Yes I think you had better use a JFET and an adjustable source resistor. With the E102s they sell in USA (made by Semitec; $2.32 each @ Mouser.com), the ratio (Imax / Imin) equals 1.5X! You can do far better with cheap 1% resistors and 5% zener diodes. If you don't mind SMD you can get 1% zener diodes and they are not expensive.
I note in passing that Igate is no larger than (Vz1 / Rs) = 1mA even with the drain not connected. Even the scrawniest little bitty VHF JFETs have Absolute Max Igate ratings of 10mA, so 1mA is nonfatal. But current limiting resistors are cheap so why not fit one and sleep soundly.
_
I note in passing that Igate is no larger than (Vz1 / Rs) = 1mA even with the drain not connected. Even the scrawniest little bitty VHF JFETs have Absolute Max Igate ratings of 10mA, so 1mA is nonfatal. But current limiting resistors are cheap so why not fit one and sleep soundly.
_
Attachments
Last edited:
> I note in passing that Igate is no larger than (Vz1 / Rs) = 1mA
In your setup it is set to 1mA.
Suppose one wants to match J111 at 10mA (e.g. for a cascoded BF852 source follower).
The gate current will then be 10mA at steady state, and perhaps higher in transient with unconnected Drain ?
By adding a diode and a gate stopper, one makes sure any unintended current will go through the protection diode and not the gate.
The JFET will always be protected.
Patrick
In your setup it is set to 1mA.
Suppose one wants to match J111 at 10mA (e.g. for a cascoded BF852 source follower).
The gate current will then be 10mA at steady state, and perhaps higher in transient with unconnected Drain ?
By adding a diode and a gate stopper, one makes sure any unintended current will go through the protection diode and not the gate.
The JFET will always be protected.
Patrick
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.