common source mosfet amplifier

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kavkav

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I'm using IRF610 Mosfet
https://pdf1.alldatasheet.com/datashe...LD/IRF610.html

to build a common source amplifier. The mosfet is n-channel enhancement mode.

I have read everything I could find on mosfets, read my lectures and still I don't understand how to do this. This is the best schematic I could find but I have no idea how to choose my resistor values:

https://forum.allaboutcircuits.com/attachment.php?attachmenti...1335610252

I would greatly appreciate if someone could could guide me through this because everything I've come across, uses formulas which require a "gm" value or "k" parameter or "W/L" parameter and non of these which I have. I can't even calculate my gain because it depends on what resistors I use.

SPECS:
Power Supply: 24 Volts DC
Signal Source: 1000Hz AC from function generator with 150 ohm internal resistance (can go from -5 to +5 volts in amplitude)
voltage Gain: voltage of signal should multiply by a factor of 3 (at least).
Current Gain: I don't know as long as power gain is significantly higher than signal source
V(th): 2 to 4 volts according to datasheet

Thanks.
 
Your most important spec is missing! What kind of load does the amplifier have to drive?
A 10,000Ω resistor?
A 4Ω speaker?
 
Hi,

Yes a few problems here

Another one, you say you have an input of plus and minus 5 volts and you want to increase that by a minimum of 3 fold which works out to plus and minus 15 volts, yet presently you only seem to have a 24vdc power supply which isnt enough. To get plus and minus 15 volts you need at least a 30 volt power supply or better yet a little higher than that like 34 volts, but better yet you should have plus and minus 16 volt power supplies as a min, and plus and minus 20 volt power supplies would be better.

When you amplify +5 volts by a factor of 3 you get +15 volts, and when you amplify -5 volts by same factor you get -15, so that would give you a max output signal of -15v to +15v which i guess is what you are after.

Seeing as you dont have the DC power supply available, would a 2x factor be good enough rather than a 3x factor? Because if not, you'll have to get a better DC power supply or else modify the one you have if possible to put out a plus and minus voltage. Plus and minus 24v instead of just plus 24 volts would be good.
 
I read the OP as saying that the function generator maximum output is ±5V.
Signal Source: 1000Hz AC from function generator with 150 ohm internal resistance (can go from -5 to +5 volts in amplitude)
Click to expand...
 
MikeMl said:
Your most important spec is missing! What kind of load does the amplifier have to drive?
A 10,000Ω resistor?
A 4Ω speaker?
Click to expand...

The load will be an 8 watt speaker (for testing). Then once I've understood it works, I will replace the speaker with a step up transformer and raise the frequency to 3MHz. The primary of the transformer has a resistance of 15 ohms from the wiring.
 
ericgibbs said:
hi,
Have you seen this link. https://en.wikipedia.org/wiki/Common_source

Your AllAbout link does not work [ for me]
Click to expand...

Thanks but I've read that and lots more. My problem is, I don't know how to calculate things without having "k" parameter like I've learned in uni. But from what I understand in practice, people don't calculate gain and their resistor values using the formulas involving "k" parameter. So I want to know the method used in practice.
 

Roff is correct. What I meant was my signal generator can go up to +5. So if I must lower it, I will. Basically, I want to obtain the most voltage swing I can from the 24 volt power supply.
 


I've attached a print-screen of my simulation. In the simulation I used an IRF520 because there was no IRF610. I have the IRF520 chip in real life that's why I chose it. As you can see in the picture, the simulation works perfectly. and this was done with trial and error with a little help from a different post. But I have no idea what formulas to use in order to get that.

Anyway I made the circuit in real life and it didn't work. I even tried the same circuit with IRF610 (three of them). One of them actually played the 1000Hz tone but the output was a few millivolts more than the input. I think I may have burned or somehow damaged all 4 of the mosfets (3 irf610s and 1 irf520). I will go to the store and buy some new mosfets. Does anyone know how to check if a FET works using a multimeter? I know you can check BJTs but not the same way for mosfet.

I appreciate it if someone could help me understand how to choose the resistor values.

Thanks.
 
Hello,


Roff said:
I read the OP as saying that the function generator maximum output is ±5V.
Click to expand...

Yes and that is what i read too.



kavkav said:
Roff is correct. What I meant was my signal generator can go up to +5. So if I must lower it, I will. Basically, I want to obtain the most voltage swing I can from the 24 volt power supply.
Click to expand...

So you want the maximum output swing. And the generator you have NOW can go plus and minus 5 volts, and you want to increase that.
 
Anyway I made the circuit in real life and it didn't work.
Click to expand...
In your sim the input voltage is 4V. The turn-on threshold voltage of an IRF610 is 4V; which means it is passing hardly any current then. But that voltage is only typical and is subject to manufacturing tolerances.
 
alec_t said:
In your sim the input voltage is 4V. The turn-on threshold voltage of an IRF610 is 4V; which means it is passing hardly any current then. But that voltage is only typical and is subject to manufacturing tolerances.
Click to expand...

So would I have to increase or decrease the 4V signal (if I want some more current)? I think it would be increase due to the formulas but I want to be sure.

In the simulation the current is 11mA without load.
 
Last edited:
Like I told you on AAC, the lower load you have, i.e. the lower current is applied when the mosfet is fully on, the lower is the difference between the full-on and full-off gate voltage, and it is in the range of mV anyway.
Mosfets don´t have voltage gain, but have transconductance which is relation of input voltage to output current. The drain resistor converts this current to output voltage, so lower value of resistor means easier control of output voltage, and also more heat generated.

Anyway the easiest solution to this is to add an opamp feedback which will stabilize the output and set the voltage gain you really want.
 
It turns out my mosfets were all damaged and I bought new ones and it worked. I didn't have a resistor for the drain lower than 82 ohms with enough wattage. So I played around with the other two resistors my simulation AND actual circuit worked. I amplified an AC 4Vpp 0.5MHz signal to DC pulse 20Vpp 0.5MHz signal.

This is the circuit I used: **broken link removed**

This was very frustrating for me so I hope others with the same problem see this and skip all the frustration.

But I still don't know how to choose my resistor values without trial and error.
 
Thanks for your help kubeek though I only got the simulation because of the stuff you told me.
 

in practice u dont calculate using 'k' and such

what u do is use the datasheet of the transistor...usually it will show plots of the Id vs. Vds and Ids vs Vgs characteristics.

Use those to figure out ur operating points. If you're curious you can use that data to work backwards and find ur k' and W/L for that transistor.

Unlike integrated circuits, in discrete components you have no control over ur W/L. It is fixed. To 'change' your W/L you would need to use multiple transistors in parallel.
 
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