RC networks

Thread Starter

bobbyk

Joined Jun 13, 2006
12
In a 3-terminal linear RC passive network for sinusoidal input what is the maximum voltage gain that can be acheived? And in particular, can the voltage
gain be greater than 1?
 

hgmjr

Joined Jan 28, 2005
9,029
Originally posted by bobbyk@Jun 13 2006, 05:19 PM
In a 3-terminal linear RC passive network for sinusoidal input what is the maximum voltage gain that can be acheived? And in particular, can the voltage
gain be greater than 1?
[post=17700]Quoted post[/post]​
A gain of unity is all that you can squeeze out of a passive RC integrator circuit.

hgmjr
 

Papabravo

Joined Feb 24, 2006
12,767
Originally posted by hgmjr@Jun 13 2006, 06:28 PM
A gain of unity is all that you can squeeze out of a passive RC integrator circuit.

hgmjr
[post=17701]Quoted post[/post]​
Passive circuits with real components cannot even have a gain of 1. There is always somne loss.
 

Thread Starter

bobbyk

Joined Jun 13, 2006
12
Originally posted by hgmjr@Jun 13 2006, 05:28 PM
A gain of unity is all that you can squeeze out of a passive RC integrator circuit.

hgmjr
[post=17701]Quoted post[/post]​
Can you prove that? Because I have a passive 3-terminal RC network with a voltage gain of 1.03! I'd sure like to see your proof!

Look, we're asking about VOLTAGE gain (output voltage/input voltage) for
sinusoidal steady-state input. Please reconsider your answer.

bobbyk
 

BladeSabre

Joined Aug 11, 2005
105
An ideal RC network cannot have a gain of more than 1. You need an LCR network for that- I guess a gain of 1.03 could be caused by stray inductance in your circuit.
 

Papabravo

Joined Feb 24, 2006
12,767
Originally posted by bobbyk@Jun 13 2006, 11:33 PM
Can you prove that? Because I have a passive 3-terminal RC network with a voltage gain of 1.03! I'd sure like to see your proof!

Look, we're asking about VOLTAGE gain (output voltage/input voltage) for
sinusoidal steady-state input. Please reconsider your answer.

bobbyk
[post=17706]Quoted post[/post]​
I don't have to reconsider my answer, in fact I stand by it. You on the other hand can believe whatever you want. Let me know when they award you the Nobel. I'll buy you a draft.
 

hgmjr

Joined Jan 28, 2005
9,029
bobbyk,

Papabravo has a good point. In the real world a passive RC network is bound to have some losses and so a gain of one is not actually obtainable. I should have qualified my earlier response as being based on an ideal RC circuit.

Like mozikluv, I think it would be helpful if you could describe how you arrived at your figure of 1.03.

hgmjr
 

mozikluv

Joined Jan 22, 2004
1,437
Originally posted by hgmjr@Jun 14 2006, 06:24 PM
bobbyk,

Papabravo has a good point. In the real world a passive RC network is bound to have some losses and so a gain of one is not actually obtainable. I should have qualified my earlier response as being based on an ideal RC circuit.

Like mozikluv, I think it would be helpful if you could describe how you arrived at your figure of 1.03.

hgmjr
[post=17723]Quoted post[/post]​

hi hgmjr,

quite a long time that i have participated again.

as to the above figures, i would wryly say the best that can be achieved is something like 0.98 all things considered like tolerance and variances and at an ideal condition.

mozikluv

:rolleyes:
 

hgmjr

Joined Jan 28, 2005
9,029
Originally posted by mozikluv@Jun 15 2006, 02:58 AM
hi hgmjr,

quite a long time that i have participated again.

as to the above figures, i would wryly say the best that can be achieved is something like 0.98 all things considered like tolerance and variances and at an ideal condition.

mozikluv

:rolleyes:
[post=17729]Quoted post[/post]​
Welcome back mozikluv.

