square pulse amplifier

Discussion in 'The Projects Forum' started by nl5hucd, Apr 17, 2011.

  1. nl5hucd

    Thread Starter New Member

    Apr 17, 2011
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    I am building a pulsing (square pulse) circuit using voltage power supplies.

    so far i am able to generate nice square pulses with a few % duty cycle using a 555 timer (i have a N914 diode that helps me get such low duty cycles). The 555 timer pulses are 5V in size. afterwards, I am trying to input this 5V square wave into an NMOS transistor from NTE:

    http://www.nteinc.com/specs/2900to2999/pdf/nte2932.pdf

    However, I am unaware of how to properly connect the set up. should i use a common source amplifier? I am trying to get about 20-25 V pulses out (after amplification). I have been trying to read Sedra/Smith but I'm not convinced I know all the parameters from the data sheet to do the calculations.

    Anyhow, any help would be appreciated. Thank you.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    I guess you are thinking more of changing switching levels rather than amplifying the 5V pulse from the 555 timer. Presumably the 555 runs from a 5V supply ...?

    Keep in mind that a single stage FET switch in Common Source mode will invert the pulse - as would a BJT switch operating in Common Emitter mode. So you'd get a long high output and a short low output - opposite to what you wanted...?

    Using the FET in switching mode might require more than the 5V peak pulse value to properly turn on the FET as a switch.

    You could do the job equally well & perhaps more effectively with a BJT as the switching device if you are constrained to using a 5V pulse.

    In the case of a BJT you drive the base through a suitable resistor value which ensures the BJT saturates. The emitter would be tied to 0V and the collector connected to a 20-25V DC supply through a load resistor. The 20-25V pulse would be taken off the collector terminal.

    If you require a low output resistance you might then feed the collector to an emitter follower stage.

    If you really want to use a FET you could run the 555 timer off a supply voltage higher than 5V so that you get sufficient output to reliably drive the FET gate, to ensure a low drain to source on resistance.
     
  3. nl5hucd

    Thread Starter New Member

    Apr 17, 2011
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    0
    I am not constrained to 5V. I can input a lot more than that. I believe my psu goes to about 25-30 V ( I am not in the lab right now to verify this however).

    we have actually tried to drive the 555 timer higher, but in checking the voltage out of the drain, our pulse completely washes out. When I last looked at the system, we had a 1M Ohm resistor on the source and the drain. So obviously this needs to be changed.

    You are also correct, I also don't want to invert it. I was also thinking of using an LM 741 noninverting amplifier instead. However, I'm not sure I'd be able to get the gain I want without running the numbers.

    If I used say, 12 V to drive the 555 timer, what sort of configuration would be best for the MOSFET amp?
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Rather than inverting the signal from the 555 timer can't you configure it to operate with a long high output and short low output? Use the diode to make a short discharge and long charge cycle.

    A 12V supply to the 555 would produce sufficient output to directly feed the gate and thereby turn the FET on properly as a switch.

    So the 555 timer pin 3 [12V pulse] would connect directly to the FET Gate. The FET Source would connect to 0V and the Drain would be connected to the 20V supply through a suitable resistor - say 2.2KΩ. The 0-20V output pulse comes off the FET Drain terminal.
     
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