How can I design a mosfet differantial amplifier with 2N7000 mosfet and how can I measure k value of 2N7000 using its datasheet to analyse mosfet differantial amplifier.

 

What type of differential amp uses 2N2700 MOSFETs?

 

Here is a picture of the inside of the CA3130 fet front end opamp...



You can see that the mosfet used are all p-channel mosfets.

You might start reading the ebook in this AAC website here.

Allen

 

I know MOSFETs are used in some IC opamps.
That's not particularly relevant to the op's post.

My question is, why would you want to make a differential amp with discrete 2n7000 MOSFETs?
That would not make a particularly good differential amp.
For example, Vth varies from 0.8v to 3V so there would likely be a significant input offset if you didn't select and match them from a large selection to match Vth.

 

I knew that you know.... That post was for the OP.

Besides, this is a homework question. Or may be 2n7000 is what is available in his lab.

Like most of the Uni Student designing using old, obsolete 741s, 8051, 16F84A etc. It is no surprise that the only mosfet that he have heard of is the 2N7000.

But I have seen opamp design based on 2n2222, 2n3904 and 2n3906 BJT. Though not perfect but it works.

Actually I thought that mosfet was only good for high current switches and not suitable for linear applications.

Allen

 

I knew that you know.... That post was for the OP.

Okay.
It seemed like your answer showing a circuit using P-MOFETs was not directed to the op since the 2N7000 is an N-channel device.

But I have seen opamp design based on 2n2222, 2n3904 and 2n3906 BJT. Though not perfect but it works.

BJT's have a much lower variance in base-emitter voltage between units than the gate-source voltage of MOSFETs, which is why I questioned their use in a differential circuit.
Certainly if you can tolerate input offsets up to a couple volts, you could use the 2N7000.

I thought that mosfet was only good for high current switches and not suitable for linear applications.

It is a small signal MOSFET as indicated here, so can be used for linear applications or as a switch up to 200mA max.

 

So does it mean that BSS138 (N channel) and BSS84(P channel) Mosfets are better choices than 2n7000 for designing the differential amplifiers? Vgs(th) is 1~2V typical=1.5V.

They are lower than the 2n7000 where Vgs(th) = (0.8~3V) .

Allen

 

So does it mean that BSS138 (N channel) and BSS84(P channel) Mosfets are better choices than 2n7000 for designing the differential amplifiers? Vgs(th) is 1~2V typical=1.5V.

Some.
It would reduce the maximum offset from 2.2V to 1V.

 

I think Datasheet You will not help. The Necessary experimental measurements.
Also You it is necessary to choose the state of working transistor. If you well study in textbooks, that You realize that possible to get the greater gain small current. I recommend 1мА - 10мА for 2N7000. Under greater current will be a best frequency characteristic, but small gain.

 

1мА - 10мА

Why you didn't use mA, but used мА?
Although the м is small print, but normally M is represent 10^6 (Mega), what's your reason?

 

Why you didn't use mA, but used мА?
Although the м is small print, but normally M is represent 10^6 (Mega), what's your reason?

I am sorry. This is a russian letter m.

 

I am sorry. This is a russian letter m.

Ok, you can use it, unless have many members report about it then our mods team will discuss it, if we get used to one kind of numbers or symbols system when we see others maybe we will feel a little strange, that was how I felt, maybe you have the same feeling as mine.

 

We all learned something then. I thought it was a smaller font capital M ... at first. I'll have to store that tidbit away. Thanks.