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  1. audioguru2

    preamp op amp circuit question

    A differential amplifier cancels the same signal on both inputs but its emitters are connected together, not its collectors. This one is the differential input of an opamp:
  2. audioguru2

    a very simple FM Tx

    No Walid. Your calculation doesn't include the +90 degrees and -90 degrees phase shifts of the capacitor and inductor and they cancel. Therefore at resonance a parallel LC has an infinitely high impedance.
  3. audioguru2

    BJT biasing

    A single transistor without negative feedback so its gain is about 170 distorts very much when its output level is high. An opamp has such a high amount of gain that a large amount of negative feedback can be used to reduce its distortion to nearly nothing, and still have high gain. Here is a...
  4. audioguru2

    preamp op amp circuit question

    Hi Walid, We were talking about the 10uF filter capacitor. If it isn't there and the supply has 40mV of mains hum then the R3 and R4 divider would reduce it to 20mV and the opamp would amplify it by 101 to 2.02V at the output. Signals at the input need to have a very low impedance or they will...
  5. audioguru2

    preamp op amp circuit question

    BEEP! Hee, hee. ;D R3 and R4 are a voltage divider. They reduce noise from the supply to half. Then it is amplified 100 times so the output is 50 times as noisy as the supply.
  6. audioguru2

    preamp op amp circuit question

    Hi Walid, I fixed the backwards inputs. The 10uF capacitor is a filter so that any noise from the supply isn't amplified 50 times. Hi Kevin, The opamp was drawn as a comparator with hysteresis. It won't oscillate, it will just latch with its output either high or low.
  7. audioguru2

    a very simple FM Tx

    Hi Walid, The calculator you found is WRONG! It has the wrong formula for the impedance at resonance of a parallel LC. You shouldn't calculate things at such a high frequency because you don't know how much stray capacitance is in parallel with your C, and you don't know how much inductance is...
  8. audioguru2

    Tan Capacitor

    Do you mean a tantalum capacitor? They are a low impedance up to a medium high frequency. They cause audio distortion if they are a coupling capacitor. If they have reversed polarity across a high current power supply then when the power is applied they explode and shoot molten metal pieces.
  9. audioguru2

    preamp op amp circuit question

    Hi Walid, The opamp has a single supply so R3 and R4 form a voltage divider to provide it with a half-supply reference voltage. Then the output of the opamp can swing symmetrically. If it had only R4 then its output would be saturated near 0V and it would be a rectifier. I just noticed that the...
  10. audioguru2

    EMC Question

    Hi Stew, I don't know any communications at the very low radio frequency of only 150KHz to 200KHz but maybe ships or submarines use them. Any ships or submarines around your electronic controller? The governments have set limits for radio emissions from electronic products but I don't know why...
  11. audioguru2

    preamp op amp circuit question

    When you swap the inputs of the opamp you will notice that the input impedance of the preamp circuit is only 2.2k ohms which is too low for an electret microphone. A non-inverting opamp circuit would have a much higher input impedance so that an electret mic is not loaded down. For a very low...
  12. audioguru2

    Microphone Switch

    Hi Tenatious, Welcome to our forum. ;D An electret microphone needs a small amout of power. Usually a 10k resistor is connected to it from a positive supply of about 9V. The output level of microphones is very low. You will need to amplify the output of the mic with a mic preamp circuit. The...
  13. audioguru2

    Comparator usage.. Help!

    Hi Salvatore, Your phototransistor is an emitter-follower without any gain. If its emitter is grounded and the 10k resistor is connected from its collector to the positive supply then its collector output would have lots of gain so it can switch quicker. Then the comparator output would be the...
  14. audioguru2

    a very simple FM Tx

    Hi Walid, A parallel resonant LC has a very high impedance, nearly infinite ohms. A series resonant LC has a very low impedance, the resistance of the coil plus the resistance of the wires on the capacitor. Look at LC Resonance in Google.
  15. audioguru2

    transmission lines

    Coax cable has series inductance and capacitance between conductors. If it is terminated with an incorrect resistance like a short or an open, then it resonates as a series resonanlt circuit with a very low resistance or as a parallel resonant circuit with a high impedance. You tune the resonant...
  16. audioguru2

    transmission lines

    My cable TV is fed from a 75 ohms coax cable. It isn't a very high resistance. Some coax transmission lines are 50 ohms.
  17. audioguru2

    a very simple FM Tx

    Hi Walid, Thanks for the white roses. They are fresh and smell nice. ;D The tuned circuit at the collector of the transistor is resonant and tries to be a high impedance. The coil is an auto-transformer and the 50 ohm antenna is multiplied by the square of the turns ratio so appears as 5 x 5 x...
  18. audioguru2

    a high pass filter question

    What are you talking about, Kevin? A single transistor can't have a voltage gain of 200 with a load resistance of 50 ohms, but two or three transistors can. When you increase the load resistance to 1k it won't affect the input. You can short the collector of a transistor with a capacitor to...
  19. audioguru2

    a very simple FM Tx

    Hi Walid, I started to calculate the effect of the 1nF capacitor on the frequency response of the electret mic at 15kHz but I made a simulation instead. It makes no difference!
  20. audioguru2

    POWER INVERTER

    No, MP. A flip-flop latches in either state and is different. This circuit is called an astable multivibrator oscillator. An astable multivibrator oscillator that uses silicon transistors usually needs a supply voltage less than the max reverse-bias voltage rating of the emitter-base junction...
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