As to the forum topic, mozikluv's estimate of 0.98 is certainly a reasonable real world upper limit. There are a number of stray components that can contribute to the loss in a passive circuit. bladesabre's mention of inductance is just one example. In addition to stray inductance there is always leakage current in the capacitor.

One thing that should also be considered is the accuracy of the measurement equipment. Digital voltmeters and oscilloscopes all have measurement tolerances that must be considered when using them to make ultra precise measurements. And while on the subject of tolerances, there are also tolerances associated with the resistor and capacitor that make up the circuit. Just because the resistor is marked as a 10K ohm resistor does not mean that it is precisely 10K. A 5% 10K resistor can range from 9.5K to 10.5K. Likewise the capacitor will have a tolerance on it as well. These component tolerance must be considered in the measurement/calculation of the gain of the RC circuit under discussion.

I generally interpret the term "passive" when used to describe an electronic component as a way of emphasizing the fact that the component does not contribute energy to the circuit. If any circuit provides a net increase in energy as in the case of bobbyk's circuit and it is not a measurement/tolerance anomally then it must be coming from some source other than the passive components that make up the circuit.

I look forward to hearing more from bobbyk on the makeup of the circuit and how he obtained the 1.03 figure for its gain.

hgmjr
 

radiosmoke

Joined May 30, 2006
17
Originally posted by hgmjr@Jun 15 2006, 02:40 AM
Welcome back mozikluv.

As to the forum topic, mozikluv's estimate of 0.98 is certainly a reasonable real world upper limit.  There are a number of stray components that can contribute to the loss in a passive circuit.  bladesabre's mention of inductance is just one example.  In addition to stray inductance there is always leakage current in the capacitor.

One thing that should also be considered is the accuracy of the measurement equipment.  Digital voltmeters and oscilloscopes all have measurement tolerances that must be considered when using them to make ultra precise measurements.  And while on the subject of tolerances, there are also tolerances associated with the resistor and capacitor that make up the circuit.  Just because the resistor is marked as a 10K ohm resistor does not mean that it is precisely 10K.  A 5% 10K resistor can range from 9.5K to 10.5K.  Likewise the capacitor will have a tolerance on it as well.  These component tolerance must be considered in the measurement/calculation of the gain of the RC circuit under discussion. 

I generally interpret the term "passive" when used to describe an electronic component as a way of emphasizing the fact that the component does not contribute energy to the circuit.  If any circuit provides a net increase in energy as in the case of bobbyk's circuit and it is not a measurement/tolerance anomally then it must be coming from some source other than the passive components that make up the circuit.

I look forward to hearing more from bobbyk on the makeup of the circuit and how he obtained the 1.03 figure for its gain.

hgmjr
[post=17732]Quoted post[/post]​
Would a magnetic amplifier (or is a Mag-Amp a different beast?) also have no gain other than approaching unity?
Or Can you have voltage gain and current loss, and current gain and voltage loss?
I haven't the foggiest, just a curiosity.
 

kubeek

Joined Sep 20, 2005
5,657
Originally posted by radiosmoke@Jun 15 2006, 11:46 PM
Would a magnetic amplifier (or is a Mag-Amp a different beast?) also have no gain other than approaching unity?
Or Can you have voltage gain and current loss, and current gain and voltage loss?
I haven't the foggiest, just a curiosity.
[post=17744]Quoted post[/post]​
Transformers amplify the mains current, while reducing the voltage. As I remember some microphones used a special transformer to increase the S/N ratio.
 

