grounded-grid MOSFET

[Ray di Tutto]

Hi

I am gathering information about how to build a class-C power amp for
SSTV operation on 20 m.

Some interesting designs have been published using inexpensive MOSFETs.

Grounded-gate triodes are a long-time classic. I never saw MOSFETs being
used in a grounded-gate circuit. I wonder why?

Angel Vilaseca HB9SLV

[email protected]

 

[Stan]

}Hi
}
}I am gathering information about how to build a class-C power amp for
}SSTV operation on 20 m.
}
}Some interesting designs have been published using inexpensive MOSFETs.
}
}Grounded-gate triodes are a long-time classic. I never saw MOSFETs being
}used in a grounded-gate circuit. I wonder why?

See: http://www.qsl.net/ab4oj/quadra/sshfamp.html for a general discussion
of what you mentioned.

Stan.

 

[JURB6006]

You know engineers lost the convenience of building a drive circuit with
practically no current behind it when bipolar came out.

The advent of FETs solved this. The only reason to use the common gate design
is to remove most of the effect of interelectrode capacitance. If it costs you
more to implement this, your are not ahead. Driving the capacitance might be
better than supplying the entire output current, which is what you must do in a
common gate circuit.

You would most likely want to use a coil, whether you apply the signal to the
source or gate, you still must give it enough drive to cause cutoff as well as
the level of saturation you need. In this case you need to supply this to a
much lower impedance.

In the 20m band, a common source circuit might be better with modern devices.
Think about it.

JURB

 

[Asimov]

"JURB6006" bravely wrote to "All" (12 May 04 00:56:27)
--- on the heady topic of "Re: grounded-grid MOSFET"

A trick to minimize input capacitance it to use a split darlington
style with the 2nd feeding back capacitively from its source to the
drain of the 1st fet which is decoupled by a resistance.

Also wouldn't a combination common source with common grid circuit
called a cascode solve each of the problems? For example the input
common source solves the current problem and the following common grid
solves the capacitance problem? With enough current a cascode circuit
still amplifies at many megahertz with minimal input capacitance.


JU> From: [email protected] (JURB6006)

JU> You know engineers lost the convenience of building a drive circuit
JU> with practically no current behind it when bipolar came out.

JU> The advent of FETs solved this. The only reason to use the common gate
JU> design is to remove most of the effect of interelectrode capacitance.
JU> If it costs you more to implement this, your are not ahead. Driving the
JU> capacitance might be better than supplying the entire output current,
JU> which is what you must do in a common gate circuit.

JU> You would most likely want to use a coil, whether you apply the signal
JU> to the source or gate, you still must give it enough drive to cause
JU> cutoff as well as the level of saturation you need. In this case you
JU> need to supply this to a much lower impedance.

JU> In the 20m band, a common source circuit might be better with modern
JU> devices. Think about it.

JU> JURB