Amplifying a small frequency

Discussion in 'The Projects Forum' started by mattswk, Jul 22, 2014.

  1. mattswk

    Thread Starter New Member

    Jul 22, 2014
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    Hello,

    I am trying to amplify a small signal with variable gain. The frequency going in is about 30 Hz. That is my limitation. Right now, I am using LT 1228. Before I get into the variable gain problem, I first want to see if the LT 1228 will even work with such a small frequency. I have attached a schematic of my circuit that should be similar to the one on the LT 1228 datasheet. My question is theoretically will this circuit be able to work with such a low frequency? I have breadboarded the circuit and I am not receving much output on the oscilloscope so I am curious if this is my fault or the LT 1228's fault. If the LT 1228 can't handle the frequency (it does so a small signal at ~300Hz), does anyone have any recommendations of a chip that I could use for this?

    Thank you
    Matt
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
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    What is Iset of LT1228?
     
  3. RichardO

    Well-Known Member

    May 4, 2013
    1,231
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    The LT1228 is a 100MHz Current Feedback Amplifier with DC Gain Control. This amplifier is about 1 million times faster than you need for 30 Hz. I suspect that you do not have proper power supply bypassing and circuit layout for such a fast part.

    I would suggest that you do some more research looking for a DC controlled variable gain preamp intended for audio frequencies. National Semiconductor (now part of TI) had such parts.

    If you want to continue experimenting with the LT1228 then you will need to put 0.1 uF ceramic capacitors from each power pin to ground. These caps must have leads less than 0.2". The 4.7 uF caps shown in the example diagram in the data sheet should also be near the LT1228 but the leads do not have to be as short.

    The rest of the wiring around the LT1228 must also be kept similarly short. The amplifier can not drive capacitive loads greater than about 200 pF. Because of this, you need to put a resistor of about 50 ohms between the amplifier output and any capacitors.

    What is the purpose of the circuit with the LT1001? I don't see what it will do.
     
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  4. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Lots of opamp models would work. Do you need your variable gain to be voltage-controlled, or is manual control fine?
     
  5. mattswk

    Thread Starter New Member

    Jul 22, 2014
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    I will continue testing with the LT 1228 unless I have a different chip on hand. I'd rather not buy something new unless I have to. The LT 1001 compares the incoming voltage from Vout to a set voltage (I made it 5V to the negative, sorry I forgot to say this). I didn't show the other part to this design, but essentially there will be a weak signal coming through which I will amplify ~200x (I don't know exactly what it will look like but I'm using a simple sine curve to test). Then the amplified signal will be compared in another LT1001 with a square wave about 30Hz. This signal will then go through the LT1228. Considering I won't know what the signal will look like, it will be voltage controlled. What I'm hoping to do is 1) amplify a weak signal, 2) variable gain it so it doesn't oversaturate or not be noticeable enough, 3) find out how much that gain was by comparing it to a fixed square wave. Right now, where it says "Freq" I am testing with a .3 mV, 30Hz square wave. I am expecting to find that signal to be simply amplified. Once I add the other components, I expected the gain to change as the signal changes. Thanks for the responses guys. I hope this clears some stuff up. I'll continue to work and let you know if I solve my problem. I appreciate any further ideas.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the only change would be the capacitors, you would need larger values for less impedance at 30 hz.
     
  7. mattswk

    Thread Starter New Member

    Jul 22, 2014
    13
    0
    Hi alfacliff,

    Could you clarify which capacitors you are referring to?

    Thanks
     
  8. mattswk

    Thread Starter New Member

    Jul 22, 2014
    13
    0
    Nevermind, it works now. I believe the signal is small enough to pick up noise and disrupt everything. Thanks everyone for the help. I also took out one of the capacitors because I suspected that the chip couldn't handle the capacitance.
     
  9. RichardO

    Well-Known Member

    May 4, 2013
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    382
    Good. Could you show us what is working for future reference?
     
  10. mattswk

    Thread Starter New Member

    Jul 22, 2014
    13
    0
    Yes, sorry Richard I'm late. It's not anything to really show. I suspect that some signal flowed back from my circuit into the function generator disrupting things. Oh by the way, a couple pins on the LT1001 didn't seem to work. I changed the pins that I connected to because there were 4 opamps in the LT1001 chip and it started to work. Odd, I couldn't detect any shorts. So I don't know why it wasn't working. Other than that, I think things were fine from the beginning, as far as the layout goes.
     
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