Op-amp supply voltages wrong way round.

[Beer-monster]

Hi

I'm somewhat new at this practical electronics stuff. So could use some help.

I'm trying to debug a circuit made by a colleague who has moved on. It's a derivative of a non-inverting amplifier designed to amplify a signal from a generator. It works, to an extent, however the output wave from usually seems deformed in some way usually in the decreasing part of the wave e.g the voltage drop on a square wave having an exponential decrease or stray sinusoids sneaking in.

After going through it again and again the circuit seems to be wired correctly, for the most part, as described in the diagram and properly grounded. The one error I can find is that the input pin for the positive supply voltage is actually connected (via some resistors and capacitors) to the input jack for the negative supply bias.

So, basically, I'm wondering what effect supplying the op-amp with the voltages the wrong way round will have on a circuit. As I stated, the circuit does work to some extent so this can't be disasterous, but could it possibly explain the problems I've been having with the distorted output signal.

Thanks

 

[Studiot]

Posting a circuit would help.

 

[berkeman]

Hi

I'm somewhat new at this practical electronics stuff. So could use some help.

I'm trying to debug a circuit made by a colleague who has moved on. It's a derivative of a non-inverting amplifier designed to amplify a signal from a generator. It works, to an extent, however the output wave from usually seems deformed in some way usually in the decreasing part of the wave e.g the voltage drop on a square wave having an exponential decrease or stray sinusoids sneaking in.

After going through it again and again the circuit seems to be wired correctly, for the most part, as described in the diagram and properly grounded. The one error I can find is that the input pin for the positive supply voltage is actually connected (via some resistors and capacitors) to the input jack for the negative supply bias.

So, basically, I'm wondering what effect supplying the op-amp with the voltages the wrong way round will have on a circuit. As I stated, the circuit does work to some extent so this can't be disasterous, but could it possibly explain the problems I've been having with the distorted output signal.

Thanks

And what happens when you replace the opamp chip and fix the supply connections and power it back up?

 

[Beer-monster]

Thank you for your replies. I checked again and turns out the problem is a little more complicated.

How the circuit should be wired is shown below. Now, unlike what I said in my first post, the wires from the sample voltages go into their correct pins on the op-amp i.e -Vs to -Vs and +Vs to +Vs pin. The route they take to get there is wrong.

The -Vs input is connected to the -Vs pin AND to the 33 pF capacitor that leads on to C1 and R4. It should be the +Vs input that is connected to these components and into its own pin. However, the +Vs input is actually connected directly to its op-amp pin as -Vs should be.

I hope that description is clear. Does anybody know if this could explain why the op-amp works but seems to distort the output signal?

Next time I'm in the lab I'll try to rewire and correct for this...hopefully it can't make thinsg worse.

 

[schip666!]

Looking at the spec sheet I found for that amp: http://www.cirrus.com/en/pubs/proDatasheet/PA78U_B.pdf I'd say that the circuit you have is right out of the app note, except of course that your schematic doesn't show which pins are what. The extra dangly bits are probably for frequency compensation and could effect the slew and ringing behaviors described, If they are not suited to your signal their values may need to be modified. I'd recommend a careful reading of the specs and perhaps a little bread-boarding time to see if you have the right component values.