Amplifying wave generator signals

Discussion in 'Analog & Mixed-Signal Design' started by kevinnas, Sep 18, 2018.

  1. ebp

    Well-Known Member

    Feb 8, 2018
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    If you are investigating behavior then you probably do need to be able to drive various waveforms, however ...

    I used to be involved with instrumentation for ultrasonic transducers used in non-destructive testing (pipeline welds). The transducers were piezo types with resonant frequencies of typically 5, 10 or 15 MHz. The transmit signal was generated by exciting the transducer with a single very narrow pulse of about 400-450 V which would yield a current of 8-9 A, at least at the input end of the 50 ohm cable (which was long enough that the driver was all done before the pulse arrived at the transducer). The circuitry to do this was quite simple.

    A piezo transducer is going to be reactive and because it is resonant the impedance will vary dramatically with frequency.
     
  2. kevinnas

    Thread Starter Member

    Jul 31, 2017
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    Yes this is something very close to what I am trying to do except it will not be 400V but 100V.
    Do u have any sketch or some sort of a rough schematic that I could work with atleast as a starting point
     
  3. danadak

    Distinguished Member

    Mar 10, 2018
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  4. Bordodynov

    Well-Known Member

    May 20, 2015
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    Look at my experiments, but I warn you that the board, the transformer must be made correctly. Also note the dissipation of heat by the output transistors. 2018-09-21_09-06-33.png 2018-09-21_09-04-22.png
     
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  5. kevinnas

    Thread Starter Member

    Jul 31, 2017
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  6. kevinnas

    Thread Starter Member

    Jul 31, 2017
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    Thank you very much! I really appreciate it. I will try the simulations and get back to you as soon as I can!
     
  7. innocent.mushonga

    New Member

    Aug 14, 2018
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    hI
    HI
    Where can I find the LT SPICE Model for the PLV668A regulator diode?
    I downloaded your libraries but I do not think this is included
     
  8. Bordodynov

    Well-Known Member

    May 20, 2015
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    http://bordodynov.ltwiki.org
    or
    https://forum.allaboutcircuits.com/...nents-models-of-ltspice-free-download.133690/
    I chose such a zener diode, because the zener diodes of this series work well and at a current of 10 μA. The model is in the Standard.dio file. You just need to go through the first link. Download the lib.zip file, and install it as written in the instructions. There are also explanations of how I build transformers.
     
  9. Bordodynov

    Well-Known Member

    May 20, 2015
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    In the electrical circuit, the parasitic inductances of the sinks and sources are not very noticeable. In a simple transistor model, there are none, but in this case these inductors are necessary. In my library they represent a short line.
     
  10. innocent.mushonga

    New Member

    Aug 14, 2018
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    Hello
    I have finally managed to get some time to simulate the circuit and it works well, however It is a little hard for me to understand what is going on without any brief explanation of how the circuit operates. Could you please just give me a brief explanation of the circuit? at the moment I am guessing 1V pulse is amplified to 12V which is then fed to a step up transformer 1:4 ratio (20/5) to make 48V.
    I just cannot quite understand (mainly) how the Mosfet Side of the circuit operates (PLVA668A, Potentiometers, IN4148, M1 and M2) why there is two of them in sort of a potential divider configuration.
     
  11. Bordodynov

    Well-Known Member

    May 20, 2015
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    The gain of the first voltage amplifier is four. The output signal amplifies to 2V. The transformer transmits a signal of 0.5V to the gate of the transistors. Transistors are included in the common source circuit. Due to the use of the transformer, the signal on the gates in antiphase. I set the initial current to 150mA. Zener diodes and potentiometers are designed to set the operating point. At the beginning, the ammeter is turned on between the source of the upper transistor and ground and then it is set to 150mA (using a potentiometer). Then, the bottom potentiometer sets half the power at the source of the upper transistor. The gain of the field-effect transistors for a given signal is ~ 100.
     
  12. kevinnas

    Thread Starter Member

    Jul 31, 2017
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    I have seen how you build the transformers from your website. Is there somewhere I where I can buy these off the shelf?
    If I want to prototype the circuit what is the nearest source I could use?
     
  13. Bordodynov

    Well-Known Member

    May 20, 2015
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    The transformer is a ferrite toroidal ring. I indicated the dimensions in mm in the diagram. It is necessary that windings (a primary 20 turns and secondary 5 turns) occupy one length. This is necessary for strong magnetic coupling. Those. the closer the magnetic coupling coefficient to unity, the smaller the leakage inductance. This inductance reduces the gain at high frequencies. The leakage inductance is a parameter of the primary winding.
    I studied the possibility of buying a toroidal ring and did not find it in Digi-key and Mouser. So I looked for high-frequency ferrite in Russia. I chose ferrite with a permeability of 30 (given in the model).
    Conductors for windings are sufficient to apply a thickness of 0.3mm or more. You can use other ferrites that can be used in transformers for 30 MHz frequencies. For example, firm Amidon.
    http://www.amidoncorp.com/ferrite-toroids/
    I did not find any ready transformers. I tried to use ready-made transformers, but did not find the right configuration.
     
