Re: gEDA-user: PCB footprints

["Harold D. Skank" <skank54@xxxxxxxxx>]

Harold Skank here.

I may have missed the point of your original query about dual gate
MOSFET's, but I just pulled up the URL listed below and found an entire
page of them listed.  I didn't bother to check, but it was a Vishay-
Siliconix page, so I expect that SPICE models would be included as well.
You should be able to tell a good deal about them from that.

URL:

	http://www.vishay.com/transistors-rf-af/dual-gate-si-mosfet/

	Harold Skank

On Tue, 2005-08-30 at 10:23 +0200, Wojciech Kazubski wrote:
> >
> > Their description is somewhat opaque, but one gate biases the device and
> > serves as the input (it's grounded thru an R so it's biased to Idss).  The
> > other gate is just a shield because it is 'biased' way higher than the
> > other gate so therefore doesn't aid in channel conduction, but does get
> > grounded thru a large cap.  It's is a capacitive shield between the drain
> > and the signal gate because of the way the part is made, like a tetrode in
> > this regard.  How to design the bias voltage on the extra gate, I'll leave
> > to Karel.
> >
> The dual gate MOSFET can be treated as cascode connection of two generic 
> MOSFETs. The second gate is usually biased 4V above the source. Reducing this 
> voltage makes smaller gain, so the AGC can be applied. Above 4V the gain 
> usually saturates.  Gate 1 is usually kept at DC ground for easy bias but 
> sometimes is biased positive a bit and a source resistor is used to control 
> the DC drain current. This is often found in mass produced circuits as TV 
> tuners since MOSFETs have significant spread of IDSS current. The source 
> resistor must be bypassed for RF by a capacitor.
> The source resistor (60-200om) can also be used with gate 1 at ground. this 
> reduces the current consumption and allows to tune the oprerating point for 
> minimum noise.
> Classic MOSFETs operate best from relatively high voltage (7-12V). Philips 
> invented a family of low voltage (5V) MOSFETs but they are biased in 
> different way.
> 
> When designing the RF amplifier, please remember for good shielding between 
> input and output circuits. Due to high gain they can easily oscillate due to 
> parasitic coupling.
> 
> Wojciech Kazubski