Re: Mosfet RF Amp "sprogging"

Thanks for the feedback (no pun intended :) I appreciate it.

I will add a shield between input and output circuit. I do remember seeing this in other designs. In my prototype construction I do have the input and output coils fairly close and in the same orientation so I imagine there very well could be coupling between them.

the fact that the circuit oscillates very close to the tuned frequency is probably a clue that positive feedback between input and output is happening. The source is grounded so Drain - Gate seems feasible.

I will also take the advice of adding a ferrite bead between G2 pin and the voltage divider for G2. (Currently 100k+100k with 1n decoupling).

Can anyone suggest a reasonably simple method to determine noise figure of this stage ? I do not have a distortion meter. I have directional couplers, sig gens, sepctrum analyzer and digital power meter. Can I use these somehow ?

Thanks in advance again


Best regards

David

nospam@xxxxxxxxxxxxxxxxxxxxxx wrote:
On Fri, 30 Jun 2006 01:20:52 GMT, David <dhuisman@xxxxxxxxxxxxxx>
wrote:

I am currently experimenting with construction of my first Dual-Gate
Mosfet Amp.

Below is an attempt to explain the circuit typology as best I can

Freq = 148 MHz to 152 MHz

Power supply is 8V, Bias on G2 is 4V. Source is grounded

Drain has Variable inductor to Vcc and Cap to ground to form first stage of a double-tuned circuit. This then couples through a small cap to the second half of the DTC.

RF input to Single tuned circuit with tapped capacitors attached to G1 of BF998.

C1 to ground is 56pF, C2 to G1 is 15pF, L from G1 to ground is variable with nominal inductance of 85nH (tunes 75nH to 110nH), Q approx. 100

The Drain has the same type of inductor to Vcc (decoupled Vcc end).
C3 is 18pF from drain to ground (This forms part of the double-tuned circuit on the output).
Coupling to second half of DTC is 1pF. Second half of DTC C4 = 18pF to ground and L is same as previous inductors (85nH nom.).

This will later go to G1 of a Dual-Gate Mosfet mixer but for testing I replaced the 15pF on second DTC with series 18pF and 56pF to form a capacitive divider down to 50 Ohms for my spectrum analyzer input.

With this configuration the circuit will oscillate on its own when the input is removed. If I replace the Capo from drain to ground with a trim cap I can adjust it so the self-oscillation stops but as soon as I try to change the tuning of the inductor from drain to Vcc, it starts to oscillate on its own.

If I remove the cap from drain to ground altogether, the oscillations stop but I almost the ability to tune the Inductor, it becomes extremely
broad tuning.(The input inductor and output DTC inductor tune fine though).

I have tried a series 33R between the Drain of the mosfet and the Inductor/Capacitor and this did not help.

The measured gain with the capacitor removed was 17dB. Drain current was 4.8mA. 3dB bandwidth was 3.2MHz.

The circuit is laid out "dead-bug" style on a flat bare PCB. I'm not sure if it is a layout problem or typology issue.
I have seen damping used across tuned circuits in output stages of
rf amps and may try this next. But, I would like to know what is happening because if I manage to stop the oscillation, I am not sure if it is marginal and will reappear with changes in temperature or drive level etc.

Any help much appreciated.

Regards

David

Usually DGFET amps are stable. But they are also very high gain.

The likely culprit is the input can see the output (incidental
coupling). I always build this with a shield partition right across
the device and also a ferrite bead in the G2 lead near the deivce
as possible (or for SMD in the Source leg bypass cap). These devices will take off at uhf due to gain and less then careful
bypassing. That includes making sure the G2 bias resistor (Gate to DC source) does not couple RF back. As part of this also insure the input and output leads to the connector are short
as possible or even coax.


Allison
.

Relevant Pages

  • Re: ringing inductor in boost circuit
    ... The circuit is running at about 15kHz with light load. ... But when the voltage at the collector drops below the voltage on the filter cap, the inductor rings. ... ringing is quite pronounced. ...
    (sci.electronics.basics)
  • Re: Mosfet RF Amp "sprogging"
    ... Below is an attempt to explain the circuit typology as best I can ... Drain has Variable inductor to Vcc and Cap to ground to form first stage ... The Drain has the same type of inductor to Vcc. ...
    (rec.radio.amateur.homebrew)
  • Re: test--question 1 power 101--buck converter
    ... To convert AC to DC you are making a simple drain to ground ... |> a high current mosfet circuit controlled by a oscillating circuit. ... |> amount of average current and use a much larger cap. ... | John Popelish ...
    (sci.electronics.design)
  • Re: ringing inductor in boost circuit
    ... The inductor has a fraction of an ohm ... The circuit is running at about 15kHz with light load. ... The diode clamp across the emitter-collector and the 68pF cap are there ... ringing is quite pronounced. ...
    (sci.electronics.basics)
  • Re: test--question 1 power 101--buck converter
    ... To convert AC to DC you are making a simple drain to ground through ... > a high current mosfet circuit controlled by a oscillating circuit. ... > amount of average current and use a much larger cap. ... John Popelish ...
    (sci.electronics.design)