Help for a simple voltage amplifier with LF411 or LM7171

Hi Aaron,

Some comments:

1. Decoupling caps next to supply pins are mandatory. It helps to use a series R in addition (like a LPF). Route decoupling cap return (ground) trace away from input(s) to avoid unwanted feedback. If you are using device sockets, beware of any capacitance they may add (it is best to solder device on board directly).

2. Probe capacitance (~12pF) may need to be isolated with a series R. Better yet, load the 2nd stage (say 500ohm by having 450ohm in series and plug to the scope 50ohm terminated). Some high speed devices behave better when loaded (at least lightly).

3. Ground plane helps (but keep it away from sensitive nodes, like the inverting input(s)). Ground lead inductance is often a stability liability.

4. Make sure you are not adding stray capacitance and inductance by keeping lead lengths to a minimum and a tight layout (remember it is not the speed of the signal you are testing with but rather the speeds that the part is capable of)

5. LM7171 requires a minimum gain of 2V/V and its feedback capacitor cannot be too large as a large RF interacts with stray capacitance to create phase shift

6. If none of these help, try a small value resistor (start with 100ohm) in series with the non-inverting inputs of both stages.

Hope above resolves your stability issue.

Regards,

Hooman

The maximum supply voltage for the LF411A is +/-22V, but for the LF411 it is +/-18V.  Are you using the "A" version?

Drop the power supply to +/-15V and see what happens.  Generally it is good practice to stay at least 10% below the absolute max rating for a part.

I have used the LM7171 frequently and it works well with a gain of +2 (as Hooman notes) but will not run off +/-20V.

Tim

Hi Tim and Hooman,

Is the LM7171 or LF411 even usable on a simple breadboard (not a PCB)? It sounds like a lot of work needs to be done to ensure that there is no capacitive coupling, etc.

My ultimate goal is to generate a 12V-amplitude sine wave (24V pk-to-pk) with frequencies ranging from 0.1 to 2000 Hz. If you have any other suggestions, I would appreciate them very much!

Aaron

That should be pretty straight forward to achieve.  You have lots of slew rate and bandwidth with either amplifier.  Just remember to put a cap across the feedback resistor for stability.  Depending on your breadboarding technique, you could have significant stray capacitance to compensate for.  The LF411 might be the better choice because with the LM7171 you need to be sure you meet the minimum noise gain requirement of +2.  That makes the selection of the feedback cap value a little trickier as you need to maintain the +2 noise gain minimum at high frequencies.

Tim

Hello,

Thanks for all your tips! I got the LM7171 to give me a stable output with +3 V/V gain (I switched to non-inverting feedback)!!  All I had to do was add a resistor after the input voltage, and also add a resistor before sending it to the oscilloscope (both circled in green). I tried adding a feedback capacitor, but it seemed to have no effect in my bandwidth of interest (0.1 to 5 kHz). Do you know the explanation for why this works? I was looking at the current value being supplied by one of my power supplies, and it seems that adding the 2k resistor to the input lowers the current to ~7 mA (without the resistor, it was 20+ mA).

Here is a photo of my breadboard. Just a simple do-it-yourself setup, probably with tons of stray capacitance...

Hi Aaron,

The push-in breadboard you are using is not recommended for high speed devices. That's probably why you measure high supply current (at least with 0ohm in series with your non-inverting input) because you are probably making the LM7171 oscillate. It is best if you use an Evaluation board meant for this purpose, like this one:

http://www.ti.com/tool/lmh730227 (for SOIC-8 single op amp)

Since you are using a DIP package, I don't have a high speed evaluation board to recommend. So, you may have to do your own board to experiment.

Regards,

Hooman