FET Amplifier Biasing

[DGBB]

Hello,

As a hobby project I have built a couple of basic AM radio circuits and they work pretty well but only with a "water pipe" type direct earth connection.
To make them more portable I was wondering if a JFET or other type of amplifier added to the input from the tuning coil would boost the incoming RF enough to eliminate the need for the direct earth.
Being a newbie I have looked at various JFET amplifier  circuits and the problem I have is while I understand the reason for biasing I have only found circuits with the components identified and their place in the circuit  shown but with various math  formula to work out the value of the components.
Can anyone suggest a simple explanation of the way these things are worked out or a website that might help?
Also are there basic rule of thumb figures you can use to determine the value of the various resistors in the JFET amplifier biasing circuit?


Cheers,

DGBB

[Hero999]

You should use a ferrite rod or loop aerial if you don't want to rely on an external earth connection.

[audioguru]

A simple crystal radio has no voltage gain so it needs the very strong signal provided by a long antenna, a ground and a strong local station.
But a real radio has a very high voltage gain so it simply amplifies the tiny signal it receives. It has AGC so it attenuates its gain when it receives a strong local station.

[Kevin Weddle]

You might consider using a bipolar transistor based receiver input, which is more common. Selectivity and gain can limit the use of simple circuits. But several tuned circuits and amplifier stages can recover weak transmissions. Many of which utilize IC's.

[Hero999]

But a real radio has a very high voltage gain so it simply amplifies the tiny signal it receives. It has AGC so it attenuates its gain when it receives a strong local station.

It also has a variable oscillator and mixer to produce a fixed intermediate frequency so a high Q tuning circuit can be used to give excellant selecxtivity.

[Kevin Weddle]

Should JFET's be eliminated from circuit designs? If not, are there any examples of circuits with characteristics only obtained by using JFET's?

[audioguru]

A Jfet is used when you want a very high input impedance but a Mosfet can also be used.
Many opamps have Jfet inputs.
sometimes a Jfet was used for an automatic level control circuit like an audio compressor, audio limiter or to stabilize the level of a sine-wave generator.

[Kevin Weddle]

The -VGS biasing is similar to a DMOSFET and their input impedance is very high. So mistakenly, a JFET should replace MOSFET's and bipolar transistors in many instances. And aren't biploar transistors prefered in low frequency circuits with low supply voltages, making the use of MOSFET's even less desireable?

[audioguru]

The -VGS biasing is similar to a DMOSFET and their input impedance is very high. So mistakenly, a JFET should replace MOSFET's and bipolar transistors in many instances.

No.
Most Mosfets are enhancement type, but most Jfets are depletion type which is completely different.
A Jfet is biased with just a gate resistor to ground and a source resistor to ground.

And aren't biploar transistors prefered in low frequency circuits with low supply voltages, making the use of MOSFET's even less desireable?

No.
Microcontrollers, microprocessors and Cmos logic ICs use Mosfets with a very low supply voltage.
A Cmos 555 the LMC555 is made with Mosfets and it is guaranteed to work when its supply is as low as 1.5V.
Electric model airplanes use Mosfets to drive the propulsion motor with low voltage and low frequency PWM.