Hi all,

Why is it not necessary to use any coupling capacitors (at input and output) for op amp circuits e.g. inverting and non inverting amplifier?

Thank you.

 

Hi all,

Why is it not necessary to use any coupling capacitors (at input and output) for op amp circuits e.g. inverting and non inverting amplifier?

Thank you.

Because they need to work at low frequencies all the way down to DC. In high pass filter stages, you may see what "look like" coupling capacitors but they are there to implement the high-pass filter characteristic which attenuates low frequencies and DC.

 

Hi all,

Why is it not necessary to use any coupling capacitors (at input and output) for op amp circuits e.g. inverting and non inverting amplifier?

Thank you.

Your question is taken out of context. If you can post a circuit diagram that would help us to provide you with a proper answer.

In general, input and output without coupling capacitors mean that the signal is DC coupled. In other words, signal voltages will depend on DC conditions within the circuit as well as voltage drifts caused by temperature fluctuations.

Coupling capacitors create high-pass filters. Low frequencies and DC are removed and only high frequencies are passed from output to input. This means that any changes in the DC operating conditions will not adversely affect the operation of the circuit.

Edit: Papa types faster than me.

 

If the op amp is powered by a dual supply coupling caps generally are not needed unless as mentioned a filter is required.
Single supply operation will normally need coupling caps.

 

Your question is taken out of context. If you can post a circuit diagram that would help us to provide you with a proper answer.

In general, input and output without coupling capacitors mean that the signal is DC coupled. In other words, signal voltages will depend on DC conditions within the circuit as well as voltage drifts caused by temperature fluctuations.

Coupling capacitors create high-pass filters. Low frequencies and DC are removed and only high frequencies are passed from output to input. This means that any changes in the DC operating conditions will not adversely affect the operation of the circuit.

Edit: Papa types faster than me.

 

Thanks for posting the op-amp circuits.

Both non-inverting and inverting circuits shown are designed to provide voltage gains of 11 and 10, respectively, for signals from 0Hz to about 100kHz. Thus it will operate on constant DC voltage inputs.

Adding input and/or output coupling capacitors will remove this DC capability.
Adding an input coupling capacitor in the non-inverting circuit presents another problem in that the non-inverting input would no longer have any DC bias voltage required to make the op-amp operate properly.

Whenever in doubt, go back to this basic differential op-amp configuration. In this circuit, Rg provides bias to GND on the non-inverting input.

 

Thanks for posting the op-amp circuits.

Both non-inverting and inverting circuits shown are designed to provide voltage gains of 11 and 10, respectively, for signals from 0Hz to about 100kHz. Thus it will operate on constant DC voltage inputs.

Adding input and/or output coupling capacitors will remove this DC capability.
Adding an input coupling capacitor in the non-inverting circuit presents another problem in that the non-inverting input would no longer have any DC bias voltage required to make the op-amp operate properly.

Whenever in doubt, go back to this basic differential op-amp configuration. In this circuit, Rg provides bias to GND on the non-inverting input.

View attachment 293451

Many thanks for the explanation.

Best regards.

 

Hi all,

Why is it not necessary to use any coupling capacitors (at input and output) for op amp circuits e.g. inverting and non inverting amplifier?

Thank you.

Some do. It depends on what the circuit is supposed to do. Coupling capacitors are used to remove DC bias from the signals so that you only amplify the signal. Amplifying the DC bias can quickly rail the opamp (or transistor-based amp -- this is not something that is unique to opamp circuits). But if you need to work with DC signals (think something like a weight scale, for instance), then you can't AC-couple the signals. Again, this is not unique to opamps.