Casscode Curent source

[ppl]

I am using a CRD (Current limiting Diode to Bias an opamp into class a and i wish to use somthing better than a resistor in series with the diode to buffer the capacetence of the Current source and i was considering a cascode type like, as Detailed in the Burr Brown AB-007 Note. Has any one done this and what results did you get. I would like to keep the Drop-out Voltage of the curent source as low as i can since the Amp is battery operated. TIA.

 

[Nelson Pass]

Are you trying to isolate the active gain stages
from the current source's capacitance per se, or
are you trying to make the current source more
constant?

 

[ppl]

Yes i am wanting to isolate the Current sources Capacitence. I am at present using a resistor to do this. Since this Current source is to just Bias the Gain-stage into Class A I think Accuracy may not be required but let me know if you think it should be.

 

[Nelson Pass]

It's up to you whether you want to do it or not.
I'm sure it will work fine, but the DC drop across
the resistor will tend to limit its range, so you
will want to keep that fairly low, say a couple
of volts.

 

[ppl]

Thank you for your reply. I am using a 1k resistor in series with a 1N5298 Current regulator Diode to Supply about 1.0 Ma of constant Current into the Opamp's output stage. The Opamp is driving the Input of a Open loop Buffer.My concern is that at large AC Voltage swings the CRD will drop out and Thus create more Distortion that the Class A Biasing would eliminate. So far the CRD and a 1K resistor works right up to the point of Clipping.I am also concerned that 1K may not be large enough to isolate the Capacitence.

This is why i am considering a casscode aproch to the Problem since this Topology has low Capacitence to start with and the Capacitence is more or less Constant with changing Voltage. TIA PPL