On Fri, 07 Jul 2006 08:29:54 GMT, David
wrote:

I am going around in circles attempting to understand the datsheet for
the BF998 Dual Gate FET.

From what I can gather, with Vcc = 8V and 4V on G1 and ID of 10mA, at
150 MHz
Input appears to be 1700 Ohms with 2.1pF par
Output appears to be 1250 Ohms with 1pF par

(I thought Fets had hundreds of K ohm input impedance - or is this only
at low Freq ?)

At DC and low frequencies the inputs are effectively open circuits
(very high R) with a small C across it. As frequency increases the
package(leads and all) and that small C combine to reflect
a decreasing impedence.

Also those numbers are for best noise performance as RF amplifier.


The typology I have is.

L/C tuned circuit to G1
100K+100K providing 4V to G2 (with 100n Bypass cap close to gate)

I'd usually use lower value resistors like 10k to ground and 47k to V+
for G2. G1 is returned to ground through tuned circuit L.

I use 12V on the drain.


The source has 100R with 100n Par to ground.
The Drain has the Inductor of the tuned circuit to Vcc (and bypassing on
Vcc side as well as 47R series resistor for additional decoupling).

The tuning cap goes from Drain to ground.

I have about 250mV on the source (only 2.5mV bias current).

How do I obtain 10mA drain current ? Or are they only referring to AC
current ?

As RF amp or mixer? As RF the bias on G1 and G2 must be correct.
For Mixer that would likely not be the case.



Allison


Would someone mind please giving me a bit of overview of these devices.

Thanks in advance

Regards

David