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SW radio using OP amplifier

Discussion in 'Radio and Wireless' started by aurelZ, Oct 19, 2020.

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  1. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    Because of unknown reason similar post is closed i open new one .
    Why i simply have lot of this chips and would be odd to throw them into
    garbage,
    In the past i have build FM transmitter with 4.5 V using two RF trasistors and
    uA741 on this 4.5V battery and work well...
    so now i am wondering how is possible that uA741 in metal case work on such a low voltage?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,591
    2,360
    Nov 17, 2011
    It may work, but possibly not well.
    The µA741 is specified for +- 5 V min. supply voltage. To ensure every chip will work at this low supply voltage, the manufacturer will test the chips to somewhat below this min. value. SO the chips leaving factory may or may not work even at +-4 V. This is however not a reliable design as the chip is operated outside its specification. Some chips will work at +-4.5 V, other will not. And some of those that work from +-4.5 V under room temperature conditions may fail in hot or cold weather.
     
  3. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    Yes that is strange to me either, in that fm transmitter uA741 was used as microphone preamplifier for oscillator
    part of fm transmitter and if i rememeber circuit work ok.
    When i looking into iner circuit of 741 chip we can see that output transitors are complementary with
    two resistors at emitters upper is 25 ohm and lower 50 ohms which is probably used to limit
    output current of chip and they have very high resistance.
     
  4. aurelZ

    aurelZ

    90
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    Jun 10, 2019
    upload_2020-10-19_13-52-0.png
     
  5. aurelZ

    aurelZ

    90
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    Jun 10, 2019
    It is stated that gain should be around 40 which gives R4- 8k2 and R3 -220 ohm
    upload_2020-10-19_14-9-42.png
     
  6. bertus

    bertus Moderator

    1,158
    439
    Nov 8, 2019
    Hello,

    That 741 amplifier circuit will not work.
    The negative going signals will be cut.
    Also the 741 will not be able to drive a speaker.

    Bertus
     
  7. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    I will try this
     
  8. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    upload_2020-10-19_17-24-25.png
     
  9. bertus

    bertus Moderator

    1,158
    439
    Nov 8, 2019
    Hello,

    The shown schematic has no bias for the opamp.
    Also there is no detector.
    It probably uses the opamp itself as detector.

    Bertus
     
  10. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    Yes Bertus you have a right ,i will add 50k trim-pot to try
    and yes it should be regenerative detektor.
    I have somwhere ..in a one old electronic book similar circuit
    so i will try ...thanks for a tip !
    upload_2020-10-19_18-24-45.png
     
  11. bertus

    bertus Moderator

    1,158
    439
    Nov 8, 2019
    Hello,

    If you want to bias the opamp correctly, have a look at fig 3 of the attached PDF.

    Bertus
     

    Attached Files:

  12. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    Yes i see it is non-inverting version where is nothing connected from + rail
    right? just a input capacitor ?
    upload_2020-10-19_19-30-26.png
     
  13. aurelZ

    aurelZ

    90
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    Jun 10, 2019
    Is that mean that i don't need any voltage on non-inverting input ...where is just a base of transistor ?
     
  14. bertus

    bertus Moderator

    1,158
    439
    Nov 8, 2019
    Hello,

    Do you see the point Vcc/2 ?
    That is on half the supply voltage.
    It can be done using two equal resistors with a capacitor of 10 uF from the middle to ground.

    Bertus
     
  15. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    I see that point of course but for working point transistor base dont need voltage splitter with
    two resistors as far as i know ...i will see
     
  16. Harald Kapp

    Harald Kapp Moderator Moderator

    10,591
    2,360
    Nov 17, 2011
    Depends on the specific circuit and can be true for single transistors. However, you're dealing with an operational amplifier here which is much more complex and requires an input voltage higher than the negative suupply voltage and lower than the positive supply voltage.
    This kind of amplifier was originally meant to be used with a positive and a negative supply, so with no input signal (0 V) the voltage at the inputs and the output would be at 0 V, which is exactly halfway between V- and V+.
    A single supply lacks the negative rail, so the input reference (formerly 0 V) needs to be brought to within the available range of the supply voltage, namely 0 V ... V+. A common way to do this is to use V+/2. One could use other values like 2/3 × V+ or 1/3 × V+, but that would lead to asymmetric behavior. Read this application note for more in detail information.

    Note that there are so called rail-to-rail opamps which can operate from a single supply with input signals reaching down to 0 V and up to V+. But even these cannot be used with AC input signal reaching below 0 V without adding an offset (see app note).
     
  17. Audioguru

    Audioguru

    2,999
    672
    Sep 24, 2016
    The design of a lousy 741 opamp is 52 years old. It has many problems.
    Use a modern opamp instead.
     
  18. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    Ha ha "lousy" if is uA741 lousy what is then with LM386 ..it is probably SUPER lousy with
    annoying hiss.
    uA741 is probably one of the best every made chips , just to let you know some special version are used in
    military applications from -50 to 150 C degrees...
     
  19. Audioguru

    Audioguru

    2,999
    672
    Sep 24, 2016
    The lousy 52 years old 741 opamp should be kissed "goodbye" then buried. It was never made for audio because its slew rate is too low therefore it cannot produce full output at the highest audio octave from 9kHz to 20kHz and its noise is so high that it is not spec'd.

    The LM386 is a power amplifier, not a preamplifier. Its noise level is fairly low at its default gain of 20 bit the noise is bad when its gain is 200. If you need the gain of a preamp then use a low noise audio opamp for it.

    New Japan radio made a NJR386 copy of the LM386. Their datasheet has horrible "Engrish" and says for lower noise then use resistors to ground on the inputs that are less than 10k ohms. The Ruby guitar amplifier project used the Japanese NJR386.
     
  20. aurelZ

    aurelZ

    90
    9
    Jun 10, 2019
    ```The LM386 is a power amplifier, not a preamplifier```
    well who know that ??? he he better noisy toy powerish amplifier which is not used in almost any comercial
    product
     
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