Always thought AC was a wave form that went both positive and negative,through zero volts. It can be any repeating form, square, sine, sawtooth, etc. But must be alternating between + and - volts.

You're right. The AC average voltage is zero, which it will always be across the secondary of an ideal transformer.

 

OK. I was wondering about the source impedance. I think most function generators are 50Ω.


Kavkav, post your schematic, and maybe we can help.

This is the schematic I was copying based on jony130's calculations on the thread he posted.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=42388&stc=1&d=1335610252

However, I got it working with a simple setup and with a different mosfet. But I still want to learn how to do the setup posted by jony130 if anyone is willing to help.

The setup that got it working is in the attachment.

 

okay nevermind I just realized the TIP only worked because it's actually a transistor even though it looks like a mosfet. I can't go to high frequencies with that.

 

Pretty much if someone could explain me how to choose my Rd, Rs, R1, and R2 in order to obtain as much of the 24 Volts as I can, that would be great. If a value for current is needed then lets 100mA. OR the overall power gain from the 24 volts should be at least a factor of 3.

 

I'm using IRF610 Mosfet
http://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:

http://forum.allaboutcircuits.com/at...8&d=1335610252

I would greatly appreciate if someone 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 link to allaboutcircuits is bad.

 

Your link to allaboutcircuits is bad.

I am looking for a very simple amplifier because I am new to mosfet amplifiers.

This is the outline of what I want:
http://en.wikipedia.org/wiki/File:N-channel_JFET_common_source.svg

But from what I've read it is going to require R1 and R2 in the first link I posted.
If you know it's bad, could you please explain why? And if there is a simpler way to amplify a signal using the irf mosftet, I'm all ears.

 

one amplifier here http://diyaudioprojects.com/Solid/IRF610-Class-A-Headphone-Amp/

 

But from what I've read it is going to require R1 and R2 in the first link I posted.

Both links in your first post don´t work.
The design is basically the same as with common emiter amplifier. First you need to spcify what load you want it to drive. There is no problem getting full 0-24V swing out of it, so start with the load, find out what reistor you need between drain and vcc to get the high level voltage on the output. Then find out what voltage swing you need on the gate to get there.

 

Both links in your first post don´t work.
The design is basically the same as with common emiter amplifier. First you need to spcify what load you want it to drive. There is no problem getting full 0-24V swing out of it, so start with the load, find out what reistor you need between drain and vcc to get the high level voltage on the output. Then find out what voltage swing you need on the gate to get there.

I don't think it works the same way as a BJT. I made the amplifier first using BJT because I thought it was a mosfet (it looked the same) but then realized its a BJT. Then I replaced the BJT with the mosfet and it didn't work. I tried two of the same mosfets.

The circuit that made it work for the BJT was just a simple common drain setup:
http://en.wikipedia.org/wiki/File:N-channel_JFET_source_follower.svg

Except I had a resistor at the before the drain and my function generatot had a 150 ohm resistance at the base. Pure simplicity and it worked like charm. But mosfet was a no go.

 

one amplifier here http://diyaudioprojects.com/Solid/IRF610-Class-A-Headphone-Amp/

Thanks but I found that as well. I'm looking for the simplest working mosfet amplifier so that I can learn from there.

 

The circuit that made it work for the BJT was just a simple common drain setup:
http://en.wikipedia.org/wiki/File:N-channel_JFET_source_follower.svg

Common drain setup is basically the same as emittor follower, but instead of 0.7Vbe drop between base and emitter you have roughly 5V Vgs drop between gate and source.

The same goes for common source amplifier, but the drive voltage on gate needs to be carefully biased close to the actual threshold voltage of your particular piece, and the amplitude of the driving signal around this level will be very small unles you have amps flowing through the source. I can try simulating this if you want.

 

Ok, so with 24V, drain resistor 100 ohms, I get it to bias around 12V on output with 3.18V on input, and input amplitude of 70mV peak sine superposed on that bias gives me almost full swing on output.
Higher values of drain resistor will lead to less power vasted, but also to much less easy to set bias point. On the opposite side of things, thermal drift from high current going through the transistor will also shift the bias point significantly.

 

Okay so when you bias output to 12V does that mean you add another 100 ohm resistor after the source like a voltage divider? Or the does the second 100 ohm resistor go in parallel with the drain and ground?

 

No, There is just one mosfet and one resistor on the drain. The bias is done by carefully setting the gate voltage to get just the right amount of current to flow through it.

 

Thanks for your help. 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: http://postimg.org/image/hh0xrgigh/

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

I just have one question. What formulas do you use to calculate your drain resistor and your bias voltage?

 

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: http://postimg.org/image/hh0xrgigh/

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.

 

Hello,

I have merged the two threads.
Please keep all info in ONE thread.
The posts will show up in time line order.

Bertus

 

And please post you images directly to this forum. At a later date they may be deleted on an off forum server. When posted to the forum they will stay with the forum.

 

Your thread answer helps a lot. So far I'm at this formula:
gm = 2 * √(Id * K) = 28.5mS

I am not sure what gm stands for. Also, the "2" in the formula, is that a constant or is that because the thread question wanted a gain of 2? In other words I want a gain of 3.5 so would I have gm = 3.5 * √(Id * K) ?
or would it still be gm = 2 * √(Id * K)?

gm - transconductance = 2 * √(Id * K)
And this "2" in the formula is a constant number.

And for this circuit

To to obtain high output voltage swing Vds must be equal to 0.5Vdd = 12V

Read this
http://forum.allaboutcircuits.com/showthread.php?p=494633#post494633

So if you want your voltage gain to be equal to 4.5.

Voltage gain Av = Rd / ( 1/gm + Rs) ≈ Rd/Rs

So if we pick Rd = 1kΩ we need Rs = Rd/Av = 1K/4.5 = ≈ 220Ω

Drain current will be equal to

Id = (Vdd - Vds)/(Rd + Rs) = (24V - 12V)/1.22K = 12V/1.22K = 9.8mA

So we need to choose R1 and R2. In real life we are force to use a voltage divider plus a potentiometer to select the right bias point (Id = 9.8mA).
But here I use a LTspice to find K factor for IRF530.
K = 4.17 and Vt = 3.89V.
So the get Id = 9.8mA we need Vgs = Vt + √Id/K = 3.89V + √(9.8mA/4.17) = 3.89V + √2.35m = 3.938V

The voltage at MOSFET gate will be

Vg = Vgs + Id*Rs ≈ 6

So we need a voltage divider (R1 and R2) with the output voltage equal to 6V

Vg = Vdd * R2/(R1+R2)

Vdd/Vg = 1 + R1/R2

24V/6V = 4

R1/R2 = 1 - 4

R1/R2 = 3 ----> R1 = 3*R2

If I choose R2 = 100KΩ then R1 = 300KΩ

Also you shoudl notice that your input signal 5V peak is too large for this amplifier.