Phase shift oscillator SUCCESS!

Discussion in 'The Projects Forum' started by KLillie, Aug 5, 2014.

  1. KLillie

    Thread Starter Member

    May 31, 2014
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    My amps at the output were 0.25ma. I did not have any luck using the second op amp to boost my signal. How should this be connected? The circuit and output are below.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Square wave output tells us that you applied too much gain. Basically you amplified signal so much that it went straight to the rail, once it hit the rail it can not go any further so you see a flat line, overall effect is you see something that looks like square wave.
     
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  3. to3metalcan

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    Jul 20, 2014
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    KLillie, can we see the circuit you tried that included the second op amp?
     
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  4. KLillie

    Thread Starter Member

    May 31, 2014
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    OK First I tried the schematic below except I used the same power supply as the 358 and a 10nf and a 2N3904 transistor. That transistor blew, so I used a 4401 and this is when I got the weird output. I tried the second op-amp in the 358. I grounded pin 6, put my output from the first amp to pin 5 and expected a good output from 7. I don't think I got anything. I can't remember. It was late.
    I wanted to get about a 5 volt pulse for my arduino frequency counter.
     
    Last edited: Aug 5, 2014
  5. to3metalcan

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    Jul 20, 2014
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    This transistor circuit is biased for maximum gain...if you want to use it, you're going to need a crash course in common-emitter transistor biasing. Look up voltage divider bias and emitter degeneration...you're not going for maximum amplitude, you have a target gain in mind.

    An op-amp circuit would probably be easier (certainly more predictable), and it also makes more sense than wasting the other op amp stage and using an extra transistor... but you need to set up the negative feedback with a pair of resistors. Look up "non-inverting amplifier."
     
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  6. KLillie

    Thread Starter Member

    May 31, 2014
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    To: to3metalcan
    Thanks that's just what I needed. :) And yes, I have a lot to learn.
     
  7. to3metalcan

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    Jul 20, 2014
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    Hooray!! I've been interested in this stuff my whole life, but I've only been hard at it the last four or five years, so I still have memory of what was easy for me to understand and get working off the bat, and what took awhile to make sense. :)
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Why don't you just output the sine from the opamp output pin that has a low impedance?
     
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  9. KLillie

    Thread Starter Member

    May 31, 2014
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    In my lm358 I tried hooking up my sine wave output from my 1st op-amp to my 2nd. (LM358 has 2 op-amps). I used the configuration below. NUTHIN! But when I used a non-inverting voltage follower circuit I got the same wave form. I did this to estimate that the 2nd op-amp had at least continuity. What needs to happen to make this amplify?
     
  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    "configuration below" does not have values of the resistors. Telling us that you used "configuration below" tells us nothing.
     
  11. KLillie

    Thread Starter Member

    May 31, 2014
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    Sorry-image didn't upload. First I used a 1M res. for RF and a 1K pot for R2, then I tried a 330K pot for RF and a 20 ohm resistor for R2. I was wondering if my problem was because I'm using a single supply?
     
    Last edited: Aug 8, 2014
  12. to3metalcan

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    Jul 20, 2014
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    Well, whatever else may be wrong, the LM358 has almost no bandwidth at 1000x (!) gain...most op amps don't; this much gain would be set up as the product of several successive gain stages (also unconvinced you actually need this much gain...explain?)
     
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  13. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Yeah, this makes sense, the second op amp becomes low pass filter, and since the pass band is tiny, frequencies higher than pass band are blocked so output is whole lot of nothing.
     
  14. KLillie

    Thread Starter Member

    May 31, 2014
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    Sometimes I need a slap in the face. I was thinking I needed a 1000 gain! Bad, quick math in my head. I had a 50mV output p-p and wanted 5v. But, ultimately I want to convert this to 5v, zero to peak positive, square wave form. WHY? They have these frequency counter programs for Arduino on the internet. So I need to retain the frequency, but I think my input needs to be a pulse. I also found the below chart in the LM358 specs. Is this the bandwidth problem? Sorry if my questions sound impulsive...they are. :)
     
  15. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Datasheet says that at unity gain (gain of 1, 0 dB) and with loading capacitor+loading resistor the bandwidth is 0.7 MHz (700 kHz). That is what your graph shows, the plot of the minimum is about 700 kHz on the frequency axis and 0 dB on the gain axis.

    Now when you jacked up the gain to 1000 (60 dB), your bandwidth shrunk to less than 1 kHz, say about 800 Hz. You got 3.6 kHz signal, which WaaaaaaaaaaY outside of your 800 Hz bandwidth so it got blocked.

    This low pass filter is actually interesting property of op amps. You don't really have to do much, just pick op amp with bandwidth you want and any signal higher than the bandwidth gets blocked.
     
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  16. KLillie

    Thread Starter Member

    May 31, 2014
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    Thanks shteii01, very cool info.
     
  17. KLillie

    Thread Starter Member

    May 31, 2014
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    I got my second op-amp in my lm358 to amplify, but it's clipping my negative wave. Thoughts? (I know I'm a pain in the butt)
     
  18. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you show a schematic of the complete setup?
    It might be that an offset in the oscillator will cause the clipping.

    Bertus
     
  19. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Two things you can do:

    1) reduce gain

    2) move the wave higher (change dc offset). remember that wave is oscillating around a dc voltage. you can increase this dc voltage and the wave will shift up, this way you will have more room for the negative cycle of the wave, the wave will not hit the bottom rail and there will be no clipping. The trouble here is that if you shift the wave up too much, then you will hit the top rail and have the positive cycle of the wave clipping.

    The worsed case scenario is that you may need to do both, reduce the gain and shift the wave.

    Ok, I just though of the third option.
    3) Increase the op amp power supply (rail). That will give you more room for the signal to oscillate, then you can shift it up. Or do the dual op amp power supplies, this way again you give the signal more room to oscillate by moving the bottom rail lower so the negative oscillations of the wave don't hit the bottom rail and don't get clipped.
     
  20. to3metalcan

    Member

    Jul 20, 2014
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    Shteii's pretty much covered the bases re: your current difficulty...I just wanted to add some preventative advice: though you're having the opposite problem at the moment, one thing about the LM358 family of op-amps is that they can go all the way down to their negative rail, but can't output quite as high as their positive rail...there's about a 1.4V internal drop.
     
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