# Need help on-parallel resonant circuit vs. bandpass filter!

These two types of circuits, parallel (LC) resonant and bandpass filter (i.e. some resistance connected with parallel resonant circuit in series). Please tell me when to use which one and what are the advantages of one over another? Thanks in advance.

• If you show us the two circuits we can try to explain the difference (if any). – LvW Jan 13 '17 at 16:54
• 'These two types of circuit ...' What two types of circuit? Draw a diagram (schematic, use the fine editor) or you're wasting our time, and yours for that matter. – Neil_UK Jan 13 '17 at 22:15

You have a misconception. Just like you try to compare "when should I use a wheel and when a vehicle - what advantages and disadvantages they have over each other?"

Offtopic: the difference may sometime have been quite small

http://images.tcj.com/2012/03/Hart-9-650x937.jpg

A parallel resonant circuit can be a part of a bandpass filter. Bandpass filters can be realized with or without resonant circuits just like vehicles can havve wheels, but for example a hovercraft doesn't.

Parallel resonant circuit added to be in parallel with a load and feeded from a signal source with non-zero internal impedance was the simplest form of bandpass filtering that was selective enough for separating different radio stations. That historical fact has made it important and was a reason for serious theoretical and empirical investigations on how the basic circuit should be built and how the circuit can be developed for better performance.

Today we have the circuit theory and we can calculate or look up the tables to find, if 2nd order bandpass filter is enough for a filtering purpose. If the answer is yes, then a resonant circuit can be ok choice. A single resonant circuit fulter definitely is not enough, if steep passband slopes are needed and the wanted bandwidth is not small when compared to the passband middle frequency.

Most practical limitations of the LC resonant circuits are caused by the non-ideal coils. They can be too bulky, have too much resistance or the needed coil is too small to be mostly a coil, not a complex composition of capacitance, inductance, resistance and working as an antenna.

Just a parallel LC can theoretically be used without a resistor if the input signal is from a current source. What you will find is that at resonance, the voltage continues to rise to infinity because there is no "loss" to dampen the circuit down.

But there are always losses (such as in the inductor). Its DC resistance is usually enough to limit Q to maybe between 10 and 100 and this will mean that with a small value AC current source at LC resonance, the resulting voltage will be a few volts. Off resonance and the resulting voltage will be much lower. By how much is determined by Q: -

$Q = \dfrac{f_0}{f_2-f_1}$ so high Q means a very peaky response.

With a resistor feeding the LC, the response is similar. This time the source is presumed to be a voltage source and the series resistor connects this to the parallel LC circuit. At resonance the output voltage will nominally equal the input voltage and, off resonance the output voltage will be smaller: -

Again, Q factor plays it's part but this time Q is mainly due to the value of the series feeding resistor and somewhat from other circuit losses.

Please tell me when to use which one and what are the advantages of one over another?

I can only tell you what I've told you because if I were to guess the different scenarios which you might be considering I'd be here all day.

For resonance, you must have a closed path for the energy to circulate.

simulate this circuit – Schematic created using CircuitLab

The left circuit is SUCKOUT; the right is "band stop"; the left circuit has 50 ohm damping; the right has 200 ohm dampening