Op-amp at DAC's output

[alphacat]

Hello.

I'd like to ask several questions please about this circuit.

1. We are dealing here with large-signal analysis.
What is the DC input resistance of an Op-amp?
If the Op-amp is MOSFET-based, then its infinity, but what if its BJT-based?

2.
Is the DC input resistance of an Op-amp depends on the DC inputs VI- and VI+?
(Its known that the small-signal input resistance depends on the DC inputs).

3.
If I connect a resistor in the feedback loop, would VO be still equal to VI-? (Since the Op-amp sources no current because there is no load connected at the output terminal).

Thank you.

 

[Hayato]

Hello.

I'd like to ask several questions please about this circuit.

1. We are dealing here with large-signal analysis.
What is the DC input resistance of an Op-amp?
If the Op-amp is MOSFET-based, then its infinity, but what if its BJT-based?

2.
Is the DC input resistance of an Op-amp depends on the DC inputs VI- and VI+?
(Its known that the small-signal input resistance depends on the DC inputs).

3.
If I connect a resistor in the feedback loop, would VO be still equal to VI-? (Since the Op-amp sources no current because there is no load connected at the output terminal).

Thank you.

1 - Well, BJT nor FET opamps have infinite input impedances. A BJT opamp can have impedances from 300 kΩ to 2MΩ. Those informations are available in the datasheet.

2 - Well, the input resistance of any opamp based circuit does not depends only on the inverting and non-inverting inputs. You must consider the impedance network connected to the inputs, for example, the picture you posted the network is formed by the resistors connected to the opamp, and the load will be part of the impedance too.

3 - Depends. All of this is found in basic opamp theory. For a pure inverting configuration, R2/R1 must be equal to 1, for a pure non-inverting configuration a 1 + R2/R1 must be equal to 1.

 

[audioguru]

Your circuit will not work because the R-2R signal source input is also connected to the output of the opamp that has a resistance of nothing.