op-amp connections, ground, power supply

Thread Starter

PG1995

Joined Apr 15, 2011
832
Hi

Please have a look on the attachment. Are the connections correct? When I was trying to build this circuit some days ago, someone was saying that I need two separate power supplies to power up the op-amp and personally I was confused too about it. The power supply had only two terminals +ve and -ve and a knob to change the voltage level. I had set the knob at 15V. One thing really confuses me that there is no reference (ground) for the voltages of power supply; e.g. +15/-15 volts with respect to what? Could you please help me? Also I didn't have access to o-scope to check the Vout waveform. Thank you.

Regards
PG
 

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Audioguru

Joined Dec 20, 2007
11,248
Usually the (-) wire of the signal generator connects to ground and the left wire of the resistor that limits the LED current usually also connects to ground.

You show the signal generator with a positive output voltage. Since it feeds the (-) input of the opamp then the output of the opamp will try to go negative.
But the output of the opamp cannot go negative without adding a negative power supply.

Note that the LED lights only when the output of the opamp is positive and the input is negative.
The LED will be damaged if the output of the opamp goes more than 5V negative.
 

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Thread Starter

PG1995

Joined Apr 15, 2011
832
Thank you, AudioGuru.

So, I really needed two separate power supplies. Thanks a lot for the help.

Best wishes
PG
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Usually the (-) wire of the signal generator connects to ground and the left wire of the resistor that limits the LED current usually also connects to ground.

You show the signal generator with a positive output voltage. Since it feeds the (-) input of the opamp then the output of the opamp will try to go negative.
But the output of the opamp cannot go negative without adding a negative power supply.


Note that the LED lights only when the output of the opamp is positive and the input is negative.
The LED will be damaged if the output of the opamp goes more than 5V negative.
Hi again

Okay. Suppose I'm not using op-amp in non-inverting (NI) configuration, then would I still need a negative power supply? I'm asking this because the output is not going to be negative. Please let me know. Thank you.

Regards
PG
 

Audioguru

Joined Dec 20, 2007
11,248
You are very confusing:
1) Does your power supply have a positive output voltage, a negative output voltage plus a 0V terminal? Then it is a dual-polarity power supply.
2) If it has only a positive and a negative terminal without a 0V terminal then it is a single-polarity supply.
3) your opamp is inverting so when the input goes positive then the output will try to go negative if the opamp has a dual-polarity power supply.
4) You said, "the output is not going to be negative" then what do you want the output to be?

Is the input swinging positive and negative?
When the input is positive what do you want the output to be?
When the input is 0V then what do you want the output to be?
When the input is negative what do you want the output to be?
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Thank you for the reply.

Yes, in this diagram the op-amp is inverting. But what if we use the op-amp in NI configuration like this (although in this diagram the op-amp doesn't use any feedback). So, I think when the +ve sine wave signal is fed to non-inverting (+ )input while the inverting (-) input is grounded, the output is going to be swinging above 0v axis. Am I any clearer? Thanks for the help.

Regards
PG
 

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BershaM

Joined Aug 22, 2011
17
if an op amp needs +Vcc and -Vcc to operate, you have to use 2 batteries (2 power supplies) for it to work in ANY mode of operation .
 

Audioguru

Joined Dec 20, 2007
11,248
Most opamps have inputs that DO NOT WORK when their voltage is less than a few volts more positive than the negative power supply pin. If your input goes less than about +3V then these opamps WILL NOT WORK. Therefore these opamps use an additional negative supply or have their inputs biased at half the positive supply voltage and have the input and output capacitor-coupled.

But a few opamps DO work. LM324 (very slow), LM358 (very slow), MC3317x (fairly fast)and MC3407x (very fast) have inputs that can go down to 0V in your circuit then their output go down almost to 0V (maybe +0.05V).

Most signal generators have their output swing positive and negative. You want to add a DC offset voltage to yours so that its signal swings from 0V to a positive voltage.
 
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