common source mosfet amplifier

Discussion in 'General Electronics Chat' started by kavkav, May 3, 2013.

  1. kavkav

    Thread Starter Member

    Jan 1, 2013
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    I'm using IRF610 Mosfet
    http://pdf1.alldatasheet.com/datasheet-pdf/view/52976/FAIRCHILD/IRF610.html

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

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

    http://www.allaboutcircuits.com/videos/69.html

    Skip to 2:50
    However in that diagram it is p-channel. I don't think it makes a difference in the setup if its p-channel or n-channel but let me know if I'm incorrect.

    I would greatly appreciate if someone could could guide me through this because everything I've come across, uses formulas which require a gain 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.

    Thanks.
     
  2. #12

    Expert

    Nov 30, 2010
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    You have to tell SOMETHING about your intentions. Do you have a voltage source? Are you trying to amplify an AC signal? What are you using for an input signal? Do you need any particular current to drive a load?
     
  3. kavkav

    Thread Starter Member

    Jan 1, 2013
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    Okay, I am using a 24 volt power source to amplify an AC signal coming from a function generator. The signal can be as strong as 10 volts peak to peak and as weak as 0 Volts.

    The end goal is to amplify a signal between 0.4MHz to 4MHz (I still haven't decided exactly). But as a test run I want to amplify a 3000Hz signal so I can hear it working before I move onto the high frequency. You asked about current, I am looking for more of a voltage gain than a current gain so I am not concerned about a specific current.

    Thanks.
     
  4. #12

    Expert

    Nov 30, 2010
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    You have a few misconceptions, but that's why you asked.

    It does matter whether you use an N type or a P type transistor.
    Mosfets aren't very good in the configuration you saw in the video because the gate to source voltage is not very predictable and it's a bit high compared to a bipolar transistor.
    Megahertz signals are outside of my comfort zone.
    We really do need some information about how much gain you need, and do you need it adjustable?
    We also need some information about the load so we can get the current right.
    Megahertz signals often use co-axial cable which must be impedance matched, and that forces a certain resistor configuration.

    Keep talking and a person familiar with your frequency band will soon be with you.
     
  5. kavkav

    Thread Starter Member

    Jan 1, 2013
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    Thanks for the reply. I'll continue elaborating.

    I'm working on a high frequency power transfer experiment. I want as much of a voltage gain from the 24V source as I can get. The amplified signal will be going into a step-up transformer in order to increase the voltage even more and reduce current to as little as possible. I know a lot of people may say high frequency power transmission is stupid or inefficient but its just an experiment for fun and for my understanding.

    The primary of the step up transformer is about 15 ohms in resistance (from 30 gage wire).

    As I mentioned before, I wish to test an audio frequency like 3000Hz just so I can tell its working without relying on multimeter or oscilloscope (my oscilloscope works with my pc and can only handle about 15 or 20Vpp).

    The reason I chose irf610 is because it can handle fast switching speeds (according to my calculations about 14.82 MHz max). I have a couple other mosfets: IRF510 & TIP120 215.

    I really want to understand how to build a high (and low) frequency amplifier using mosfets.

    Thanks.
     
  6. Jony130

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    Feb 17, 2009
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  7. shortbus

    AAC Fanatic!

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    If you want AC you will need both a positive 24VDc and a negative 24VDC source. A transformer doesn't magically change DC to AC. A class "D" amplifier is something for you look into. DC to AC inverters are based on the class D amplifier.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A transformer does magically remove the DC bias from an AC signal, just like a capacitor does.
    That doesn't mean I endorse the idea of using a power MOSFET as a 4MHz linear amplifier. He's barking up the wrong tree, IMHO.
     
    shortbus likes this.
  9. kavkav

    Thread Starter Member

    Jan 1, 2013
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    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)?
     
  10. kavkav

    Thread Starter Member

    Jan 1, 2013
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    Because I'm hooking up the output to a transformer it's fine. I've done my experiment with low voltages and with pulsed dc as well as AC and the output does not change much. However, now that you mention it, my power supply is +/- 24V and instead of ground I've been connecting it to that. I don't think it would make a difference but correct me if I'm wrong.

    Also, I thought if you add a capacitor before the Vout like in this diagram:

    http://s3.amazonaws.com/answer-board-image/02ed3a99-d99e-4758-8188-d7a0affcc78a.jpeg

    the signal output would be AC.
     
  11. kavkav

    Thread Starter Member

    Jan 1, 2013
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    Do you say this because of the power losses at high frequency?
     
  12. kavkav

    Thread Starter Member

    Jan 1, 2013
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    I have another question at this part:

    " Now we chose Id current

    Vds = 7.5V

    Id = ( 15V - 7.5V) / ( 1.47K) = 5.1mA "

    How do you choose Vds to be 7.5 just like that. Isn't Vds dependent on Rd and Rs? And how can you change your resistors without doing the (Rd + Rs) formula because you had a specific current for that (5mA).
     
  13. shortbus

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    What I got from his post was he was expecting to put a pulsing DC, 0 to +24V, into a transformer and get AC out of the secondary. Don't think that will happen.
     
  14. Ron H

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    Well, he certainly won't get DC current out of the transformer secondary (without a rectifier).
    What is your definition of "AC"?
     
  15. kavkav

    Thread Starter Member

    Jan 1, 2013
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    Okay I'm stumped. I did everything I could to follow the formulas from the other thread but the output is weaker than my signal source.
     
  16. Ron H

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    What is your signal source?
     
  17. kavkav

    Thread Starter Member

    Jan 1, 2013
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    My signal source (Vg) is 3000hz AC 5.9 V amplitude. I played around with the the Vg value and all it did was make the sound louder or quieter but still weaker than the Vg.
     
  18. #12

    Expert

    Nov 30, 2010
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    Here it is in post #3

     
  19. Ron H

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


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

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    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.
     
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