Op-amp design Question

[baby_1]

Thanks so much dear Lvw about your explanation
could you tell me how can design or modify this circuit?
http://obrazki.elektroda.pl/3273457400_1353316265.jpg[/img
because i use "David bell " Pulse techniques books to design above circuit step by step with the book designing strategy

 

[LvW]

baby_1, who forces you to use such large and uncommon resistor values?
Why not use the values as shown in the first diagram in post#19 ?

 

[baby_1]

Dear Lvw
thank you that you follow this topics and my posts
my op-amp is AD822 and the input current should be more than the min of AD822 current that explain in datasheet (min current=2pA) i assume that the best choice of current is 10ua , but it seems not work fine!!! why?

because i see in some circuit that they use 1Mohm resistor for their op-amp and seems they work.
what is the best op-amp current input to calculate my resistor? how can select resistor and input current for good designing?

Thanks

 

[barry]

You don't 'select' the input current; that's a characteristic of the opamp. Ideally, you want the input resistance low so that the input current doesn't develop much voltage across that resistance (causing an error). This is why sometimes the input resistances on both inputs are chosen to be the same: so that the voltage generated by input (bias/offset) currents cancel each other. Other circuit operating conditions may dictate that the resistors can't be TOO low.

 

[baby_1]

Thanks dear barry
you tell me good about the reason that we use another resistor in other pin of op-amp .
it means the input current shows me leakage current from op-amp to resistor or they tell me that we should designing our circuit with consider them?

because as i read some text they told me we should choose current of resistor >> the min of input current of op-amp and calculate they resistor with consider the input current.


could you introduce me a reference or book to tell me very fast and common in designing op-amps circuit?

 

[barry]

Yes, your source needs to supply the input current, but it's usually so small that any nominal resistor value will work. In other words, the input current requirements contribute very little to your choice of value. As I said, smaller is generally better for the input resistor, but if you have small feedback resistors, your output will have to supply the necessary current through that.

As far a reference, there is virtually unlimited information on the web. Just do a search on "opamp fundamentals". The manufacturers (TI, Analog Devices, etc.) also have very good information.

 

[LvW]

baby_1, there is a general rule as follows:

The resistors of the feedback network should be selected (if possible):
* much larger than the output resistance of the opamp unit (which in most cases is between 20 and 100 ohms);
* but at the same time much smaller than the input resistance of the opamp.

In most cases, this leads to values in the lower and mid kilo-ohms range.

Another disadvantage of very large resistors (Mega-ohm range) is the fact that parasitic parallel capacitances play already a role at lower frequencies.