  14. kevinnas

    Thread Starter Member

    Jul 31, 2017
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    Thank you very much. May I also ask why for the first amplifier you used a combination of a Common Collector and Common Emitter, Usually it is the other way around like in the attached image, is there a reason for that? Also what is the purpose of C12, R18, R15? I have not seen that in any circuits online before, is it some sort of feedback? I am not sure I also understand this " At the beginning, the ammeter is turned on between the source of the upper transistor and ground and then it is set to 150mA (using a potentiometer). Then, the bottom potentiometer sets half the power at the source of the upper transistor"
    why is it set to 150mA initially?
    I understand how the First amplifier gives 4V which is then stepped down to 0.5V by the transformer, but why is the transformer giving 0.5V at the gate of M1 and M2? First of all why the Value 0.5V? is that just so that when it is multiplied by the gain 100 it will be 50V? If this is true then
    What is the reason for having the upper side and the lower side for the 2nd amplifier Mosfets?
     
  15. Bordodynov

    Well-Known Member

    May 20, 2015
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    I'm not limited to conventional circuits. I am limited only by my knowledge. I can draw a lot of circuits on two transistors, but I chose this one, it seemed to me the easiest to get the required gain, the required input impedance (~ 50 Ohm), and the lowest possible output impedance. Yes, you are right - this is negative feedback. You do not correctly calculate the signal levels. I took the source impedance of 50 Ohm. Together with the input resistance 50Ω, a divider is obtained into two. As a result, 0.5V is amplified by 4, and is obtained at the output of 2V. Transformer ratio 1: 4 ==> variable signal on the 0.5V gate. For other source resistances, other voltage levels are obtained.
    The gain is not less than 100 and it depends on the frequency.
    This is a push-pull amplifier in the AB class. The current is selected as a compromise. The limiting factor is the power dissipation. With amplification of substantially smaller signals, the gain decreases (nonlinearity). At high resting currents, linearity is higher.
     
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  16. kevinnas

    Thread Starter Member

    Jul 31, 2017
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    Thanks that makes sense I will start building the circuit up in the coming week. Is there any other off the shelf transformer I can use which may not be as good or may be limited in terms of frequency range but functional that I can use just to build up a prototype? or any specific DC supply to give the 150V? I have never built up a transformer before so I do not know if it is a good idea to do so myself. I could settle for a little worse performance if something off the shelf can be available
     
  17. Bordodynov

    Well-Known Member

    May 20, 2015
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    To manufacture a transformer is the simplest in this project (if you buy a suitable toroidal ferrite core, for a core with other dimensions and permeability, recalculation of the circuit is necessary). Here the main difficulty with the design of the board. With heat dissipation from transistors (how to use heat sinks). Without heat sinks, transistors will burn.
     
  18. kevinnas

    Thread Starter Member

    Jul 31, 2017
    73
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    Okay thanks, The heatsink issue will be okay I am not too worried about that. I will look into building the transformer, do you know of any good step by step tutorials available that I could follow in constructing one? I just want to make sure I do it right and not blow anything up in the process.
    Is something like this overkill? since the transformer will be much smaller there wont be space for this?
     
    Last edited: Oct 1, 2018
  19. Bordodynov

    Well-Known Member

    May 20, 2015
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    For example, I took FT-140-61.
    Isolate the ring. It is used for non-conductive varnish or polyethylene film. Then wind 20 turns of insulated wire (turn to turn). The thickness of the wire is not very critical. Secure the ends of the winding so that the winding does not disperse (for example, sewing thread). Then take a thicker insulated wire. Fold it in half and wind 5 turns of this double wire so that the length (or width) of these two secondary windings almost coincides with the length of the primary winding. This achieves a good magnetic coupling. Note that on the electronic circuit dots indicate the beginning of the windings. The secondary windings are connected to the transistors in antiphase! Try to make the supply of windings of minimum length.
    Here's a new circuit with a new core. I had to reduce the amplitude of the input signal.
    2018-10-01_15-06-58.png

     
  20. Bordodynov

    Well-Known Member

    May 20, 2015
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    To power the amplifier, you can use the simplest circuit on the transformer and diode rectifier. See transforms: A41-130-230 or FD8-120
     
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