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High GBW OP AMP Selection

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vinothe83

New Member
Hi all,

I am doing an inverting amplifier in my present design. I am not able to choose an wright op amp for that application. The requirements are listed below.

1. Gain = 1 to 12 (variable)
2. Frequency = 1 hz TO 100khz
3. Rf & Rin = 1 ohm to 1 Mohn

If you select an high gain bandwidth OP AMP, it will not support the higher feedback resistance(100 Kohm). So i am not able to finalize.

1.Which paramenter is affecting the GBW Vs high value feedback resistance?
2. Any simulation tool is available to simulate this condition?
If somebody helps, it will great.


Tnx,
Vino
 

unclejed613

Well-Known Member
Most Helpful Member
TL2072 has more than enough bandwidth, and is a fet input with over 10 E+13 ohms input impedance. max gain of 12 is not a problem even with high values of Rf.

i use LTSpice available for free from Linear Technology - Linear Home Page

any particular reason the resistor values need to be high?

are you running into DC offset problems? that can be fixed by adding a resistor from the + input to ground equal to the input resistor, so if your amp is a gain of 10, with Rf= 1Meg, and the input resistor = 100k, then putting a 100k resistor to ground from the + input will balance the input bias currents and minimize the DC offset.
 
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crutschow

Well-Known Member
Most Helpful Member
1. You can not use a high value resistance and get a high frequency response because of the parasitic capacitance at the summing junction which consists of the op amp input capacitance and circuit layout capacitance. This reduces the high frequency response. You can minimize this capacitance by using a PCB layout with very short connections between the resistors and the summing junction and remove any ground plane near the junction. Summing junction capacitance is your enemy.

2. Use a Spice simulator (such as LTSpice). Add a small amount of stray capacitance (probably in the neighborhood of 10pF) at the summing junction to ground and you will see the effect on the frequency response and peaking in the response curve. The peaking can be minimized by adding a small capacitance across the op amp feedback resistor.
 

unclejed613

Well-Known Member
Most Helpful Member
10pf at 100khz has an Xc of 159k, so a 100k input resistor would be about the highest resistance you could go. the -3db corner frequency of 100k and 10pf would be 159k. so you might get away with a 100k input resistor with extreme caution in your board layout, but i wouldn't think you would want to go any higher than that. there are few things you would need such a high input impedance for, one of them would be a piezo sensor, another would be a crystal radio preamp. since you're looking for 100khz response, my guess is a piezo element.
 

vinothe83

New Member
Thanks for the information. Now i got some ideas. First i will try your suggestions and will gome back to you. I downloaded LTspice IV and intalled.
is it the correct one? Linear devices components only available in the library.
Any possiblity is there to run a simulation with OP07?

Thx,
Vinoth
 
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unclejed613

Well-Known Member
Most Helpful Member
there are models you can use available from TI for the OP07. make sure you use the PSPICE or SPICE3 models. the TINA and ORCAD models are binary, and won't work in LTSpice. sometimes when adding models of op amps, you may have to edit an op amp symbol so that LTSpice will draw it on the schematic. there are a few things about editing symbols you need to read about in the help file. also i highly recommend you join the LTSpice yahoo group if there's something you need to learn, you will find a lot more info there, as well as help from experienced users of LTSpice.
 
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