LME 49713 + LME 49600 pre amp

[klewis]

I tried this thread out in the Solid State section and didn't get much response, so, I thought I would try here. I want to build a pre amp utilizing the CFB LME49713 op amp. As I understand it, it's a bit tricker than a VFB op amp. The attached is my current schematic.

The CFB op amps seem sensitive to input impedance and stray capacitance, so I thought that I might try the pot after the device and then into a buffer which should be more tolarant of variable impedance and variable drive loads and capacitance(s). Note that I'm not particularily well versed on this stuff, so, I could be all wet - I sure someone here will let me know...

I've also found a similar op amp to try a spice model with and will show the results in the next few post.

Ken

[paulb]

Have you got a feedback loop around the potentiometer? What is JP4 for?
Ah, I bet it's just for bypassing the buffer. Sorry can't help with your CFB design.

[klewis]

JP4 is to make a feed back loop including the buffer. And I think I need another jumper to take the feed back away from the front of U3. So, if JP4 is in place the other one would need to be open.

[klewis]

For the spice modeling, I found a similar CFB op amp sub circuit from National and used this to try out different resistor values. The buffer is not shown and the pot is represented by a fixed resistor - R7. I tried several runs with R7 set at values from 1k to 5K without much change. In retrospect R7 should be set at 5k if it's a 5k pot... I've also shown stray capacitance at C3 and C10, which would represent stray board layout capacitance. With out C3 the square wave overshoots about 1/4 the height of the square wave. C11 is a real capacitor to compensate for ringing in the square wave - if necessary.

As I write this stuff, I realize how good some of you are at writting... being clear on this stuff is hard work.

Ken

[klewis]

Here's the 10kHz square wave from the prior simulation.

[klewis]

AC analysis for same

[klewis]

Finally, THD for same.

[pacificblue]

The LME49713 is very tolerant towards variable drive loads and capacitance. According to the datasheet it is able to drive up to 100 pF loads without oscillating and to deliver 100 mA output current. It does not seem to need a buffer after it.

A potentiometer would replace R11. What evil could it do?

Are all your sources able to drive R11||(R12+(R16||C18||U3)), which results in less than 700 Ohms? If so, you do not need a preamplifier.

Is the RF filter at 265 MHz a typo?

[klewis]

Originally posted by pacificblue
The LME49713 is very tolerant towards variable drive loads and capacitance. According to the datasheet it is able to drive up to 100 pF loads without oscillating and to deliver 100 mA output current. It does not seem to need a buffer after it.


A potentiometer would replace R11. What evil could it do?

Are all your sources able to drive R11||(R12+(R16||C18||U3)), which results in less than 700 Ohms? If so, you do not need a preamplifier.

Is the RF filter at 265 MHz a typo?

It will be quite easy to try it without the buffer.

Regarding putting the pot to replace R11 - The input is another matter, R12 is matched to R13 along with the R12/C18 work together against stray capacitance. When I simulated variable resistance at the R11 position, the results varied. That said, I intend to try it with the pot replacing R11 as well.

It was my understanding that R11 would be the dominate contributor to the input impedance, please help me with the math. I don't understand what || means.

By the RF filter your refering to R12/C18 combo?

Ken

[klewis]

When I look at the input impedance in spice divide the input voltage by the input current I get a bit more than 27k. Am I doing something wrong. Any help is always appreciated.