# buffer circuit before rlc

Discussion in 'General Electronics Discussion' started by bhuvanesh, May 2, 2014.

1. ### bhuvanesh

201
0
Aug 29, 2013
what is the purpose buffer circuit(op-amp).and do we use same buffer circuit for low pass RC circuit or this is only for RLC circuit.thank you in advance

2. ### Harald KappModeratorModerator

10,560
2,354
Nov 17, 2011
A buffer is used to provide a low impedance source to the next stage, in this case to the filter. You can use the buffer with any filter, if the impedance of your source is too high.

bhuvanesh likes this.
3. ### bhuvanesh

201
0
Aug 29, 2013
ya my question is why we need current source(low impedance source) here.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010

A filter imposes a load on the signal that is input to it. If the impedance of that signal is high, the filter's impedance will load the signal down, and the source's impedance may cause a change in the frequency response as the loading may differ with frequency.

Conversely, the load on a filter may need to have a high impedance so it does not load down the output of the filter. This will essentially cause the same problems, lower signal and change in frequency response.

So if you want the frequency response you calculate, you might use a buffer to drive the filter from a moderate or high impedance source, and another on the output if the load is not already a high impedance.

Active filters may reduce the requirement for a buffer amplifier because of the inbuilt gain element.

bhuvanesh likes this.
5. ### LvW

604
146
Apr 12, 2014
A low-impedance source is called "voltage source". A current source has a very large internal source resistance.

5,164
1,081
Dec 18, 2013
Just to clarify. I think Lvw means a constant voltage and constant current source. A battery is a voltage and a current source.

7. ### LvW

604
146
Apr 12, 2014
Yes - correct.
On the other hand, for my opinion, if we speak about a voltage or current source we ALWAYS mean a constant voltage resp. current source, don´t we?
Remember, for example, the classical abbreviations VCVS, VCCS.

I don´t think so. Does a battery deliver a certain current - independent on the connected load? No - it is a voltage source (not ideal, of course, but nothing in electronics is ideal).

8. ### Harald KappModeratorModerator

10,560
2,354
Nov 17, 2011
A battery is a source of current, not a current source - a fine difference.

Arouse1973 likes this.
9. ### LvW

604
146
Apr 12, 2014
Harald - may I ask you: What is the difference?
For my opinion, it is not a "source of current" but a (nearly) constant voltage source. And such a source can, of course, allow a current, which however is determined by the connected load only.

5,164
1,081
Dec 18, 2013
I use constant current term for just that. Other than that I use just current source for somthing that can supply current that is not constant. That's just what I do.
Yes that is what I was trying to say.
Thanks

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
I would say that "voltage source" and "current source" refer to the theoretical constructs. A battery approximates a voltage source for some range of loads, and a current source for another set of loads (of course we would rarely employ those sort of loads on a battery -- where the load resistance is small compared to the internal resistance)

"source of current" really says to me not a lot more than there is a potential difference which can withstand some sort of load without falling to zero. It's a very relative term.

I think the fineness of difference is in the wording, not the meaning.

I think you would say "constant current source", where I would simply say current source. This may be getting into pot-a-to/pot-ah-to

12. ### Harald KappModeratorModerator

10,560
2,354
Nov 17, 2011
I think we all agree thatneither voltage alone nor current alone are vrey useful. Power derives from the product of both. So when we use a current source, power derives from the fact that the currrent develops a voltage drop across a load.
When we use a voltage source, power derives from the current that develops through the load.

Connect a load to a voltage source (e.g. resistor to a battery). Then a current will flow through the load. This current is sourced by the voltage source. The difference to a current source being that the voltage is (ideally) constant whereas the current varies with the load. Still the voltage source is the source of the current, other wise one would have to assume that the current is sourced by the load, which is humbug.

For a currrent source it is vice versa.

13. ### LvW

604
146
Apr 12, 2014
Harald - I agree to all of your explanations regarding the voltage source.
As far as the term "current source" is concerned: This leads to the question if - physically spoken - such a thing like a "current source" really does exist.
For my opinion, it is just a wording to describe a voltage source having a very large internal resistor (large in comparison to the connected load) - nothing else.
Do you agree?

14. ### Harald KappModeratorModerator

10,560
2,354
Nov 17, 2011
To a limited extend, yes.
You can use the same argument and say a voltage source is a current source with a very low internal conductance. Mathematically both are equivalent

We neither have ideal current nor ideal voltage sources. We do have approximations using (mostly electronically) stabilized circuits. Within the operating rangeof these circuits they can approximate ideal sources - up to the point when limiting sets in (current limiting for voltage sources, voltage limiting for current sources).

15. ### LvW

604
146
Apr 12, 2014
Yes, of course - mathematically!.
But - as you see - I had good reason in my last reply to use the comment "physically spoken".
For my opinion, something like a current source can be, of course, defined and matheatically treated a such. However, it was my only intention to point to the fact that a current source does not EXIST in practice. The source of a current is ALWAYS a kind of "charge unbalance" (potential difference) called "voltage" - otherwise no current is possible.

5,164
1,081
Dec 18, 2013
I have always used constant current to determine the fact that the current is constant through a circuit and a source of current or current source as something different. A source of current is just that something that can supply current.

A battery can supply a current to a resistor connected across it so it is a source of current but that does not mean it is constant. Put another way if I said to you connect a resistor to a battery and asked you where was the current coming from.

You would say the battery. And if I said so the battery is providing current then you wold no doubt agree. Then would it be correct to say the battery is the source of this current.

17. ### LvW

604
146
Apr 12, 2014
Three times: No!
I wouldn´t.
In any case I would say: The VOLTAGE provided by the battery is the source (better: is the cause) of current.

5,164
1,081
Dec 18, 2013
I would. That's my opinion and I am intitled to it.