OPA445: OPA445 schematics

Schéma.pdf

Hi Druilhe,

where did you find this circuit?

druilhe_opa445.TSC

Kai

it's a setup used on amp cards.

The simulation shows that it works...!

We find the same schema on the lm3886 datasheet

See figure 2 of the attached documentSchémas 1.pdf

Hello Thomas,

I agree with your analysis however if you look at figure 2 of the attached document (Schémas1.pdf)  the counter reaction is on the + pin of the AOP. Which is shocking.

The idea of putting 2 AOPs in series is to simulate the same operation as schematic 2. This may not be a good idea.

However, this counter-reaction on the + pin of the AOP in diagram 2 is suspicious.

Do you have an idea?

Regards Yvan.

Hi Yvan,

you said that you did find the same scheme in the datasheet of LM3886? So you mean the "DC Electrical Test Circuit" from page 5?

Well, this is a simplified schematic of the standard test circuit which is used since decades in the OPAmp industry to test input offset voltage, input bias current, input offset current, open-loop gain, common-mode rejection, gain-bandwidth product, etc. All use the same test loop with only slight variations. What you have overlooked is the "NULL" in the second OPAmp. It means that this OPAmp forms an integrator :-)

So a less simplified schematic of this standard test loop looks like this:

Kai 

I'm okay with what you said.

See attached schematic my question remains the same why the assembly is stable with a feeback on the + pin of the AOP.
The simulation shows this stabilityMontage AB.pdf

Hi Yvan,

Certainly the feedback in an op amp circuit needs to be predominantly negative to assure stability. Most single op amp circuits use only negative feedback and load conditions that assure a stable operating case. The Improved Howland Current Pump (V-to-I converter) is an example of a single op amp circuit that uses both negative and positive feedback, but the predominant influence on the circuit that keeps it stable is the negative feedback.

Once additional components are added to the output of an op amp circuit either within, or external to the feedback loop additional phase shift is added to the loop which can degrade the phase margin and destabilize the circuit. Adding a second op amp within the loop as in the LM3886 circuit you reference adds much phase shift into the loop. Very likely the feed back to the usual inverting input in that case would reduce the phase margin to the point where the circuit is unstable. That is where the reversal of the two op amp inputs comes about in the LM3886 test circuit. The second amplifier adds so much phase shift to the loop that the function of the inverting and non-inverting inputs is switched. Doing so, the phase margin can be reestablished to good positive value such as ~45 degrees and the circuit is stabilized.

Often, and I suspect this the case for the LM3886 there are additional poles and zeros in the two-amplifier loop that degrade the phase margin. A very careful Bode analysis of the circuit must be accomplished to see where they occur and what needs to done to the circuit to make sure the circuit is stable. Looking at the LM3886 Figure 1 circuit, I expect that the 220 pF capacitor across the first DUTs inputs, the parallel 0.7 uH and 10 Ohm combination at its output and the capacitor that Kai proposes adding across the null amplifier input and output, are the necessary additions to achieve stability. Note that these are in addition to the reversal of the DUT amplifier's inputs seen in the circuit schematic.

As I mentioned previously, the OPA445 and LM3886 are completely different amplifiers with completely different electrical characteristics and intended uses. Do not expect that the same compensation techniques will necessarily work for the circuit comprised of OPA445 op amps. The compensation may need to be very different.

Regards, Thomas

Precision Amplifiers Applications Engineering

Bon jour Ivan,

can you draw your schematic in TINA-TI? Then we could do some simulations for you.

Kai

Hello Kai,

You will find the design under tina.
On the other hand I do not know integrated new model of transistor the 2N5667 and the 2N5416.

I replaced 2N5667 with 2N5769 and 2N5416 with 2N5447.

Thank you for your help

Ampli_OPA445.TSC

Hello
You will find enclosed the TINA design of the amplifier.

5672.Ampli_OPA445.TSC