Increasing current/amperes from ac output

Discussion in 'The Projects Forum' started by rand, Jan 13, 2009.

  1. rand

    Thread Starter Member

    Jan 13, 2009
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    Alright so I need some help trying to increase the current of or maybe even voltage (not sure) of the output of a function generator (ac) that I built using a kit
    (specifically, from allspectrum).
    Its a function generator kit Model # FG-500K.

    Specs:
    • OUTPUT: Waveforms: Sine, Square, Triangle
    • Frequency: 1Hz -1MHz
    • Impedance: 600Ω±10%
    • Amplitude: Sine/Triangle 0-3V at 12V DC input
    • Squarewave *V (no load)
    • Frequency Variable Range: 100:1 or more
    • Frequency Multiplier: x1, x10, x100
    • SINE WAVE
    • Distortion: Less than 1%
    • Flatness: ±0.5dB 1Hz-100kHz
    • Temperature Stability: ±20ppm/°C Typical

    For the application I am trying to do, it seems as if its just not getting any electricity. When I used my teacher's more expensive (maybe like greater than $400) function generator it worked fine... and I don't think the frequency was the problem so I wanted to know if there was an easy way (to create/ cheap to buy) to amplify the current or if something else should be changed. I would rather use the one we have so that we can use it at anytime and don't have to go to school to use it. Have any ideas? It outputs by clipping 2 alligator clips for pos and neg I think. Wiki says that transistors amplify signal and something about op amps but I dont really know..

    Thanks..
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    You forgot to tell us your application of the function generator.
    The kit has 3V p-p into 600 ohms.
    How many volts p-p do you need into what resistance?
     
  3. rand

    Thread Starter Member

    Jan 13, 2009
    14
    0
    Oh i'm sending the output to a coil at a freq and then another coil (identical) should get electricity sent to it (lights up an led) Resonant inductive coupling. I dont really know what it needs to be but I know the other function generator is here um 20Vpp output into open circuit (10Vpp into 50 Ohms)
     
    Last edited: Jan 13, 2009
  4. SgtWookie

    Expert

    Jul 17, 2007
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    What frequency are you using?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Do you need DC coupling? An emitter follow could do that pretty well.

    Otherwise you could use a high power op amp (I've seen them, but don't have time to look them up). They may not handle square waves or triangle waves with perfect fidelity though.

    What impedance are you feeding?
     
  6. rand

    Thread Starter Member

    Jan 13, 2009
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    um there was actually a lot of frequencies that worked (multiple resonance frequencies?) although they were pretty high like 1,616kHz. but I can't remember the lower freq ones..

    um were using sine waves though. and explain impedance (related to resistance?)

    um a little searching this lol and this from radioshack? only like $2. I don't know how I would use it though.
     
    Last edited: Jan 13, 2009
  7. SgtWookie

    Expert

    Jul 17, 2007
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    An LM324 is an ancient and slow opamp. It's tounge is hanging out at just a few kHz.
    The TL082 is better, but it's not going to provide much current output.

    An LT1210CT opamp can output in excess of 1A at frequencies in the MHz range (35MHz bandwidth) with up to +/-15V supply rails.
    http://www.newark.com/jsp/search/productdetail.jsp?SKU=19P0824&CMP=AFC-OP
    At $17.32, not cheap - but will turn your meek signal generator into something much more capable. You'll need to build or buy a suitable power supply for it, of course.
    [eta]
    Yeesh - they add a $20 handling fee for shipping from Farnell UK - forget it.
     
  8. rand

    Thread Starter Member

    Jan 13, 2009
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    ok thanks but how exactly would I hook it up to my setup anyway because I just figured out what a lot of stuff does.. and is there any way to create an op amp because I might need to use this by this weekend and if not its okay too..(the shipping says about a week so its too long although i'l probably still order it if its not too expensive)
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    This circuit has no gain, but it will change the output impedance from 600Ω to an ohm or less. I've never build it, but it should work. Voltage regulation is critical, which is why it includes a 7812 regulator. Adjust R2 for 0Ω, and slowly tweak it up until R5 and R6 are dropping 20mv. At this point the amp is linear. It is about 600Ω input impedance and very low output impedance.

