Can you make a current source function as a high impedance until a certain cutoff frequency, then it functions as a low impedance?. I want this impedance drop to be relatively sharp, not gradual so I don't lose performance in the frequencies I want to keep.
This is the circuit I'm trying to work with.

The current source is working into a DC voltage, only reflecting away the AC currents. I want it to ignore currents above a certain frequency threshold so they get shunted into the supply.
I know as per the usually you guys are going to offer me a different solution than I asked but please just stick to the question at hand.

 

Um, you DO realize your "Ref (in)" and "Ref (Out)" are both tied to ground? Your references can only be whatever potential your ground is. In most standard cases ground is zero volts. No voltage, no current.

 

So? Are you even looking at the schematic?

 

So? Are you even looking at the schematic?

Kinda chippy attitude.
I have and it makes little sense (besides what Tony mentioned).
What's the purpose of the right (unlabeled) transistor?
What's the input to the left (unlabeled) transistor?
What exactly is that circuit supposed to do?

The current source is working into a DC voltage, only reflecting away the AC currents.

What DC voltage?
AC currents from where?
What do you mean "reflecting"?

 

Kinda chippy attitude.

I know, I'm irritated because I asked this question in other forum and it was an uphill battle to get anything resembling a straightforward response to any of my questions.

What's the purpose of the right (unlabeled) transistor?
What's the input to the left (unlabeled) transistor?
What exactly is that circuit supposed to do?

Never seen a folded cascode? It's just a normal cascode that folds back to a lower voltage instead of a higher one. This might actually be a better representation of the mechanism I'm trying to get working

I want U2 to shunt frequencies above 20khz with a low impedance and reflect impedances 20khz and below with a high impedance. I want the cutoff to be reasonably sharp.
The hope is that the currents above 20khz will go into B- instead of the load.
I put C1 there to limit the bandwidth of U2 but it's just not working.

 

Sounds to me like you are talking about a low pass filter...but it's hard to tell.

I'm sorry I can't give you a straight forward answer, because I really don't understand the question.

 

No I want to make the current source act as a current shunt above 20khz and a high impedance below 20khz.

 

Can you make a current source function as a high impedance until a certain cutoff frequency, then it functions as a low impedance?. .

Do you mean that you want a source that is a current source below a certain cutoff frequency, and above the cutoff is a voltage source? That is, relative to impedance of the load, a current source can be thought of as high impedance, and a voltage source as low impedance.

 

Yes. That is exactly what I want but I only need it to handle the AC signal currents. The DC voltage is determined by cascode device. The current/voltage source would ideally be feeding a pure DC line.

 

Sorry, it still doesn't make much sense.
Where is the AC coming from?
What is the current/voltage source?
What is the "pure DC line"?
What is the input and what is the output?

If you explain exactly what input and output signals you are dealing with and what the exact purpose (not function) of this circuit is, it would be a big help.
Your insistence on working with this particular (apparently pet) circuit of yours without stating its exact purpose in your system (not what you think you want it to do) is leading us in circles and a game of 20 questions.
I'm beginning to understand why you had a problem in getting answers from the other forum.

If you don't want to give us any details about its use in your system, then you need to present us with a proper set of engineering specifications including but not limited to:

• Every Input's voltage, current, frequency, source impedance.
• Power available.
• Every Output's voltage, current, frequency, load impedance, as related to each input.

 

I want this impedance drop to be relatively sharp, not gradual

I think that is going to need something more than a simple RC filter. Perhaps a multi-pole filter, or a frequency-to-voltage converter followed by a comparator? What is the required response time of the frequency discrimination arrangement?

 

"Chippy"? Wasn't meant to be. Sometimes pointing out something that has been overlooked sparks a thought of why the circuit doesn't work. Sorry we can't give you the answer you expect. Until you open your mind to the idea that there might be a different way, and possibly better way of achieving what it is you want to do then you're going to continue to meet with resistance. We don't mean to impede your progress but if you don't have the capacitance to tell us what the circuit runs on then the current conditions are not going to change. We can't answer your question with the information you've provided us so far. A little help from you will help us help you a whole lot more. You'll gain a lot with just a little feedback.