Potentiometer - LTSpice

Discussion in 'Homework Help' started by PsySc0rpi0n, Jun 4, 2014.

  1. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    I need to use a potentiometer in an amplifier circuit in LTSpice...

    I have found an .asc, .asy and .sub files but these are a stand alone LTSpice circuit...

    I need to use it in a circuit of mine but don't know how to do it!

    Need some guidance here!

    Files are attached!
    The image is the circuit we need to test.
    The .asc file is my circuit in LTSpice.
    The zip file is EricGibb's!

    I need how to use EricGibbs potentiometer in my circuit!

    thanks


    Edited;

    Ok, I got part of it...
    I had to restart LSpice to be able to open the Rtot symbol...

    Now I need to know if I need to connect the RL to the symbol or if comes already attached to itand if I need to add all those directives inside EricGibbs .asc file!!!
     
    Last edited: Jun 5, 2014
  2. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
    1,184
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    Ok, what I have so far is in the attached files...

    I don't know if the potentiometer is correctly set up in the circuit.

    I also need to understand the "B" and "w" values and the RL resistor role in the potentiometer!
     
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  3. ericgibbs

    Senior Member

    Jan 29, 2010
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    380
    hi,
    The 'w' is the ratio of the 'potentiometer wiper' rotation from one end to the other end, the 'w' value can be from around 0.001 thru .999 , where .001 is the lowest value of resistance between the wiper and the bottom end of the pot and .999 is the highest value.

    The 'B' value is your LIST values.

    The 'RL' is a ratio of the RPOT =10K divided by your 'B' LIST.

    Exactly what are you trying to simulate.??

    E
     
  4. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
    1,184
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    Sir Eric, I can see that. What I'm looking for is to understand what each parameters means.

    For instance, I know that a potentiometer is a variable resistor. In my case, it goes from 0kΩ up to 10kΩ. We rotate it clockwise and the resistance increases and vice-versa.

    So, can you help me to understand what those parameters are in the potentiometer?

    We are trying to simulate a Common Emitter amplifier!
     
  5. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi,
    Look at this marked up image of your circuit.

    Whats the purpose of the RL value stepping when you have Rb2.?

    Remove RL

    E

    CHECK the collector current!!! , 22R is far too low
     
    Last edited: Jun 4, 2014
  6. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
    1,184
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    22 should be 2K2Ω. My mistake typing!

    I'll try to figure out what's RL rule in the circuit later today!

    Thanks for potentiometer params explanation. Looks like clear to me now!
     
  7. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
    1,184
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    Morning...

    I am still trying to understand what is the correct setup for the image in the first post.

    I have 2 versions of the circuit but can't decide if they are equivalent or not!

    Versions are attached.

    Please, help me decide which one is the correct setup!

    Edited;

    I also have a version of a friend of mine which is the last .asc file.
     
    Last edited: Jun 5, 2014
  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Hola Eric,

    When Scorpion has finally everything clear, could you please post how you would do it?

    I am always confused in what is the best way to test variable parameters.

    I do not mean derailing the thread but getting a good complementary moral of all this, even for Scorpion.

    If you ask me to start a new thread, no problem!

    Gracias.
     
  9. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi Psy,
    You friends circuit is an error!,, check this image.

    I prefer the version without RL, adding the RL, in my opinion does not give any extra information from the sim

    E
     
  10. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
    1,184
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    Ok, thank you a lot...

    From now on, I'm going to do the simulations asked in the report!
    Ill post any questions I have in the process.

    Thanks
    Psy
     
  11. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi Psy,
    Its important to know that the potentiometer model is 'polarised'.

    Which means if required, you can rotate the models circuit symbol thru 180 degrees, so that the 'w' steps increase the pot resistance at the wiper from the Top end towards the Lower end of the pot.
     
  12. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    I have another question.

    How do I simulate the potentiometer at position "0 Ω" (zero)?

    Our teacher is asking us to simulate the circuit with "0 Ω" at the potentiometer!
     
  13. ericgibbs

    Senior Member

    Jan 29, 2010
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    make 'w' =0

    You have already done this in your earlier sims

    E
     
  14. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
    1,184
    3
    Morning...

    Sir EricGibbs, I know I have already used this potentiometer but I didn't have the time to look in my previous reports as I had to hand this report to my teacher yesterday night.

    Anyway, I need some more question answered.

    The circuit is now with the potentiometer at "0Ω".

    As far as I could understand, we have at least 2 key points in the circuit. Ve and Vout. (see in the attached circuit).
    So, we want to amplify the Vin so that the Vout is bigger than Vin and also that the output wave form be symmetric, right?

    At Ve, we are looking to have the smallest variation possible, correct???

    I know that calculating the DC Bias Point will give us the correct setup of resistors and that will help us achieving those 2 goals. Also a smaller Vin will make the circuit behave correctly, making the output waveform symmetric!

    I have made 2 simulations. One with 5mV for Vin and another with 20mV. Files are attached!

    Can you explain me why we have these 2 goals at this circuit???
     
  15. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi Psy,
    The DC operating point for you BJT Class A amp is wrong, so it will suffer from distortion when driven close to its maximum Gain value.

    The attached image shows the biasing I would use for a BC547C, [ which has a very high Beta of ~450]

    Note the value of Vc and Ve in the two circuits.

    The image also shows a simple way of determining the resistor values for the biasing.
    Its not intended to replace the conventional formula's, its to show what you are trying to realise as DC biasing values for a Class A amplifier.

    E
    Note: in you circuit the Pot is now rotated 180deg to give 0 Ohms at 'w'=0.
     
  16. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Hola Eric,

    How do you define values for Rc and Re?
     
  17. ericgibbs

    Senior Member

    Jan 29, 2010
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    hola,:)

    As a guide for Rc I assume that Vc should be close to Vsup/2
    The Rc is also determined by the required output impedance ie: loading by the next stage.

    The value for Re sets the transistor DC gain and Vb biasing, knowing the value of Ic, an estimate of the Re value can be calculated.
    This will give a Vb voltage when 0.7V is added.

    Knowing the Beta of the transistor the required DC point Ib can be calculated, I then allow base bias resistor chain current of say 10 times Ib.

    Knowing Vb, the lower bias chain resistor can calculated.

    Its advisable to re-iterate your calc's to tidy up the component values

    Is this what you are asking.?

    Eric
     
  18. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Yes. What is not clear to me what comes first: Ic or RC and how.

    All I know that output impedance for this configuration is said to be "high"
     
  19. ericgibbs

    Senior Member

    Jan 29, 2010
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    hola at,
    To avoid lots of typing
    This link best describes the method I use to get the Q-point, getting data from the transistor datasheet and knowing what is the BJT amps application.

    http://www.zen22142.zen.co.uk/Design/bjtbias.htm

    Eric
     
    atferrari likes this.
  20. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Gracias Eric. Clear now!
     
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