Field Effect Transistor tester for small signal FETS and MOSFETS, via the oscillation principle

Опубликовано: 10 Ноябрь 2024
на канале: radiofun232

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Please read the description first.

Schematic of the FET tester is here • Universal tester for MOSFETS and smal...

I got a question of one of my YT followers: why does a FET in a circuit that I (Radiofun232) have published on YouTube does not work? It does not amplify, it even “damps” the signal that I have sent in to its Gate. Is it a “passive” buffer?

My answer: that is a problem that I also found in the past. With other FET types.

A small signal FET can/will act as an amplifier or a buffer, but that depends on the type of FET and, of course, the supply voltage.

When slowly adding a higher (+) voltage to the Drain of certain (read: certain….) small signal N-FETS (BF 245, BF 256 A, J 310) it changes its behaviour from a (passive) buffer to a circuit that really can amplify (the radio signal, or whatever).

That is the reason why you find in many circuits on my YT Channel a 4K7 potentiometer as a Voltage divider in the power supply lead, between (+) and (-).

Via the setting of the the wiper to the Drain of the small signal FET, you can change its (+) Voltage to the Drain, in case of a N-FET.

So you can change the Voltage to the Drain and thus change the properties of the small signal FET. From a passive “buffer” to an active amplifier with an extremely high input impedance at its Gate, in the order of many Mega Ohms.

Anyway: with only a few types of FETS I have tested this. Of course (theoretical) with many other type numbers of N-FETS it can be done....

I can say that it works nice with that BF 245, BF 256 and J 310 (tested)

The FET theory is quite complicated. Enhancement types, depletion types, etc. Of course also N-types and P-types, combined with certain specific properties. Could be too specific for hobby purposes.

My conclusion (also in this video): a few small signal FETS always work as an amplifier: the BF 245 (a,b,c) and the BF 256 A and the J 310.

The FET tester tells you whether these FETS are dead or alive. When the circuit oscillates you can be sure that your FET (also MOSFET) is “alive” and will work in an electronic circuit.

And I found that the test circuit also can work for testing NPN and PNP Silicon transistors. By the way: not all of them....Anyway, not specific made for them.

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Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 16 October 2024.