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Old 11th December 2012, 11:35 PM   #1
Jmac222 is offline Jmac222  United States
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Default Op amp virtual earth

If a op amp is used to make a non invertting audio amplifier with a coupling capacitor included before it's input and is biased in order to operate with a single supply, will the resistor that provides voltage to the Vin- input affect the bandwidth calculation?
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Old 11th December 2012, 11:58 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Yes. The network that biases the input determines input impedance.

rgds, sreten.
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Old 12th December 2012, 12:18 AM   #3
Minion is offline Minion  Canada
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the output capacitor in conjunction with the speaker ohms will also limit the frequency response as they also form a filter ......
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Old 12th December 2012, 12:35 AM   #4
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The low-end bandwidth (as in bass roll-off) is a function of the time constant formed by the +input biassing resistor and the input coupling capacitor.
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Old 12th December 2012, 12:35 AM   #5
Jmac222 is offline Jmac222  United States
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My question was will the resistors in the feedback portion of the amp affect the bandwidth since V+and V - are kept at the same voltage virtually. iis there a path through the resistor that attenuates feedback to ground?
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Old 12th December 2012, 12:38 AM   #6
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The absolute values of the resistors won't affect the bandwidth (provided they're not extremely large, like > 100k) but the relative values will because the gain-bandwidth product is a constant for an opamp.
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Old 12th December 2012, 12:45 AM   #7
Jmac222 is offline Jmac222  United States
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But gain bandwidth applies to the opamp itself right. When it's biased or coupled everything changes as far as I have read.
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Old 12th December 2012, 12:48 AM   #8
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Yep the GBW you'll find in the datasheet for the chip - normally there will be a minimum and a typical value. Nothing in the external circuit changes this, you choose the gain with the ratio of the resistor values and this sets the bandwidth because GBW is constant. That is unless you use a cap across the feedback resistor, that will change the bandwidth of your circuit, but not the GBW.
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Old 12th December 2012, 12:54 AM   #9
Jmac222 is offline Jmac222  United States
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If the op amp is dual supllied and does not require a bias then doesn't that change it's bandwidth in relation to a biased op amp? So external cuircitry changed bamdwitdth without regard to the chips band width.
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Old 12th December 2012, 01:02 AM   #10
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If the opamp is supplied from dual (+ and - rails with gnd) supplies it still needs a biassing resistor. It will go to gnd in this case, rather than to the rail splitting network.
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