why this preamplifier doesn't work ??

Discussion in 'General Electronics Chat' started by alarassi, Apr 22, 2010.

  1. alarassi

    Thread Starter Member

    Feb 6, 2010
    hello guys
    i have already finished a project that detects and amplifies heart sounds successfully!

    i wanted to try different circuit and test it but i found something weird on it,
    i can test the output signal from the microphone clearly but once i feed the signal into an amplifier i got nothing in the output!! two batteries as voltage supply to amplify the bipolar signal but i get no signal at the output!!
    i used different types of op-amplifier tl074, OPA2134 and also lm324 just to check the signal and get the same results!! i just connected the first stage of the preamplifier and also got no results!
    is there anything wrong in the schematic??
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Your schematic does not have the correct pins called out for the LM324 inputs and outputs except for the first portion. The power pins are good for the quad op amps (LM324 & TL074, but incorrect for the OPA2134, which is a dual. Perhaps you meant to use an OPA4124?

    Are the voltages applied in the correct polarity to the power pins?

    It's a good idea to explicitly note all voltage connections with the magnitude and polarity of the voltage. Same with correct pin labeling for the device used.
  3. alarassi

    Thread Starter Member

    Feb 6, 2010
    the connection were made correctly to each pin of the different op-amplifier! i refer to the datasheet for each amplifier before i connect!

    yes it applied correctly i think! the problem some times it works and then suddenly the signal appear!!!
  4. bertus


    Apr 5, 2008

    The input of the first opamp is floating.
    Connect a resistor of 100 K Ohm to 1 M Ohm to ground.

  5. alarassi

    Thread Starter Member

    Feb 6, 2010
    where should i connect the resistor? i did connect it after the capacitor before the op-amplifier but i got nothing!! it's weird problem and i don't know why!
  6. SgtWookie


    Jul 17, 2007
    Are you using a bipolar supply? It's not going to work well with a single supply.

    The diode D1 being in there is a bit odd.
  7. bitrex

    Active Member

    Dec 13, 2009
    Good catch. I'd suggest that perhaps that size input resistor is too large for the LM324 in this circuit - it sources a worst-case bias current at room temperature of 150 nA, which across a 1M resistor would cause a 150 mV voltage rise. The first stage has a gain of 101, with no provision to roll the gain off at DC! :eek:
  8. hgmjr


    Jan 28, 2005
    My first reaction to the schematic was identical to sgtwookie's. The schematic leads one to believe that the value of the power supply voltages are up to the user. A single-ended supply will pretty much cause the circuit to run like a three-legged dog.

  9. SgtWookie


    Jul 17, 2007
    If there are high-frequency oscillations in the circuit, U3 will charge the cap via the diode that is in the path to the noninverting input of U4.

    You need 0.1uF caps from the supply rails to ground on the opamp's power supply
  10. alarassi

    Thread Starter Member

    Feb 6, 2010
    thank you all guys
    the problem was as Bertus said the input of first op-amp is floating!
    i added 1M resistor and it works now

    thank you
  11. Audioguru


    Dec 20, 2007
    Since the entire circuit is non-inverting, since the LM324 opamp has inputs that work well at ground and since the output is a DC voltage then the circuit will work (when the bias resistor is used for the first opamp).

    But it won't work with a TL074 or an OPA4134 unless a dual-polarity supply is used.