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Why op-amp needs a dual supply?

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Ravi

Member
Hello friends :)

1. Manufactures recommend using regulated dual rails for op-amps.Why?
What would be the impact if operated from a single rail.

2. Besides a well regulated supply it also needs a well balanced supply. Meaning the (+) and (-) should be equal in magnitude. How does the power supply accomplish this task?

3. If I use two separate supplies to power the op-amp, as explain below, what would be the impact?

a) Using two completely separate step-down transformers, one with a (+) stabiliser and the other with a (-).

b) Using two separate windings in a single transformer with appropriate stabilisers.

These questions are haunting my mind for quite some time now, but I was not been able to get answers. i would be very much thankful if someone would explain.
 

kinjalgp

Active Member
1) Dual supply is used to bias the inetrnal differential amplifier's transistors in active region.
2) For equal magnitude (balanced) supply, the centre-point is always zero volt and your output voltage will swing between ~ +Vcc to ~ -Vee from this centre point. This means the swing is equal on both sides.
If supply is not balanced, the swing on one side will be less than the other side and may cause clipping in the output waveform. Also this will add DC offset to the output.
3) The effect will be same if you use either of the methods. But note that grounds of + and - supply should be tied together.
 

Ravi

Member
Thank you very much Kinjalgp, but little confused.So if a single rail supply is being used, the internal transistors will not opreate in their active region.right? Does this mean that a op-amp cannot or do not recommend to operate from a single rail supply?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ravi said:
Thank you very much Kinjalgp, but little confused.So if a single rail supply is being used, the internal transistors will not opreate in their active region.right? Does this mean that a op-amp cannot or do not recommend to operate from a single rail supply?
Basically for single rail use you would normally generate a split rail, often using just two resistors. So for a 9V supply the chip would run off +/-4.5V, or more accurately 0, 4.5V, 9V. Depending on the application it usually makes life a lot easier with a split supply, and can reduce the component count substantially.
 

kinjalgp

Active Member
It is not that all Op-amps use dual supply. Some op-amps have internal bisaing resistors which makes them go in active region with single supply. One such op-amp is LM324.

You can also use uA741 with single supply by using potential divider as Nigel suggested. For e.g in inverting mode, instead of grounding non-onverting terminal, you have to connect it to a potential divider network haivng equal resistors say 10k. This will set the reference point at Vcc/2 and output voltage will swing from 0 to Vcc/2 to Vcc.
 
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