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# finding circuit elements values

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##### Full Member level 2
hello
I read a paper about active inductor circuit and now i decide to simulate its circuit but the paper doesnt have elements value and transistor ratio and now I want to find these values , can help me from which equation or equations I can calculate the best ratio of transistors?

Active inductor...

That's the op amp circuit that mimics inductor behavior. It uses a capacitor and a few resistors.

The same as, or nearly the same as a gyrator, or C-gyrator.

It can duplicate some but not all features of an inductor.

I've never seen a circuit built around a transistor that does this.

you can see in this picture the circuit of active inductor (its basic circuit)

Only circuit b would exhibit a time constant. It has a capacitor.

Not sure why the other 2 circuits are included in a diagram called 'active inductor'.

The components labeled with letter 'M' suggest they're mosfets. Probably n-type enhancement mode.

'Z in' suggests an output which can sink current, but not source current. Attach a pullup resistor to obtain voltage swing.

These do not seem like working circuits to me. Are those current sources at the upper supply node? Current limiters? Current meters? Looks like something in a textbook. To illustrate the concept. But a lot has been omitted.

If you simulate these, apply pulses at input Vb. Component values are not critical. You can experiment. Start with a 1uF capacitor in parallel with a 1K resistor.

To see simulated inductors using op amps, you can search on 'simulated inductor'. I see a circuit diagram in my book which reverses the inverting and non-inverting inputs. Consider trying both configurations.

You should calculate the impedance looking into that node with the help of small signal model. You will get the value with the form of X+sY, in which s=jw. What you should do is to chose the parameter of the circuit elements to make sure Y is positive, which acts like the inductor device.

threekingtiger: you tell correctly I put small signal model of transistors ofcourse with its parazitics capacitors and then calculate Zin impedance that it is like LS, but I do it in teory and in the parameteric way (for Cgs and ...), but my question is when I want simulate this circuit , how I know the optimum W/L of transistors?
(I use 0.18um technology in ADS)

I suppose you can connect a one-volt magnitude AC voltage source with the point that you wanna look into and perform the AC simulation from 1Hz to 1GHz. Then check the AC current flowing out of the AC voltage source. If it is a curve that decreases as frequency becomes higher, then you get your inductorl like circuit. According to the current value of the current-frequency curve indicates at every frequency, you can adjust your device parameters to obtain the exact impedance that you want.

Points: 2
@oermens: thanks for your reply , I have this ebook and read it but my problem is in simulation and choose w/l ratio of transistors

@threekingtiger: its good idea, your mean is I sweep the w/l of transistors and find the best ratio, I dont know that are you familier with ADS or no? it has a block as S-parameter simulation whats your idea about that? use S-parameter instead of AC-simulation?

Well, honestly speeking, I am not familier with ADS, though it's a excellent tool for simulating RF circuits. However, the method that I provided for you can be realized in Cadence or Hspice. But I think ADS should have the ability to perform this job too. Just have a try.

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