Amplifier stability and compensation

[rodeo]

Hi guys, I've a project on amplifier stability and compensation. I'm about half way through it now, anyboody got any info or anything interesting. It's a fairly simple project, what i have to do is complie notes that will be used to teach 3rd year electronics students about amplifier stability. I've to create 4 laboratory experiments as well. As i said i'm only half way through the thing yet, but if anybody has or knows of anything that would be good to add let me know, cheers!

[Guest Alun]

The first attached PDF is an extract from an assignment I did for college a couple of years ago, I hope it's what you're looking for.

The second is the 741 datasheet that I refered to in the assignment, on page 6 is the schematic for the 741, C1 is the internal compensation capacitor.

Compensation_of_amplifiers.pdf

NS_LM741.pdf

[rodeo]

Excellent, much appreciated!

[rodeo]

Can anybody give me a ling to a site that has a lab or a demonstration on it how loop-gain varies with frequency?

gracias

[audioguru]

Como estas, Rodeo,
You already have a graph of how loop-gain varies with frequency. I marked it to make it clearer:

[rodeo]

Deadly, i'll use that, it's perfect.

The next stage is where i create four one-hour lab session to demonstrate how to;

calculate the loop-gain,
show how the loop gain varies with frequency,
show how a amplifier becomes unstable,
demonstrate how to stabilise it.

I haven't completely decided on these labs so if anybody thinks they'd do it differently please let me know.

I've already decided that i'm going to use simple op-amp circuits in these labs. What i need to do is create these circuits. Does anyboddy have any examples / suggestions/ ideas on what circuits would suit the labs?

[audioguru]

Hi Rodeo,
Here's how to power and bias the inputs of opamps:

[Guest Alun]

rodeo,

I think I understand,

You want to devise a series of experiments to:

Calculate the open-loop gain of an op-amp.

Construct a simple inverting amplifier with a gain of 2, use the old 741 op-amp for this.

Connect an amplifier with a gain of about 1000, to the invertin input of the 741, you'll probably need a better quality op-amp (open loop gain > million), I can't think of anything at the moment but I'm sure someone else here will, I'll have a look at my data sheets when I have the time.  You should now be able to see the output on an oscillosope. You can now calculate the open loop gain as you know the input  and output voltage.

Demonstrate stability and compensation.

A bit more trickey, I don't know how well the off the shelf op-amps oscilate when given complete negative feedback or whether they oscilate at a low enough frequency to pickup with fairly cheap equiptment.

To do this you need to build a multistage (3 stages might be enough) inverting amplifier with an open loop gain of 100 or so at a low frequency say 1KHz. Build a filter in to each stage that it will have a 60^o phase shift at a much higher frequency say 100KHz and a unity gain at at bit higher frequency say 110kHz but no little at 1kHz.

Now when the amplifier is open loop it will behave normally and be quite stable, and will have a phase shift of 180^o as it's inverting. If you increase the frequency the phase shift will go down and at 100kHz it will have a phase shift of 0^o and become non-inverting also the gain will be near unity. If you now introduce negative feedback to try and make a unity inverter it will oscillate at 100kHz.

You can demonstrate that adding a capacitor in the feedback loop to reduce the gain to unity at a lower frequency of say 50kHz then the amplifier will become stable and no longer oscillate since it is now compensated.