# Bandwidth

Thread Starter

#### racerx

Joined Jan 8, 2005
7
Hi,

Having some problems with a class C amplifier. Well not so much the amplifier just the 'tank' part of it in the collector. I need to calculate the bandwidth of this circuit, it comprises of a 50mH inductor in parallel with a 0.22uf capacitor. Please note that there is no resistance in the circuit. The equations have been for the Q factor which is needed to be able to calculate the bandwidth.All the equations I have come across use resistance in the calculation but this circuit doesn't have any. So this circuit is LC and not LCR like you would normally see.

Someone please help. I 've looked in tons of books and can find no examples.

Thanks

Matt

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by racerx@Apr 5 2005, 01:03 AM
Hi,

Having some problems with a class C amplifier. Well not so much the amplifier just the 'tank' part of it in the collector. I need to calculate the bandwidth of this circuit, it comprises of a 50mH inductor in parallel with a 0.22uf capacitor. Please note that there is no resistance in the circuit. The equations have been for the Q factor which is needed to be able to calculate the bandwidth.All the equations I have come across use resistance in the calculation but this circuit doesn't have any. So this circuit is LC and not LCR like you would normally see.

Someone please help. I 've looked in tons of books and can find no examples.

Thanks

Matt
[post=6719]Quoted post[/post]​
In real world components, with every inductor you get a free resistor. This is due to the inherent resistance of the wire out of which the inductor is made. You can model a real world inductor as an ideal inductor in series with a resistor the value of which is the DC resistance of the inductor.

You can consult the inductor manufacturer's data sheet where you will find the DC resistance of the inductor or you can temporarily disconnect one end of the inductor and measure the DC resistance using an ohmmeter. The DC resistance of the inductor in a tank circuit should always be taken into account when analyzing the circuit. Sometimes it is insignificant. However, in the case such as the one you have described it is the primary source of resistance in your tank circuit.

Hope thie is helpful.

Thread Starter

#### racerx

Joined Jan 8, 2005
7
Originally posted by hgmjr@Apr 5 2005, 08:15 AM
In real world components, with every inductor you get a free resistor. This is due to the inherent resistance of the wire out of which the inductor is made. You can model a real world inductor as an ideal inductor in series with a resistor the value of which is the DC resistance of the inductor.

You can consult the inductor manufacturer's data sheet where you will find the DC resistance of the inductor or you can temporarily disconnect one end of the inductor and measure the DC resistance using an ohmmeter. The DC resistance of the inductor in a tank circuit should always be taken into account when analyzing the circuit. Sometimes it is insignificant. However, in the case such as the one you have describe it is the primary source of resistance in your tank circuit.

Hope thie is helpful.
[post=6732]Quoted post[/post]​
Hi,

Thanks for your reply. I fully understand that an inductor will have inherant resistance, but , as far as I know the equations are looking for resistance in parallel with the tank circuit. Bearing in mind this is a purely theortical investigation, is it possible to ignore the resistance of the inductor in the calculations? If so which equations would be used in Q factor and Bandwidth determination.

Thanks

Matt

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by racerx@Apr 5 2005, 12:13 PM
Hi,

Thanks for your reply. I fully understand that an inductor will have inherant resistance, but , as far as I know the equations are looking for resistance in parallel with the tank circuit. Bearing in mind this is a purely theortical investigation, is it possible to ignore the resistance of the inductor in the calculations? If so which equations would be used in Q factor and Bandwidth determination.

Thanks

Matt
[post=6733]Quoted post[/post]​
What if you go ahead and include a resistor and then analyze the resulting expression as the value of the resistor approaches infinity in the limit. This assumes you have a knowledge of calculus.

#### David Bridgen

Joined Feb 10, 2005
278
If the homework presents a purely theoretical but practically impossible question I think your answer should follow suit, vis:

The circuit has infinitely high Q and infinitely narrow bandwidth.

#### mozikluv

Joined Jan 22, 2004
1,435
Originally posted by David Bridgen@Apr 5 2005, 08:10 PM
If the homework presents a purely theoretical but practically impossible question I think your answer should follow suit, vis:

The circuit has infinitely high Q and infinitely narrow bandwidth.
[post=6743]Quoted post[/post]​
i totally agree with "david" and i might add that at resonance the impedance is theoritically infinite with ideal components (is there such thing?)

the basic formula at resonance for series or parallel connection is fo = 2pi * sq. root of LC

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