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Opamp and circuit help please.

Discussion in 'Electronic Basics' started by John Popelish, Aug 8, 2004.

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  1. Eliminate C14 and tie pins 3 and 5 together.
    The LM6132 has a wider input common mode range and output voltage
    swing but this may not be a problem if you expect the final amplitude
    to be well below the supply rails.
    Sounds reasonable.
    Then you will need to replace the supply with a stable reference as
    the source of the decision voltage at TR1. You may be able ot build a
    single voltage reference that is shared by all functions that need
    one.
    I like the 15 mA dual Schottky, BAS70-04.

    http://rocky.digikey.com/WebLib/Zetex/Web Data/BAS70.pdf
    I doubt R4 is necessary. C2 has to have a much lower impedance around
    40 kHz than the parallel combination of R1,5 to clean the amplifiers
    of supply feedback. Of course R 1 and 5 could be higher values before
    the two amplifier bias currents cause much of a bias point shift.
    With the LM6132 they could easily each be a meg ohm (reduces battery
    current a bit). The LM833 has a worst case bias current of 1 uA, so a
    pair of 1 meg resistors supplying that to two stages would shift the
    bias point a volt. I would probably use 100k resistors with that
    amp. This gives a 50k divider impedance. At 40k, a capacitor with
    500 ohms impedance (100 times reduction of any 40 kHz getting in
    through the positive supply) would be .008 uf. So 10 uf is pretty
    generous. I would probably use a .01 or .1 uf ceramic (not Y5V or Z5U
    because of their microphonics) or film.

    As I said earlier, eliminate C14.
    Put the 100 ohm resistor below the transducer, and tie the shield to
    the positive rail.
    You may want ot start thinking about how to transform these two gain
    stages into band pass filters with gain to reduce interference.
    Depending on your construction it may have more to do with stray
    capacitance across the feedback resistors, which reduces gain. Lower
    values of resistance reduce this effect.
    Connect a signal diode (1N4148, etc.) in series with your digital volt
    meter and adjust the frequency for maximum positive voltage at the
    drain of MOS1.
     
  2. Jamie

    Jamie Guest

    its simple.
    look for a single chip DC-DC inverter.
    design your circuit to operate like at
    5 volts.
    they have Converters that will maintain
    5 volts even when the input voltages gets
    down as low as 1 volt.
    if you don't want to do that then use a
    high efficiency type.
    below is a link to a reg that should handle the operation of
    at least your reciver and comparator.

    http://www.micrel.com/product-info/products/mic2954.shtml
     
  3. Rubicon

    Rubicon Guest

    Hello,

    I have been trying to make a 40kHz ultrasonic transmitter and receiver
    without much success until the recent "Opamp problem" post and the
    subsequent replies. It answered many questions for me.
    My desire is to have the receiver battery powered and be reliable.

    I have put a schematic on the net at:
    http://www.geocities.com/talionis.geo/Temp/temp.html

    Unable to locate a suggested LM6132 low power opamp locally I am
    trying a LM833 in its place. With the LM833 (GBW of 10MHz min - 15MHz
    max, slew rate of 7V/uS) I set both halves of the opamp with a 1K and
    33K for a gain of 33*33=1089.
    The square root of a desired 1000 stage gain = 31.6 so a standard
    value 33K negative feedback resistor was chosen for each stage. The
    GBP would then be 10*33*40000=13.2MHz which is within the upper and
    lower limits of the LM833. Correct so far?

    There is a LM393 comparator in place of an LM358 opamp as with a
    transistor based receiver circuit I built with a CA3140 opamp as a
    comparator (1.5m range) I found that a drop in battery voltage caused
    it to stop working. Re-adjusting the threshold trimmer fixed this but
    I don't want to have to keep doing that as the battery depletes.

    I've made the D1, D2 diodes schottky types, not quite the right ones I
    know. 1SS106 barrier diodes would be better but are unavailable.

    The yellow dots on the schematic are some things I'm not very sure
    about. What are the functions of R4, C2 and C14 and are their values
    correct?
    Do I need to add anything to have a long shielded cable to TX1?

    I have a basic 555 transmitter I made for the transistor based
    receiver circuit and using it on the above LM833 circuit it didn't
    work at all. Changing the opamps resistor values to Part-A 10k/1M and
    Part-B 10k/100K from a printed off rangefinder circuit it did but at
    only 2-3cm.
    I am not sure why the transmitter worked on the second setup and not
    the first. I read where it'd be better to stay away from large value
    resistors to decrease noise with such high gain. It's not the best
    transmitter but it must have been roughly tuned to 40kHz to work at
    all. I do not have a scope to measure it but I have seen a frequency
    meter kitset that I think might just do to tune it or its replacement.

    The transmitter on the schematic I haven't yet built.

    Any help always appreciated.

    Andrew.
     
  4. Rubicon wrote:
    (snip)
    The mosfet has a very fast fall time that has nothing useful to do
    with the emitted frequency but causes noise emission from the
    transducer lines. If you move the resistor to the drain lead of the
    transducer, you slow the rise and fall times on both transducer leads,
    reducing RFI. Since then, the transducer is connected directly to the
    positive rail you might as well just use that as the shield return.
    Still probably best to put a twisted pair inside the shield, and not
    connect the transducer end of the shield to anything.
    Agreed, as long as the long term state does not pass current through a
    pull up resistor. If you include that current, the opamp may win.
    Agreed. The filter is there just to keep wide band electrical noise
    from being amplified as much as the transducer signal is.
     
  5. Rubicon

    Rubicon Guest

    John,

    Thanks for the help there. I've made the alterations as you suggested
    and its range with the basic transmitter is now approx 20cm using my
    original 1K/33K per stage resistor ratios and trying the LM358 instead
    of the LM393. A significant percentage increase in range to me but its
    been suggested by a "noddy" that if I'm that close I'd might as well
    lean forward and turn it on myself.
    I may well have damaged the combined Tx/Rx transducers in my first
    attempts but I'm not sure about that.


    You advised to "Put the 100 ohm resistor below the transducer, and tie
    the shield to the positive rail" I'm not 100% sure what you mean here.
    Tie the long cables shield wire to V+ and then to the transducers
    negative pin while the cables core goes from the mosfets drain, 100
    ohm resistor and to the transducers positive pin? Doesn't sound right,
    to me the shield wire is negative. Could you please clarify?

    I understand that with an opamp as a comparator without negative
    feedback a little swing in input voltage causes the opamps output to
    saturate to almost V+ or V-. Which would be best to use here for a
    sensitive ultrasonic receiver, an LM358 opamp configured as a
    comparator or an LM393 comparator? My liking of the LM393 is due to
    its lower power consumption.

    I'm looking into bandpass filtering. I had read somewhere that
    ultrasonic transducers were a mechanical filter and that additional
    electronic filtering wouldn't be needed. Wrong I take it. I have done
    a little searching and I think that the type of filter required here
    is the active bandpass type with a high Ho and low Q.

    Not yet looked at the TR1 stable supply reference but I will.

    Thanks for the tuning tip

    Regards,

    Andrew.
     
  6. Rubicon

    Rubicon Guest

    Jamie,

    Thankyou for the advice and the link. I have it up on screen at the
    moment.

    Regards,

    Andrew.
     
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