NPN replacement with an enhancement mode mosfet

NPN replacement with an encancement mode mosfet

Hi,
In my circuit **broken link removed** which works from 1-30MHz, I wonder if I can replace the 2sc9018 with a 2n7000 without changing any other components at first try.
I have seen on the net that the enhancement mode mosfets are biased very similarly to bjt.
Will it make any good relating to the gain at RF and AF?

Re: NPN replacement with an encancement mode mosfet

I remember you wanted to try darlington transistor to replace it. I still don't get it why you want higher gain for any stage in a regenerative circuit. Anyway, the mosfet can ruin the linearity if it is diode connected, maybe doesn't matter, but 2n7000 has got much higher input capacitance. I don't think so it can make things better. Forward transconductance isn't too high for it. If you want to test it you should simulate the RF amplifier only.

Re: NPN replacement with an encancement mode mosfet

frankrose said:

I remember you wanted to try darlington transistor to replace it. I still don't get it why you want higher gain for any stage in a regenerative circuit. Anyway, the mosfet can ruin the linearity if it is diode connected, maybe doesn't matter, but 2n7000 has got much higher input capacitance. I don't think so it can make things better. Forward transconductance isn't too high for it. If you want to test it you should simulate the RF amplifier only.

Click to expand...

Thanks a lot!
Higher gain is needed for the next reason: The only way I can find of, that ensures Isolation of the LC from the RF preamplifier is the placement of a very small value coupling capacitor (4.7pF in this case). However, besides isolation, this couples less signal from the rf preamplifier to the detector. Thus, the only way I can think of, to let more signal pass through this small capacitor and at the same time to keep the isolation, is to supply the capacitor with more signal, which means a higher gain rf preamplifier.

My minimalism approach in this circuit does not allow for a second rf preamplifier stage to be used, so I am trying to find all shorts of "tricks". Maybe a broadband transformer between the preamplifier and this capacitor could step up the voltage more?

Re: NPN replacement with an encancement mode mosfet

It is regenerative. If you increase the gain of RF amplifier, you have to decrease the detector's gain to prevent oscillation. I am not sure, but I think you will be at the same place if you want to increase the detector's input voltage.
And I don't understand this isolation role of the C. In the crystal there is a series LC, which isolates DC between RF preamp and detector. I thought that cap is just a voltage divider with the MV209, and actually it is needed to decrease the AC level at the output of the RF preamp, so I mean after the RF amplifier you have to attenuate the signal not gain it.

Re: NPN replacement with an encancement mode mosfet

I think it's purpose is to maximize tuning range. If the value is made larger, all the components to the right of the crystal add extra capacitance across it. A crystal will only pull one or two KHz anyway so maximizing the effect the varicap is essential.

Brian.

Re: NPN replacement with an encancement mode mosfet

betwixt said:

I think it's purpose is to maximize tuning range. If the value is made larger, all the components to the right of the crystal add extra capacitance across it. A crystal will only pull one or two KHz anyway so maximizing the effect the varicap is essential.

Brian.

Click to expand...

Yes, I did not mention that at first. A small capacitor there will also maximize tuning (which is about 2KHz at 7MHz when using crystals)

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frankrose said:

It is regenerative. If you increase the gain of RF amplifier, you have to decrease the detector's gain to prevent oscillation. I am not sure, but I think you will be at the same place if you want to increase the detector's input voltage.
And I don't understand this isolation role of the C. In the crystal there is a series LC, which isolates DC between RF preamp and detector. I thought that cap is just a voltage divider with the MV209, and actually it is needed to decrease the AC level at the output of the RF preamp, so I mean after the RF amplifier you have to attenuate the signal not gain it.

Click to expand...

It does have enough headroom for input signal leveling up before locks to the incoming signal frequency. The small C also loads down the LC less which is important for frequency stability and other frequency pulling effects, when ceramic resonators are used. see this https://youtu.be/gcr7hSjTqd8?t=10m44s it is the second topology

Re: NPN replacement with an encancement mode mosfet

I disagree!
It is a reflex topology with regeneration so it doesn't match the one in the video.

Signal path is:
antenna --> 2SC9018 (RF amp) --> through 4.7pF cap --> self oscillating mixer --> LPF --> 2SC9018 (AF amp) --> output

The nature of the design doesn't allow efficient signal coupling or filtering and the tuning range is very narrow.

Brian.

Re: NPN replacement with an encancement mode mosfet

betwixt said:

I disagree!
It is a reflex topology with regeneration so it doesn't match the one in the video.

Signal path is:
antenna --> 2SC9018 (RF amp) --> through 4.7pF cap --> self oscillating mixer --> LPF --> 2SC9018 (AF amp) --> output

The nature of the design doesn't allow efficient signal coupling or filtering and the tuning range is very narrow.

Brian.

Click to expand...

I assumed the reflex switch is switched to off. With this switch to on, yes it is like you said indeed.
I have tried LC with a 2MHz or so range, but the stability was not good, so I went back to crystals. With crystals, single signal reception is possible, which is great for such a small rx. With ceramic resonators the range is a few 10s of KHz.

Ok I got this, nothing else can be improved in terms of coupling.