# How to get the gain in db against frequency curve for this parallel LC circuit in pspice?

simulate this circuit – Schematic created using CircuitLab

I have been trying to get the frequency response (magnitude of Vout/Vin on y axis and frequency on the x axis)for the circuit shown below, however I am unable to get this. Please could someone help/instruct on how to get the frequency response for this circuit ?, and where exactly do I measure Vout and Vin ?. I have been taking Vout as the voltage across C1 and Vin near the source. Also I cannot add a AC voltage source for this circuit in pspice.

• You need to show your attempt. – Chu Nov 16 '15 at 0:42
• I am unsure as to how can I attach the output stimulation I got from pspice but I have tried a lot. Although I always get a straight line that passes through 0dB when I use this to get the trace DB(V2(C2)/V(I2:-)) (I use this as I want to get the output voltage over the input voltage). any help or suggestions ?. – user63377 Nov 16 '15 at 1:25
• any help please Chu? or anyone else? – user63377 Nov 16 '15 at 2:21
• It looks like you used CircuitLab to draw that circuit, why not use CircuitLab to get the plots you need? – Tyler Nov 16 '15 at 2:27

Not a PSpice user so I can't comment there, but here's how it's done in LTSpice. I assumed you wanted a voltage source instead of a current source as you've drawn it, but your post isn't very clear. The following instructions will also work for a current source.

First, make the schematic, setting the voltage on the source to "AC 1":

Then, create the simulation by going to Simulation -> Edit Simulation Command. I did a sweep from 1 Hz to 1 MHz in 100 pts/decade.

Now you can click the output node to get the Bode plot:

Hope this helps.

Side note: as your circuit is purely reactive, the gain is very large at resonance. This is voltage gain only - this is a passive network and cannot exhibit any power gain. To demonstrate this, try assigning an impedance to the source and loading up the output.

• Thanks a lot this has helped if I cannot get on in pspice I will switch to ltspice – user63377 Nov 16 '15 at 10:21

1.) The bottom of the circuit must be grounded.

2.) You must use an ac source (Vac) for performing an ac analysis

3.) Assign node numbers for Vout and Vin.

4.) After the analysis, display dB(V(out)/V(in)).

5.) What is the purpose of C2? (it does not influence the result in the frequency range of interest)

• Thanks, although I want to use vac I cannot do so in pspice as it always comes up with an error message. Also I need the C2 as I have to vary that capacitance to see when the circuit is over and under coupled – user63377 Nov 16 '15 at 10:23
• I think, it would be helpful to know the text of the error message. Did you ground the lower part of the circuit? – LvW Nov 16 '15 at 10:32
• yes I did ground the lower part of the circuit(forgot to add that in the schematic above), the error message is along the lines of voltage and or inductor loop involving Vv2 add a series resistance to remove the error – user63377 Nov 16 '15 at 11:28
• If C2 is present, there is no inductor loop; perhaps you have shortened C2 unintentionally? Or place a very small resistor 1mohm in series to L. This will not disturb the function remarkably. – LvW Nov 16 '15 at 12:47