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What Causes Opamps to Blow? - RF Cafe Forums

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ifatfat

Post subject: What Causes Opamps to Blow? Posted: Tue Oct 07, 2008 5:00 am

Lieutenant

Joined: Tue Oct 07, 2008 4:56 am

Posts: 3

A big hi to everyone...

I would like to know what causes opamps to blow?

If I have a non-inverting amplifier, and I have the output constantly hitting the power rails, would it cause the opamp to blow?

Thanks in advance!

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darcyrandall2004

Post subject: Posted: Tue Oct 07, 2008 10:50 pm

Colonel

Joined: Tue Feb 27, 2007 6:16 am

Posts: 46

Hello,

The opamp will amplify the difference between its input terminals. The output can not exceed the power rails.

If the op amp amplifies the difference between its input terminals and the output does reach the power rail voltage, it will not be destroyed as a result of this.

If your opamp is blowing up, I think you will find that the output reaching the rail voltage is only a sympton not a cause.

Cheers

_________________

Regards, Darcy Randall, Perth, Western Australia

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ifatfat

Post subject: Posted: Tue Oct 07, 2008 11:15 pm

Lieutenant

Joined: Tue Oct 07, 2008 4:56 am

Posts: 3

Hi...

Thanks for the reply!

I'm wondering what might cause the opamps to blow? I'm not 100% sure... but I guess excessive current at the input might blow the amps? Are there other stuff I should look out for?

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IR

Post subject: Posted: Tue Oct 07, 2008 11:56 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

An excessive output current can cause the op-amp to blow, this can happen if the amplifier drives a load which is too small.

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nubbage

Post subject: Posted: Wed Oct 08, 2008 4:18 am

General

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi

I agree with both diagnoses and add that the differential voltage at the inputs has a spec that should not be exceeded, and I have had trouble like this in the past due to excessive voltage spikes on one of the two inputs that resulted in this spec being exceeded for a few microseconds. This led to unpredictable consequences within the structure, basically "punch through" that resulted in thermal run-away and permanent unrecoverable damage. Put a good scope on all interface pins and take a look.

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yendori

Post subject: Posted: Wed Oct 08, 2008 10:19 am

General

Joined: Thu Sep 25, 2003 1:19 am

Posts: 50

Location: texarcana

What IR said.

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ifatfat

Post subject: Posted: Thu Oct 09, 2008 8:08 pm

Lieutenant

Joined: Tue Oct 07, 2008 4:56 am

Posts: 3

Thanks for all the replies!

The output of the op amp circuit is going into a 0.01uF cap followed by a 50ohm resistor in series. How do I determine if this 50ohm is too small a load for the op amp to drive?

In the schematic design, the resistor to ground at the negative input is absent, so there's only a resistor in the feeback loop (from output to negative input). Would this design cause a problem?

nubbage, currently there's a resistor to ground that's parallel to the positive input. Would adding a cap in parallel to this resistor (to ground too) help out with the sudden voltage spikes?

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IR

Post subject: Posted: Fri Oct 10, 2008 4:31 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

From your description, it looks like you are using your op-amp as buffer.

The absence of the resistor to ground should not cause a problem.

This resistor is used to set the gain of the op-amp in non-inverting configuration.

You can determine if the load is too low by simply finding out from the data sheet the maximal output current that the op-amp is capable to source/sink. By dividing the output voltage to 50-ohm, you will see if the current exceeds the value mentioned in the data sheet.

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nubbage

Post subject: Posted: Mon Oct 13, 2008 4:58 am

General

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi

It is difficult to advise regarding spike suppression without knowing the exact application. A spike has a frequency spectrum, parts of which may overlap the bandwidth of your input signal.

In such circumstances it is possible to low-pass filter the input using a small passive element LC filter, so that the spike spectrum and the wanted signal are frequency-separated. Many circuits I have seen also use a degree of capacitive shunt across the gain-controlling feedback resistor acting as a kind of integrator, an integrator being a kind of low-pass filter.

Putting a good high frequency hi impedance oscilloscope across the input, the inverting input, the power rail, and the output should reveal if spikes are the problem.

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Philip_WNL

Post subject: Posted: Fri Oct 31, 2008 1:29 am

Lieutenant

Joined: Thu Oct 30, 2008 2:16 am

Posts: 2

Hi ifatfat,

some few comments:

- indeed the opamp amplifies the difference, but the gain is made so high...that, in open loop, this statement doesnt translate into useful/measurable info(your output usually stick to the supply). The Opamp is considered most of the time in closed loop configuration (as it is the case here with the non-inverting configuration).

-Therefore in closed loop ONLY, try to find the right common mode voltage by puting a small AC signal (few millivolts) over a DC voltage -as an input to your non-inverting buffer- that you shall vary starting from vdd/2 until you see a convenient output.

- an opamp to the contrary to operational transconductance amplifier (OTA) have an output buffer to drive small load. How small ? this is what you should investigate. try with 1 kOhm first. to be in the safe side, then bit by bit decrease the resistance and augment the amplitude of your AC signal to see how much you can get of your opamp.

Also pay attention to the unity gain bandwith that will tell you the maximum frequency that your overall buffer can sustain.

-finally look carefully into the datasheet to check what type of load your opamp can accomodate: for instance the output capacitor may not be suitable for opamp stability reasons.

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