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Jmac222 11th December 2012 11:35 PM

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?

sreten 11th December 2012 11:58 PM


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

rgds, sreten.

Minion 12th December 2012 12:18 AM

the output capacitor in conjunction with the speaker ohms will also limit the frequency response as they also form a filter ......

abraxalito 12th December 2012 12:35 AM

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.

Jmac222 12th December 2012 12:35 AM

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?

abraxalito 12th December 2012 12:38 AM

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.

Jmac222 12th December 2012 12:45 AM

But gain bandwidth applies to the opamp itself right. When it's biased or coupled everything changes as far as I have read.

abraxalito 12th December 2012 12:48 AM

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.

Jmac222 12th December 2012 12:54 AM

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.

abraxalito 12th December 2012 01:02 AM

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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