Re: gEDA-user: PCB footprints

[Wojciech Kazubski <wk@xxxxxxxxxxxxx>]

>
> 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