    [​IMG]

    Since it has no voltage gain you will be limited to the 3VP-P your function generator is speced at.
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The above circuit will not work correctly with output load impedance as low as 1Ω.

    It will provide power gain but with reduced output voltage.

    Simulation shows that anything less than 56Ω would give a lower voltage output than input. The 120Ω series resistor with the 200Ω variable resistor should be increased to 200Ω as the present values cannot provide the required 20mA transistor current(20mV across 1Ω emitter resistor) even with 200Ω VR set at maximum.
     
  11. rand

    Thread Starter Member

    Jan 13, 2009
    14
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    umm yea now I have no idea.. although I went ahead and bought a Dual BiFET OP Amp TLO82.. need to search how to use it..
     
    Last edited: Jan 14, 2009
  12. rand

    Thread Starter Member

    Jan 13, 2009
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  13. eblc1388

    Senior Member

    Nov 28, 2008
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    With frequency pass above 1MHz and output loading down to 50Ω, you just can't achieve the same result with a few components.

    You may try power opamps but they are expensive.

    Commercial Frequency Generator usually has an amplifier made up of many components(transistors) at the output to boost the signal power.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Try this instead.

    See the attached.

    R1 and R2 set the gain of the opamp; gain = 1 + (R2/R1)
    See this section of our E-books:
    http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

    Q1 and Q2 are voltage followers that provide a good deal more current than the TL082 by itself could supply. D1 and D2 in conjunction with R3 and R4 keep the transistors slightly conducting.

    A TL082 by itself might be able to sink or source 20mA to 30mA. With the voltage follower circuit, 300mA is achievable.

    The practical upper limit is going to be somewhere around 200kHz. You'd need a faster opamp than a TL082 to go higher in frequency with amplification.
     
  15. rand

    Thread Starter Member

    Jan 13, 2009
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    Ok so i tried getting the circuit up to the B part , and checking if that output connected to the led and gnd would light the LED up, and one time it did but then i tried hooking it to the coil (no load) and maybe it screwed something up? Instead of 10v i did 36v by hooking 4 9v battery in series and it worked but now I have no idea why it doesn't work anymore and i checked everything like 3 times.. the supply voltage for the op amp is 36VDC or +- 18VDC. i really don't know how to make it work again even though everything is the same... unless something just broke
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    What do you mean by this? Just to the (B) signal output portion?
    That will not likely have enough current to drive your coil; it'll only be around 20mA drive.
    Why don't you post your circuit exactly how you connected it up.

    If you tried to use an LED on the output of the opamp without an appropriate current limiting resistor AND a diode to prevent reverse current, you probably fried the LED.

    Adding more voltage would not increase the amplification of the opamp; it would extend it's range if your input signal was greater than 2v peak to peak.

    9v "transistor" batteries have very little current sourcing capabilities. You really need something more like a bench power supply, or at least a number of "AA", "C" or "D" batteries in series.
     
    Last edited: Jan 18, 2009
  17. rand

    Thread Starter Member

    Jan 13, 2009
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    alright i found a way to get 18v for power supply (ac adapter for cell phone or something) for the op amp on both positive and neg and got it to from like .35 mA to 50mA haha! however the voltage becomes like .2 mV when originally it was like 2v. the problem is that the coils still don't work although its possible the frequency is wrong.. i found out you need to do a lot more calculations with frequency and capicitance plates or something i dont know about.. but the other expensive function generator did it fine without them
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Did you try building the complete circuit, like I showed in the schematic?

    The circuit requires at least +10v and -10v to operate properly, with a minimum 350mA current available on both rails.

    If you have an old ATX form factor computer power supply available, you can convert it into a good bench supply for this purpose. You can use the +12v, -12v and ground to supply all of the power that you will need for this circuit.

    Google "ATX bench supply" for lots of ideas and instructions.
     
    Last edited: Jan 18, 2009
  19. rand

    Thread Starter Member

    Jan 13, 2009
    14
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    thanks for all the help.. yea im still working on the b->c part but its good that its already working for the b part.
     
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