difference between resistive and capacitive loads

we often hear that,
OP-AMPS are used to drive resistive loads.
OTA are used to drive capacitive loads.

Can anyone explain me,what is the difference with some real worl examples.

If a purely resistive load is connected to a power supply, current and voltage will change polarity in phase, the power factor will be unity (1), and the electrical energy flows in a single direction across the network in each cycle. Inductive loads such as transformers and motors (any type of wound coil) generate reactive power with current waveform lagging the voltage. Capacitive loads such as capacitor banks or buried cable generate reactive power with current phase leading the voltage. Both types of loads will absorb energy during part of the AC cycle, only to send this energy back to the source during the rest of the cycle.

Click to expand...

Quoted from:
http://en.wikipedia.org/wiki/Power_factor

Also see:
"Resistive and capacitive loads"
http://patents.ame.nd.edu/microcontroller/main/node19.html
and
http://www.ecircuitcenter.com/Circuits/op_cload/op_cload.htm

Regards,
IanP

avlsi said:

we often hear that,
OP-AMPS are used to drive resistive loads.
OTA are used to drive capacitive loads.

Can anyone explain me,what is the difference with some real worl examples.

Click to expand...

Opamp's purpose is to realize Vout=GAIN*Vin. It has small ouput impedance and can provide any current in ideal case. So, a resistor normally use to establish an output dc bias point.

But OTA's purpose is to realize Iout=Gm*Vin. It has big output impedance and only provide limited current. Due to its large output impedance, a small cap on the output can establish a low-frequency pole to make compensation. A resistor at output can corrupt its dc point.

avlsi said:

we often hear that,
OP-AMPS are used to drive resistive loads.
OTA are used to drive capacitive loads.

Can anyone explain me,what is the difference with some real worl examples.

Click to expand...

one easy way to to know the diffence is :

1. Resistive laod- there is a path to ground like driving a resistor or drain or source of the CMOS transistor.

2. capacitive laod- there is no path to ground like driving capacitor or gate of the
CMOS transistor

OP-AMP = OTA + Buffer or Output Stage.

OP-AMP has a buffer to drive resistive load. The resistive load looking into the buffer stage of OP-AMP is like looking into a voltage source (controlled voltage source to be exact). Hence given a resistive load and a voltage source, you get current flowing in this resistive load.

OTA is a controlled current source. Back to fundamentals, an ideal current source has infinite output impedance. Thus a practical current source has high output impedance. To match this high output impedance, you need to interface it with a capacitive load to achieve maximum power transfer from source to load. This is because a capacitive load or simply a capacitor has high impedance at DC or low frequency. Such capacititve load can be the gate of MOS transistor, a capacitor, a battery or even a high-pass filter.

avlsi,
OP-Amps are voltage output devices. Because of their non-zero open-loop output resistance, a capacitive load introduces an extra pole in the transfer function. This can result in instabilities when feedback is used. Output current due to load capcitance is proportional to the output voltage slew rate capability of the OP Amp. I = Cdv/dt. As long as the load current does not cause the OP Amp to try to exceed its output voltage slew rate capability, the OP Amp can drive a capacitive load (As long is the extra pole described above does not cause instability).
~
OTAs are current output devices (Io = gmVin). They have inherently high output resistance (ideally it is infinite). So as long as the output current due to the capacitive load does not cause the OTA to try to exceed its output voltage compliance capability, it has no trouble driving a capacitive load.
Regards,
Kral