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Voltage biased amplifier

  1. Sep 30, 2009 #1
    1. The problem statement, all variables and given/known data

    I need help!! My teacher just assumes I know everything and he doesn't "teach" me very much. He expects me to a) determine characteristic equation of a MPF102, but I don't understand how to do it....when I ask he just says, "go have fun with it" It might be fun if I understood it! Then he b) wants me to build a simple voltage divider biased amplifier that works. I don't really understand where to start or how to go about knowing if something is going to work. I don't understand what parameters I am supposed to be looking for when I'm throwing numbers into equations.

    If anyone understands and could maybe give me a path to follow or some help, I would be very greatful!! Thanks in advance!

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 30, 2009 #2

    turbo

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    Gold Member

    Google MPF102 for data-sheets, applications, etc.
     
  4. Oct 2, 2009 #3

    vk6kro

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    Science Advisor

    Looks like this isn't homework and not coursework you have done yet, so I'll just show you a way of doing it.
    I haven't heard of "just have fun with it" as a method of teaching.

    If you are not given any design requirements, you can just make up some of your own.

    Assume a 1K load resistor. Assume a 10 volt supply voltage. My choice.

    Now, if this is going to be a linear amplifier, the voltage across the FET should be about half the supply voltage. So about 5 volts.

    This means the voltage across the 1K resistor is 5 volts. So the drain current is 5 mA.

    Now you do have to go to data sheets. What voltage does the FET have to have on its gate to produce 5 mA of drain current? Suppose you look it up and it is -1.7 Volts.

    You can either find -1.7 volts from somewhere (not easy if you have a +10 volt supply) or you can put in a source resistor that will give +1.7 volts at the source relative to the gate.
    You know the drain current is 5 mA so the source resistor is 1.7/.005 =........ ohms?

    You can increase the supply voltage to 11.7 to compensate for this extra 1.7 volts if you like, or you could recalculate to allow for the slightly reduced drain current. I'd increase the supply voltage.

    This source resistor will reduce the gain of the amplifier if you leave it like that, so it should have a capacitor across it. The capacitor should have a reactance of 10% of the source resistance at the lowest frequency being amplified.

    Now, all you have to do is make sure there is a suitable resistor from the gate to ground. 100 K is a good value. It isn't critical, but there has to be something there.

    Notice that I did not consult the data sheets for an MPF102 so you should do this to obtain an actual bias value.

    I found the following site that looks helpful as well:
    http://www.rason.org/Projects/jfetamp/jfetamp.htm
    They may cover some points I have forgotten to mention.
     

    Attached Files:

  5. Oct 7, 2009 #4
    Thank you so much vk6kro! I am gonna try that. Although if you could check back sometime I may have more questions.... Thanks again!
     
  6. Oct 14, 2009 #5
    Just a couple points to elaborate on,

    You may want to figure in another resistor on the left side of vk6kro's schematic and connect the gate between the two resistors to comply with the voltage divider specification that the teacher asked for.

    If you need to graphically plot the transfer and source characteristics of the specific MPF102 you are using, use DC to do this, not AC. If you need a couple of schematics to help plot these characteristics, let me know. I have a feeling we are taking the same class??!!
     
  7. Oct 14, 2009 #6

    vk6kro

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    Putting more positive voltage on the FET gate would just mean you had to develop more source voltage with a bigger resistor.

    This is already voltage dividing. The FET is in series with a source resistor and a 1 K drain resistor. The bias comes from the voltage across the source resistor.

    Nobody said the bias had to come from a resistive voltage divider on the gate. The teacher just meant you couldn't just call up a different negative supply to bias the FET.
     
  8. Oct 14, 2009 #7
    vk6kro, I agree that your circuit will perform just fine. I know the teacher and have the same assignment however, and know he is looking for a certain configuration for the circuit to be modeled after.

    imeener, here's how I attacked this problem, maybe it will help. Once you decide on your circuit configuration and resistors, do a DC type analysis on the circuit just like last semester. First determine the source and transfer characteristics for the specific mpf102 FET that you are using in your circuit. Each of these have their own graphs. These graphs are essentially the plots of the device's "characteristic equation" that the teacher is looking for. You need two vital pieces of info from this step, Idss and Vpinch.

    Use Idss and Vp to plot a load line on the normalized curve and find the Quiescent point. Then you can can do some AC calculations, like finding the transconductance factor (gm), Zi, Zo, and your gain (Av).

    Next, I would build a model of it in Multisim and use the AC analysis feature to get the Bode plots, and see what you can hope to expect with a real circuit. Next, build and test the real circuit and take data to build new Bode plots off of measurement values. You can also measure the actual Zi, Zo and Av on the real circuit and compare these to the calculated values you found earlier. Do some comparisons between the theoretical circuit and the actual circuit and make them part of your conclusions. I hope this helps and good luck.
     
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