How To Use A Mosfet

You probably missed a small-scale just crucial item in this schematic.

The crucial detail is that Q2 needs to have a larger West/50 than Q1.

In the books example the ratios of the geometries (Due west/50) is virtually:

v kohm / 667 ohm = 7.5

So the W/50 of Q2 needs to be seven.5 times larger than the West/L of Q1.

So for example if Q2 = 100um/1um then Q1 = 750um/1um.

In that way the voltage (\$V_{GS}\$) to electric current (\$I_D\$) relation will be different for Q1 and Q2. You could say that Q2 will be "more sensitive".

Also this circuit is ordinarily only used on a flake because the just way that you can get a predictable behavior from this circuit is when the transistors have identical beliefs (except from the W/Fifty ratios betwixt them). Also the transistors need to have well-nigh identical temperatures. So in practice these weather condition can only be met on a flake.

Another issue is that on a scrap you tin can make a MOSFET in almost any size and geometry. You cannot do that when using discrete components like a 2N7000. The 2N7000 is an NMOS designed for switching purposes and it volition have a very large W/L so that \$R_{DS,on}\$ becomes small.

Yous can get around the W/L ratio that you demand by connecting MOSFETs in parallel, and then y'all essentially double W merely L remains the aforementioned. But every bit then \$R_{DS,on}\$ will go down even farther, this will not help.

As a outcome it is extremely hard to make a 2N7000 operate such that it has an \$R_{DS,on}\$ of a few hundred ohms or fifty-fifty a couple of kilo ohms. The 2N7000 simply isn't made for that. So that'southward why you get very large currents and the MOSFET will oestrus up because of that.

The only way I can run across this work using discrete MOSFETs is if y'all use MOSFETs that are not designed for switching simply instead are designed to amplify signals. There are some MOSFETs for amplification of RF signals. Yet these are rare and not piece of cake to utilize.

Some other option is to utilize a MOSFET array, that's a chip with some MOSFETs on it. 1 case is the HEF4007 but it is express in what y'all tin can practice with information technology due to the mode that the MOSFETs are connected.

If you want to experiment with this "MOSFET as an active load" circuit so I recommend that you beginning using a circuit simulator (Like LTSPice or QUCS) every bit there you can use a "generic MOSFET" and freely choose W/L.

How To Use A Mosfet,

Source: https://electronics.stackexchange.com/questions/502525/how-to-use-mosfet-as-an-active-load-resistor

Posted by: glessnersopland.blogspot.com

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