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This transistor is very cheap, easy to obtain and has very good characteristics. And that is not so trivial because this figure must belong to the front end LNA The LNA from Fig. This is not bad but it should be better. The FET operates at Idss as this is the best operating point for lowest noise Idss is specified to be between 2 to 18 mA - I never got the 2 mA devices, the most common Idss was from 10 to 15 mA. Input coil L1 is wound on 5 mm diameter core with 1,0 mm AgCu wire; it has 6 turns with a total length of 11 mm.
The coil should be mounted mm above the ground and at least 10 mm from any metal wall. Trimmer capacitor should be of high quality.
Other materials of other manufacturers could be tried but special attention should be given to the obtained gain and P1dB. The wire lengths from Tr1 to the FET and capacitors should not be longer than 5 mm and should be run near the PCB ground or oscillations can occur. After a lot of experimenting I found another interesting characteristic of FETs well known in the audio amplifier scene as I found out latter - when you parallel two identical FETs you get lower NF than with a single device!
So if you piggy-back another BF to the original one both should have the same Idss you can get 0,6 dB of NF, 26,5 dB of gain and the same or worse!? The IP3 problem was solved with proper output matching - the transformer was changed to The circuit is shown in the Fig. To obtain this value of IP3 the Tr1 should be changed into 1, transformer.
To get the 0,5 dB of NF the input should be tapped to the L1 at the middle that is at the 3rd turn. At the first attempt I just piggy-backed four BF's but some weak oscillations occured above 3,5 GHz not affecting the amplifier characteristics on the working frequency. Then I made new arrangement with two times 2xBF piggy-backed side-by-side; both pairs are connected together with short wires, and have separate G2 bias network the circuit layout and G2 bias blocking is very critical.
After that modification the oscillations disappeared but it should be noted that all the designs described here are only conditionaly stable.
Overall component layout is very critical, specially the 1nF G2 blocking C. The variable values are given in the Table 1. The input and output matching are different see text.
The core for the Tr1 should be of a larger size material U17, size A4 when more than one BF is to be installed. The amplifiers can be approximately tuned for minimum NF without instruments with a little trick: they should be tuned for the maximum gain at MHz the minimum NF tuning point is quite broad. This data is from feb. It turned out that the amplifier has stability problems when terminated with narrowband filter as this is the case in XVRT.
After a lot of experimenting it turned out that increasing the number of turns on output transformer but maintaining the turns ratio heals the problem. As a by-product of this modification the 1dB improvement in input IP3 was acheived. When you construct the push-pull amplifier from two single stage amplifiers you can get 3 dB higher IP3 as with the parallel configuration.
The circuit is actually the push-pull arrangement of two circuits from Fig. Input coil L1 is wound on 11 mm diameter core with 1,0 mm AgCu wire; it has 4 turns with a total length of 7 mm. The input coupling is done with a single turn coil wound on the same diamater as L1 with enameled Cu wire so that the shorting of the L1 winding is prevented. The transformers Tr1 and Tr2 use the same core as described in Fig. The output combining transformer Tr2 has 2 turn bifiliar winding.
Then I tried the push-pull configuration of four BF For other purposes the obtained NF could well be tolerable. Anyway, the circuit stays open for further optimization Those amplifiers are not well suited for preamplifiers in front of receiver or xverter as they have much too much gain.
In order to use them effectivelly although I don't recommend using preamplifiers at all one shoul add an attenuator at the output to lower the gain. In most occasions dB of gain is enough for a preamplifier if it really has to be mounted. Six LNA's were built so far No substitution is available yet, but any core of similar size and material with initial permeability from 20 to 30 should work That's it!
Tuning is not critical as the input is very broad in the sense of minimum noise something like 10 MHz. More than 40 LNA's were measured in feb. The latest datasheet
BF981 - BF981 N-Channel Dual-Gate MOSFET SMD Transistor
This article originally appeared in the Australian, June, issue of Amateur Radio magazine. The design will be useful for constructors more comfortable working with larger size mosFETs. The BF can still be sourced via eBay. On a performance for dollar basis it was probably the best device as of available. However, circuits published in some overseas magazines have failed to perform as well as expected. This article suggests a possible reason for this and provides information on how to realise the device's potential. Performance Claimed.
10pcs BF981 PHILIPS/NXP Silicon dual gate MOSFET Transistors Business & Industrial
The lowest-priced, brand-new, unused, unopened, undamaged item in its original packaging where packaging is applicable. Packaging should be the same as what is found in a retail store, unless the item is handmade or was packaged by the manufacturer in non-retail packaging, such as an unprinted box or plastic bag. See details for additional description. Good value and excellent E Bayer. Will deal with any problems in a proactive, swift and effective manner. Verified purchase: Yes Condition: New. Skip to main content.
BF981 MOSFET. Datasheet pdf. Equivalent
By continuing to use the forum you conform your acceptance of these. If you are not happy to accept these you must stop using the forum and delete our cookies from your browser. BF mosfet lead identification. I am trying to identify the leads of a BF correctly. The longest lead is the drain and easy to identify. But all of the legs seem to have notches, making it difficult to identify the source. Therefore I've taken a picture and marked out the resistance readings between adjacent leads.
N-Channel MOSFET low noise amplifier
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