open loop gain VS close loop gain

Discussion in 'General Electronics Chat' started by relicmarks, May 17, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    How do you measure open loop gain? i put my dbm meter where?
    How do you measure close loop gain? i put my dbm meter where?

    IF a op-amp has a 40db of open loop gain and 20db of close loop gain , why is that bad VS a op-amp at 90db of open loop gain and 60db close loop gain?

    What does the open loop gain and close loop gain do really?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You feed a known signal level to the inverting input, ground on the noninverting input, and put your dBm meter on the output. Subtract the known input level from the measured output level.

    Same thing, except now you'll need to have the feedback path connected from the output to the inverting input.

    It's whatever the specs call for. If it's out of specfication limitations, then it's bad.

    Open loop gain is the maximum gain (whether current or voltage, depending upon the type of opamp) that can be obtained from the amp.

    When you go closed loop, there is an R/C/L (various combination thereof) feedback path from the output to the inverting input.

    If the feedback path to the inverting input is a zero-ohm connection, the amplifier is then a unity-gain buffer; whatever appears on the noninverting input will appear on the output; within the limits of the device.
     
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Thanks alot for the information

    If i don't have a meter that measures dbm

    Whats the easist way to take a AC voltage reading and convert it to a db reading?

    6 volts AC is what dbm?
    12 volts AC is what dbm?

    Or is dbs not relative at all to a AC voltage value its seperate and different?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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  5. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    seems like its best just to go buy a DBM meter its easier it seems
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    If you buy an old-fashioned meter with a needle movement, you'll find dBmv on the scale.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Most ordinary opamps have an open-loop voltage gain of typically 200,000 which is 106dB from DC to about 12Hz. Then the gain rolls off at the rate of 6dB per octave until the gain is 1 at about 1MHz.
    Negative feedback is used to reduce the gain to a usable amount (closed-loop gain), decrease the distortion, decrease the output impedance and widen the bandwidth.
     
  8. Distort10n

    Active Member

    Dec 25, 2006
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    The problem I see with this technique is that the output of the amplifier will rail because of the offset voltage being multiplied by the open loop gain. It would be very difficult to de-mingle this from a small input signal.

    What I have done on the bench for first order analysis is to use a non-inverting op-amp with a small, 100mVpp, signal on the input and a 100nF capacitor from IN- to ground with a 100k feedback resistor. I sweep the frequency of the input signal from DC to 1 MHz.

    There will be a cut-off frequency before the open loop gain intersect so that data before f(c) is invalid.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    This is a very good point.

    The open loop gain is really only valid at DC. At any frequency above the the open loop gain will be less.

    The the open loop frequency response of an amplifier is not 'flat' - constant with frequency - not a desireable characteristic.

    The purpose of the negative feedback is control the circuit gain to maintain a flat frequency response up to what ever is the desired maximum operating frequency, albeit at reduced gain level.
     
  10. techroomt

    Senior Member

    May 19, 2004
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    a voltmeter can be used to read the gain, and converted to dB. voltage gain (Av = Vout/Vin) can be expressed in a unitless numer (e.g. 100), or in Db, Av (dB) = 20 x log Av. so if the voltage gain where 100, it could be expressed in dB as 20 x log 100, or 20 x 2, or 40 dB.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Most ordinary voltmeters are calibrated at 50Hz or 60Hz and are inaccurate at higher frequencies.
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Don't confuse dB and dBm. dBm is referenced to 1 milliwatt. Notice I didn't specify an impedance. If you looked at some dBm voltages published on the web, you will find some are related to 600 ohms and others are related to 50 ohms. There was a time when receiver sensitivity was rated at -x dBm, and transmitter power was rated at +dBm. When working with a system, it's easier to add the gains (expressed as dB) and subtract the losses (expressed as dB) to quickly calculate the overall gain.

    There are various dB type reference. dBfW, dBm, dBV, dBu, et al. All are the logrithmic ratio of the output / input.
     
  13. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    what cheap hand held meter measures dB type reference. dBfW, dBm, dBV, dBu?

    I can't find a digital hand held meter that measure these that is cheap
     
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