Created by
Created November 13, 2011
Last modified June 08, 2017
Tags frequency-domain   resonance   rf  


A square wave into a resistor, inductor, and capacitor can produce ringing and narrowband amplification


At low frequencies, L1 looks like a short circuit, and C1 looks like an open circuit, so V(in) and V(out) are closely linked by R1. At high frequencies, L1 looks like an open circuit and C1 looks like a short circuit, so V(out) approaches zero. But in the middle range of frequencies, something interesting happens.

In fact, for a narrow range of frequencies around 1MHz, this circuit actually produces a tremendous amount of voltage amplification!

A few things to try in CircuitLab

This circuit highlights several cool features of CircuitLab simulations that you can quickly try. Load this circuit with the "Open in editor" button above, and then click the "Simulate" tab to get started.


We've configured V1 to put a square wave into the input, and if we plot the output, we can see a tremendous amount of ringing in the output. Try changing R1 and seeing how it affects the shape of the ringing! Also adjust L1 and or C1 to tune the exact frequency of the resonance.


Run the frequency domain analysis to see a sharp peak of about +36dB around 1MHz. That means that a signal around 1 MHz will get amplified by a factor of about 63x (10^(36/20) = 63)! That's a lot of gain, all from a passive circuit. That's why this kind of inductor/capacitor combination is often found in radio equipment, as it can select a narrow band of frequencies and amplify them without amplifying the rest of the spectrum.


ahahah sehr gut!

by pietrod21
June 10, 2012

What is creating the total resonance? Why would 1MHz be some universal frequency spike? Is there some molecular resonance in copper, mass of copper, cross sectional area, length of the copper conductor?

If all components are tuned to 20kHz, can the same 63x gain also be achieved?

Is this a special case for 1MHz? Is this a 'ring' constant in nature?

by mythhealer
October 01, 2012

How can i change the preassigned values of R,L,etc..? please help.

by sabya
May 10, 2013

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