capacitive load vs. resistive load

resistive vs capacitive loads

What types of opamps are suitable for capacitive load, and what for resistive load? What's the difference? Thanks.

capacitive vs resistive load

The difference is that the capacitive load forms another pole with the amp output impedance. If there is feedback from the output, this pole affects the stability.

So what's the implication in amplifier architecture? Is a Class-AB output stage acceptable for capacitive load, such as a switched capacitor feedback?

Output stages with inherently low impedance are used for the capacitive load types. These would be source/emitter followers in the complimentary N and P types in a totem poll.

The CMOS types with the drains connected together and to the output will have high impedance.

opamps with high output impedance can not drive a too low resistive load and are more suitable for capacitive load,
opamps with low output impedance is suitable for resistive load, but it also can drive capacitive load.

In a word, for resistive load, you should choose your op amp carefully . But for capacitive load, it doen't matter.

I am trying to design an opamp to be used with a resistor feedback. I first though using a folded cascode, but someone told me that it's useless because the feedback resistor is becoming the load (it's value is 5k).
What is your opinion? Thanks

Hi, I am confused again. resistive load needs low output impedance. Then why capacitive load doesn't need? Does low output impedance mean faster charge-up? or am I wrong?

And the lower the output impedance, the higher the voltage the resistive load can get. So their requiements are the same. What's the difference?

By the way stupid question: how do you define the output resistance. Are you using a current source with a parallel output resistance or a voltage source with serie output resistance? Thanks

skal81 said:

By the way stupid question: how do you define the output resistance. Are you using a current source with a parallel output resistance or a voltage source with serie output resistance? Thanks

Click to expand...

Output resistance is calculated supposing you do not drive anythig, (or you dirive an infinit resistor.) you can get it from the small signal model.

Added after 7 minutes:

Dear vistapoint,
Yes, you are right, low output impedance means higher output pole and normally means higher bandwidth.
However, what I said was about the effect of different kinds of load on the DC gain of the op amp.
The gain of your op amp(suppose you do not have buffer stage) is gm*ro, ro is your output resistor.
Then when you drive a resistive load with a value RL, This RL is in parellel with your op amp and now your true "output resistance" is Ro//RL and your DC gain is G=gm*(Ro//RL) )If Ro>>RL, G=gm*RL<<gm*Ro. Your DC gain is drastically decreased.
While capacitor is an open at low frequency, it won't affect your DC gain.
In the case you have the bandwidth requirement and the output pole is your dominant pole, of couse it is better to have a low output resistance in order to get a higher bandwidth.
Can I get a "helped me" button click for this reply? just joking.
Regards,

vistapoint said:

Hi, I am confused again. resistive load needs low output impedance. Then why capacitive load doesn't need? Does low output impedance mean faster charge-up? or am I wrong?

And the lower the output impedance, the higher the voltage the resistive load can get. So their requiements are the same. What's the difference?

Click to expand...

I have another point of view. For resistive load, the output driver is become a voltage divider with the load resistor. If the output impedance is high, it means the peak to peak voltage swing is low. However, for a capacitive load. It is open at low frequency. It means, the even with high output impedance. The output voltage swing is high.

some times capacitive load are used to avoid DC current, thus lower down the power.

the capacitive load will decrease the bandwidth of an OPAMP. So, in order to get high bandwidth, the opamp should have a low output impedance.

Common drain/Source follower's/output stage amps drive a resistor
load with first stage providing gain .Pole due to resistor load must be
out of the GBP .All OTA's can drive cap load .

Resistive load requires the OPAMP to have low output impedance, thus a dedicated output driver stage is typically needed.

For capacitive load, which is quite common in switched-capacitor circuitry, the OTA does not need an output stage, which simplifies the Opamp implementation and tends to save power.

Which load to use really depends on the application.

If you have opamp followed by a low impedance output stage, then you can drive low resistance load OR a capacitive load. The pole in later would be at a very high frequency and will have little or no effect. So it is correct to say that low impedance output stage opamp will drive low resistance load or capacitive load effectively!