Question about high speed op amps driving a capacitive load

[obrien135]

Hi again. As I was informed on another thread, certain high speed op amps should not be used to drive a capacitive load. I built a 4 stage amp using op amps that have caps between stages for coupling. Should I short them out or remove them and replace with a short, or would the biasing of adjacent stages throw each other off? If it is a chip that doesn't have an offset null input, does this mean that if ones stage is biased correctly it will have a 0V (or for single supply applications, 1/2 supply voltage) at it's output. If this was the case , would this be sufficient to not upset the successive stage if it is biased the same way (with 0V or 1/2 supply at + and - inputs and output)? If not how would I correct this problem? With a chip that has an offset null ?

Also if an op amp with a single supply is used to drive a transformer with one end of the primary connected to ground, would you need some kind of buffering , such as an emmitor follower to drive the transformer, so it doesn't upset the biasing of the op amp (if you don't use a coupling capacitor)?

George

 

[obrien135]

Actually, as another member, FvM, mentioned on the other thread, maybe I can use LM6171's and keep the caps in. That is if I understood correctly the point he was trying to make. If not I apologise to him.

 

[keith1200rs]

Series coupling capacitors shouldn't be a problem because they are not a capacitive load. They are coupling the output of one opamp to the input of the next, which should be a high impedance. It does depend on the exact circuit though.

Keith

 

[FvM]

Indeed a coupling capacitor isn't neccessarily acting as a capacitive load, although it may, in some cases. To know exactly, you have to calculate or measure the load impedance. A large capacitor can be treated as short AC-wise, so except for DC, the load behind it matters.

The sketched circuit surely can't work without a coupling capacitor, because the transformer is a DC short to the OP output. If the transformer is a suitable load for the OP - depends on. The winding capacitance may e.g. present a considerable capacitive load, or the inductance be too low to be driven by the OP. Also the circuit at the secondary must be taken into consideration. Adding a voltage follower would reduce the load to the OP output. If the follower can drive the load in the intended way is a different question.

I suggested LM6171 instead of LM7171 because it's unity gain stable and promises somewhat more circuit stability, if you don't need the LM7171's GBW. It has been mentioned also by other contributors, that these > 100 MHz bandwidth OPs are likely to design unwanted oscillators. Their requirements for low inductance ground and bypassing are considerably different from gentle general purpose devices. It's like driving a family or a racing car.