Op-amp question

[stlouisembedded]

I have a question if somone could help.

I am supposed to design a filter circuit for school. I have chosen to use an op-amp because i have used them many times on my own.

My question is: the design says the the output impedence of the previous stage will be 100k ohms. I am assuming that my filter needs to match that value to allow maximum transfer of the signal.

So, my op-amp circuit has to have an input impedance of 100k ohms. So do i achieve that by having a 100k ohm resistor at the inverting input (assuming inverting amp)? or do i have a 100k shorted to ground where the signal comes in?

OR... do i not need to worry about impedance matching? Should i be satisfied that the really high input impedance of the opamp will not "load" the previous stage?

Sorry for the long post, any help appreciated.
thanks

 

[ljcox]

I have a question if somone could help.

I am supposed to design a filter circuit for school. I have chosen to use an op-amp because i have used them many times on my own.

My question is: the design says the the output impedence of the previous stage will be 100k ohms. I am assuming that my filter needs to match that value to allow maximum transfer of the signal.

So, my op-amp circuit has to have an input impedance of 100k ohms. So do i achieve that by having a 100k ohm resistor at the inverting input (assuming inverting amp)? or do i have a 100k shorted to ground where the signal comes in?

OR... do i not need to worry about impedance matching? Should i be satisfied that the really high input impedance of the opamp will not "load" the previous stage?

Sorry for the long post, any help appreciated.
thanks

You said "the design says the the output impedence of the previous stage will be 100k ohms". Do you have a circuit of the previous stage or is this a requirement imposed on you by the teacher?

It sounds to me like the previous stage has a current output (hence the relatively high output Z) If so, then input Z of the filter could be low.

Normally it is not necessary to match the impedances except where you want to maximise power transfer. You normally design for a mismatch. For example, if the source has a low Z (ie. a voltage source) then the next stage requires high Z and vice versa.

Len

 

[Russlk]

It is not normally necessary to match impedances in a signal circuit, high impedance load will maximize the signal voltage, which is usually what you want. The problem here is that with high impedance load, the voltage may be so high that the signal is distorted. You can't really do a good design without knowing more about the previous stage. But, assuming the signal does not distort, I would use a unit gain buffer to provide high Z load and low output Z.

 

[Nigel Goodwin]

My question is: the design says the the output impedence of the previous stage will be 100k ohms. I am assuming that my filter needs to match that value to allow maximum transfer of the signal.

Impedance matching is generally done to maximise power transfer, not signal transfer, for your purpose it's a bad idea - it will attenuate the input signal by 50%.

As the others have said, you really need to know more about the source, 100K output impedance sounds pretty silly - if you tell us more about your design it might make more sense. But, in general, audio amplifier stages will have a high input impedance, and a low output impedance.

 

[stlouisembedded]

wow, thanks for clearing up my "Z matching" problem.

Just so you know, the previous stage to my filter is a D/A converter. Outputing the neighborhood of 5kHz.

I am going to go with 2 opamps, one unity gain...followed by the lowpass filter part.

thanks again!