give you another degree of freedom that allows you to select the filter's selectivity (its 3-dB bandwidth). Again, if you look at the transfer fuctions, you can see where this comes from. You can find these kinds of low order circuits in matching networks.

I should also clear up something from my explanation of the LC tank in amplifiers. As I said above, the LC tank only acts as a tuned load; meaning that it is a high impedance at some frequency and low impedance at other frequencies. If you place this node at the collector (or drain) of a transistor, this will create an amplifier that is effective only in one band and so the amplifier will only have gain in the a restricted band. This gives the bandpass-like response I was talking about.

 

[TuxDave]

Originally posted by: Jassi
I'll need some time to absorb all of that but my literature research in the field of on chip inductors looks very promising. We have managed to design a 0.789 micro henri inductor will good properties and low internal resistance. Since we are in the MHz range, the frequency response is pretty good. I have been told that in the GHz range is where inductors act funny 🙂

Thanks a lot for your help, 1 final question - Give me an example of a 2 element band-pass filter. All the ones I have seen are 3 element 🙂

2 elements meaning only two types of devices or 2 elements meaning only use 2 devices?