BladeSabre

Joined Aug 11, 2005
105
Originally posted by hgmjr@Jun 15 2006, 09:40 AM
As to the forum topic, mozikluv's estimate of 0.98 is certainly a reasonable real world upper limit. There are a number of stray components that can contribute to the loss in a passive circuit. bladesabre's mention of inductance is just one example. In addition to stray inductance there is always leakage current in the capacitor.
...
I generally interpret the term "passive" when used to describe an electronic component as a way of emphasizing the fact that the component does not contribute energy to the circuit. If any circuit provides a net increase in energy as in the case of bobbyk's circuit and it is not a measurement/tolerance anomally then it must be coming from some source other than the passive components that make up the circuit.
I did not get the impression that bobbyk's circuit necessarily showed a net increase in energy. The transformer example is a good one. And I was suggesting that the stray inductance may be able to cause a voltage gain of more than 1 (rather than as a source of loss). I'm not sure if that's right, and trying to visualise what would happen, it's coming out very weird. I know it's best not to trust my intuition with this stuff and just do the math. I'll come back to this later.

Edited to add: OK, some math later, I don't think that's going to happen with the tiny values of inductance that you'd find in a circuit by accident. Put a larger inductor in there and the voltage across the capacitor can go above the supply voltage, but not in this case I don't think =P
 

kubeek

Joined Sep 20, 2005
5,657
Originally posted by BladeSabre@Jun 16 2006, 10:01 AM
I did not get the impression that bobbyk's circuit necessarily showed a net increase in energy. The transformer example is a good one. And I was suggesting that the stray inductance may be able to cause a voltage gain of more than 1 (rather than as a source of loss). I'm not sure if that's right, and trying to visualise what would happen, it's coming out very weird. I know it's best not to trust my intuition with this stuff and just do the math. I'll come back to this later.

Edited to add: OK, some math later, I don't think that's going to happen with the tiny values of inductance that you'd find in a circuit by accident. Put a larger inductor in there and the voltage across the capacitor can go above the supply voltage, but not in this case I don't think =P
[post=17750]Quoted post[/post]​
I think it´s just error of measurement.
 

hgmjr

Joined Jan 28, 2005
9,029
In the absense of additional details on bobbyk's circuit and his measurement method, I agree with kubeek and bladesabre that the gain anomaly most likely can be traced to a measurement error.

hgmjr (700)
 

Thread Starter

bobbyk

Joined Jun 13, 2006
12
An ideal RC network cannot have a gain of more than 1. You need an LCR network for that- I guess a gain of 1.03 could be caused by stray inductance in your circuit.


Not true! Can you give a proof or references to your statement? And I don't think a circuit drawn on paper can have stray inductance!
bobbyk
 

Thread Starter

bobbyk

Joined Jun 13, 2006
12
Originally posted by hgmjr@Jun 15 2006, 04:40 AM
Welcome back mozikluv.

As to the forum topic, mozikluv's estimate of 0.98 is certainly a reasonable real world upper limit. There are a number of stray components that can contribute to the loss in a passive circuit. bladesabre's mention of inductance is just one example. In addition to stray inductance there is always leakage current in the capacitor.

One thing that should also be considered is the accuracy of the measurement equipment. Digital voltmeters and oscilloscopes all have measurement tolerances that must be considered when using them to make ultra precise measurements. And while on the subject of tolerances, there are also tolerances associated with the resistor and capacitor that make up the circuit. Just because the resistor is marked as a 10K ohm resistor does not mean that it is precisely 10K. A 5% 10K resistor can range from 9.5K to 10.5K. Likewise the capacitor will have a tolerance on it as well. These component tolerance must be considered in the measurement/calculation of the gain of the RC circuit under discussion.

I generally interpret the term "passive" when used to describe an electronic component as a way of emphasizing the fact that the component does not contribute energy to the circuit. If any circuit provides a net increase in energy as in the case of bobbyk's circuit and it is not a measurement/tolerance anomally then it must be coming from some source other than the passive components that make up the circuit.

I look forward to hearing more from bobbyk on the makeup of the circuit and how he obtained the 1.03 figure for its gain.

hgmjr
[post=17732]Quoted post[/post]​

All your comments on precision, tolerances, etc. are interesting (and well known)
but have no bearing on my question. I'm talking about IDEAL components
and not a MEASURED network but a CALCULATED one!

I didn't claim my circuit had an "energy gain", just an open-circuit VOLTAGE gain.
 
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