Need to create a circuit with a linear relationship between Vin and Vout

[EasyGoing1]

OK, given this circuit:



Which has this actual relationship between the voltage at the base of the MOSFET and the voltage across the resistor at the drain:



Notice that the saturation happens rather quickly at low gate voltages. What I'd like is to be able to vary the gate voltage from 0 to 5 volts and have the voltage drop across the resistor be more proportional across that range so that the graph would look more like this:



How would I go about figuring out how to make a circuit like that?

[eccerr0r]

Re: Need to create a circuit with a linear relationship between Vin and Vout

You're using a p-channel schematic drawing again for the n-channel irf520...

Based on what you're doing here, you have absolutely no advantage to using a MOSFET. If you insist on using a MOSFET in the configuration presented, you need to make a (FAST) negative feedback loop.

That being said, it's easier with BJTs as they have self negative feedback in emitter follower configuration. And the other hint is that we'll be describing to you something that sounds sort of similar to something else...

hint is :
SPEAKER

...

[EasyGoing1]

Re: Need to create a circuit with a linear relationship between Vin and Vout

You're using a p-channel schematic drawing again for the n-channel irf520...

I'm not sure why then the cad program lets me chose by part number so it must have the schematic symbol wrong for that part but anyway...

Based on what you're doing here, you have absolutely no advantage to using a MOSFET. If you insist on using a MOSFET in the configuration presented, you need to make a (FAST) negative feedback loop.

That being said, it's easier with BJTs as they have self negative feedback in emitter follower configuration. And the other hint is that we'll be describing to you something that sounds sort of similar to something else...

hint is :
SPEAKER

...

I'm not married to ANY specific components ... I just need the results that I need whatever it takes to make that happen, I'm all in.

[eccerr0r]

Re: Need to create a circuit with a linear relationship between Vin and Vout

Well you seem to be married to a method at the moment. I can only guide you through that path now.

The hint of what you're asking for, well, aren't audio equipment supposed to be linear to faithfully reproduce sound.

So what you need is to build an audio amplifier - which would be linear. Bipolar transistors behavior in the forward active region is linear with respect to base current which is what you want. MOSFETs are second order so you'll need second order feedback to return to linear.

A power audio amplifier IC or even a LM317T would easily do the job you want minus the limitations in power handling, which is another side effect to be included why this approach to solving your initial problem is nonideal, but enough said.

[budm]

Re: Need to create a circuit with a linear relationship between Vin and Vout

I am curious about your schematic capture and simulation program you are using, I cannot see how you can get those graphs from the circuit simulation when the wrong MOSFET is used to simulate the circuit, this is the second time your schematic shows the wrong MOSFET.

[eccerr0r]

Re: Need to create a circuit with a linear relationship between Vin and Vout

Actually I think his waveform is correct ... sort of ... for how he simulated it, providing the model used in the schematic indeed is a p-channel device.

The waveform is really bad and shows nothing like what you'd want for a real circuit indeed because the transistor being used incorrectly...

[budm]

Re: Need to create a circuit with a linear relationship between Vin and Vout

You mean N-CH.

[eccerr0r]

Re: Need to create a circuit with a linear relationship between Vin and Vout

After looking at the scale of the initial waveform something is indeed messed up even for N-channel as the threshold voltage hasn't been reached though subthreshold leakage may be occurring.

I'd still say throw this all away, go back to the drawing board, and forget about sinusoidal output prior the the output stage.

[redwire]

Re: Need to create a circuit with a linear relationship between Vin and Vout

The simulator is out to lunch, the 10MEG is too large and the mosfet drawn as P-ch, and the body diode would just stay on, never letting Vout go above 0.7V but the graph does not agree. It looks like a poor simulator.

The Falstad simulator is better for understanding circuits:
https://falstad.com/circuit with many examples https://falstad.com/circuit/e-index.html

IRF520 has leakage current of over 10uA so a 10MEG would not work.

For sanity, the IRF9520 is not a IRF520.

[budm]

Re: Need to create a circuit with a linear relationship between Vin and Vout

It looks like crappy schematic capture/simulator program with bad component libraries.
I mean you look at the schematic you can just see what is not right right away, I.E. the effect of body diode of that P-CH MOSFET. This where basic understanding of of how components works come in instead of on relying on the CAD program alone.
Like I said, this is not he first time OP shows us bad schematic using this CAD program.
https://www.badcaps.net/forum/showpo...08&postcount=1