Op amp virtual earth

[Jmac222]

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]

Hi,

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

rgds, sreten.

 

[Minion]

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

 

[abraxalito]

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]

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]

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]

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

 

[abraxalito]

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]

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]

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.

 

[Jmac222]

I appreciate but don't get it. I've read that the opamp has a flat response but I did not take that to mean that adding external components would not effect it's bandwidth. My original question was does the attenuating feedback resistor effect the lower frequency response because There is a virtual path between the V+ and V - inputs.

 

[Fast Eddie D]

The bandwidth of the circuit and the gain bandwidth product are two different things. Increasing gain decreases bandwidth. The term gain bandwidth product is self explanatory; the product of the circuit gain and the circuit bandwidth is theoretically constant for a given device. So a device with a GBP of 5.5 mHz (like the 1875) can provide a gain of 10 with a 550 kHz bandwidth, a gain of 20 with a 275 kHz bandwith, etc, theoretically. Of course for an audio circuit we might limit that bandwidth by using a snubber on the output, which decreases loop gain with increasing frequency and helps to prevent oscillation.

 

[Jmac222]

The rail splitting network should go to ground , correct?

 

[Fast Eddie D]

The rail splitting network should go to ground , correct?

For your AC Thevenin equivalent circuit? Yes.

 

[abraxalito]

My original question was does the attenuating feedback resistor effect the lower frequency response because There is a virtual path between the V+ and V - inputs.

I'm not sure what you mean by 'virtual path' here. There's quite a high differential input impedance in an opamp, above 100k typically. Its true that they stay at the same voltage by virtue of feedback, but I can't see how that's a 'virtual path'. Are you getting 'virtual path' from the notion of 'virtual earth' in the inverting gain case? In which case there's not a path between the + and - inputs in that case, just because the -input looks like a virtual ground. You could put a resistor in the +ve input and the -ve input would still look like ground, the resistor in the +ve input wouldn't have any effect on the characteristics of the -input.