resistive loading vs. capacitive loading for op amp

capacitive loading and output

Hi all,

Maybe it is a very simple question for you

Some types of op amp can drive only capacitive loading while some types of op amp can drive both resistive loading and capacitive loading.

How to decide which type of op amp can drive resistive loading or capacitive loading? How to design it?

regards,
jordan76

In most cases, OpAmps that drive other OpAmps (or ADC's etc) inside an integrated circuit have to drive only capacitive load (the input capacity of the next stage). Op amps that have to drive a circuit outside the integrated circuit usually have the capability to drive current according to its expected use (and load) and so it has the capaility to drive resistive loads.

Actually, if you have to drive capacitive loads, your amplifier can have a large output impedance and behave mostly as a current source. Such amplifiers are called "Operatioanal Transconductance Amplifiers" (OTAs).
There is nod need for a low impedance output stage. Speed (ft, SR) of OTA is given by the effective transconductance (gm) they represent and the effective capacitance seen. The term effective capacitance stands because in some architectures (Miller like), the dominant pole of your circuit is dominated by the Miller compensation capacitance. In architectures such as symetric or folded cascoded OTAs, the dominant pole is determined by gm and the load capacitance. The gain of OTAs is determined by gm times the effective output impedance (taking into account conductance of output stage and resistive load, if any, attached to the output node). A good picture of OTAs is given by its NOrton equivalent circuit.

To drive resistive loads, your output impedance must be as low as possible. Just think now of your OpAmp as an equivalent Thevenin scheme. A voltage source with an output resistance connected in series must drive another resistor. The resistive divided made by the output resistance of your OpAmp and your load resistor reduce the voltage applied to the latter resistor. That is why your ousput resistance must be lowered by using output stages such as class AB and push-pull architectures. In this case gain and speed of your OpAmp are mostly dependent of internal components (within certain limits). As you must have noticed it, in this case we talk about "Operational Amplifiers" (OpAmps).

Agree to Humungus

I think the most difference is the output resistance and capability of current providing.
If the output resistance is large, then resistive loading will be parallel to the output resistance of Amplifier and change your dc gain. if your amplifier cannot provide current to resistive loading , it can extract current from your amplifier and cause system offset to your circuit, also because I = V / R , this causes distortion