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Op amps CSS check pls.

Discussion in 'Electronic Design' started by Robert Quinn, Mar 15, 2007.

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  1. Robert Quinn

    Robert Quinn Guest

    The circuit linked below is intended to be a CCS amp with a gain of 2
    and double pole LPF. The latter smooths an eight step, digitally
    simulated sinewave (8 Hz, 1.5V swing at pin 3). Load is a 200 Ohm air
    core solenoid.

    The finished device will be a field calibration source for my Earth
    micropulsation sensing coils. It operates from 4 x AA cells.

    This is as far as I have progessed, based upon a combination of
    existing circuits. I am having trouble finding the relevant
    information to proceed.

    http://home.iprimus.com.au/loungecinema/css.gif

    1. It only outputs unity gain. With the LM358 I am hoping for at least
    5V.

    2. The 1.5V sinewave looks fine, until a load is connected. At 200R
    the upper right half the the waveform distorts. At 25R the output goes
    flat. As monitored across the load.

    Any advice would be sincerely appreciated.

    Robert Quinn
     
  2. The FET is shown with source and drain interchanged. Even connected
    correctly, the 2N2700 has a typical gate cutoff voltage of 2.1 V,
    which means it needs to stay that far above the transistor base, and
    2.8 V higher than the output on the coil. You would do better with a
    higher power supply voltage, like 9 or 12 V. (Unless you've selected a
    transistor with especially low cutoff voltage- the range is pretty
    wide.) That it works at all with a 200 ohm load probably means you've
    got a FET with low cutoff voltage.

    The amplifier, besides being a filter, is also a unity-gain voltage
    follower below cutoff. It's a constant voltage source for the output
    on the top of the load coil, so there's no gain of 2. You would need a
    different circuit configuration to get a constant current source.

    The filter has a cutoff frequency of about 4 Hz, a little low for an 8
    Hz input signal.

    With a load of only 25 ohms and 3 V p-p output, you might be pushing
    the 2N2222 a little hard without a heat sink.

    If I can suggest something, you might split up the functions- filter,
    gain amplifier, constant current source- into separate stages until
    everything is working, and then condense it if possible. It's easier
    to troubleshoot things piecemeal.

    I simulated the circuit in LTspice, which you can download for free
    (just Google it). The .asc file, which I'll send in another post, can
    be loaded and will display the schematic, and you can simulate it. You
    can get an undistorted output with either a 200 ohm or 25 ohm load by
    changing the power supply voltage from 6 to 12V, or by changing the
    FET to an FDS6574A, which has a low (0.65 V) cutoff voltage. You can
    change this by right-clicking the FET symbol.
     
  3. The simulator file- copy the text and save it into notepad, and save
    it with an extension of .asc, and then load it into LTspice.

    Version 4
    SHEET 1 880 680
    WIRE -1312 -1008 -1504 -1008
    WIRE -1168 -1008 -1312 -1008
    WIRE -800 -1008 -1056 -1008
    WIRE -688 -1008 -800 -1008
    WIRE -1312 -992 -1312 -1008
    WIRE -800 -992 -800 -1008
    WIRE -1504 -976 -1504 -1008
    WIRE -1056 -976 -1056 -1008
    WIRE -1056 -976 -1232 -976
    WIRE -1056 -960 -1056 -976
    WIRE -912 -944 -992 -944
    WIRE -864 -944 -912 -944
    WIRE -688 -928 -688 -1008
    WIRE -1232 -912 -1232 -976
    WIRE -912 -912 -912 -944
    WIRE -1312 -896 -1312 -912
    WIRE -1264 -896 -1312 -896
    WIRE -1168 -880 -1168 -1008
    WIRE -1168 -880 -1200 -880
    WIRE -1104 -880 -1168 -880
    WIRE -1600 -864 -1632 -864
    WIRE -1504 -864 -1504 -912
    WIRE -1504 -864 -1520 -864
    WIRE -1488 -864 -1504 -864
    WIRE -1376 -864 -1408 -864
    WIRE -1264 -864 -1376 -864
    WIRE -992 -864 -992 -944
    WIRE -992 -864 -1056 -864
    WIRE -1632 -848 -1632 -864
    WIRE -1376 -816 -1376 -864
    WIRE -1312 -816 -1312 -896
    WIRE -912 -816 -912 -832
    WIRE -912 -816 -1312 -816
    WIRE -800 -816 -800 -896
    WIRE -800 -816 -912 -816
    WIRE -688 -816 -688 -848
    WIRE -912 -800 -912 -816
    WIRE -1232 -768 -1232 -848
    WIRE -1184 -768 -1232 -768
    WIRE -1184 -736 -1184 -768
    WIRE -1632 -704 -1632 -768
    WIRE -1376 -704 -1376 -752
    WIRE -1376 -704 -1632 -704
    WIRE -1232 -704 -1232 -768
    WIRE -1232 -704 -1376 -704
    WIRE -912 -704 -912 -720
    WIRE -912 -704 -1232 -704
    FLAG -688 -816 0
    FLAG -1184 -736 0
    SYMBOL npn -864 -992 R0
    SYMATTR InstName Q1
    SYMATTR Value 2N2222
    SYMBOL res -1504 -880 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 33k
    SYMBOL res -1392 -880 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R2
    SYMATTR Value 33k
    SYMBOL cap -1520 -976 R0
    WINDOW 3 38 44 Left 0
    SYMATTR Value 1µ
    SYMATTR InstName C1
    SYMBOL res -1296 -896 R180
    WINDOW 0 36 76 Left 0
    WINDOW 3 36 40 Left 0
    SYMATTR InstName R3
    SYMATTR Value 22k
    SYMBOL Opamps\\UniversalOpamp -1232 -880 R0
    SYMATTR InstName U1
    SYMBOL cap -1392 -816 R0
    SYMATTR InstName C2
    SYMATTR Value 1µ
    SYMBOL res -928 -928 R0
    WINDOW 3 38 56 Left 0
    SYMATTR Value 10k
    SYMATTR InstName R4
    SYMBOL res -928 -816 R0
    SYMATTR InstName R5
    SYMATTR Value 200
    SYMBOL voltage -688 -944 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 6
    SYMBOL voltage -1632 -864 R0
    WINDOW 0 -43 6 Left 0
    WINDOW 3 -59 105 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V2
    SYMATTR Value SINE(3 1.5 8)
    SYMBOL nmos -1104 -960 R0
    SYMATTR InstName M2
    SYMATTR Value IRF3706
    TEXT -1664 -1080 Left 0 !.lib opamp.sub
    TEXT -1640 -992 Left 0 !.tran 10
     
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