I am learning about opamps. Now I want to experiment with them but the problem is with -Vee (that negative power supply). I felt like I couldn't design it so opted for ground instead of negative supply, as I heard about single supply. Now anyone please elaborate what is the use of two power supplies also the consequences of making the -ve supply as ground. My college course prefers IC 741 OPAMP.
Electronic – the use of two power supplies in an opamp
operational-amplifier
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Best Answer
See this:
The ground line isn't normally connected to supply a 741. There's no pin for it. Thus the 741 cannot have a slightest idea what happens to be your ground potential compared to VEE line, it's happy if there's high enough voltage between VCC and VEE lines.
Let's assume your 741 does something useful and does it as we expect an opamp to work in its linear region. Then internal transistors Q3 and Q5 need proper operating voltage. It comes from Q1.
Let's assume UE=2V is enough, more is allowed but less than 2V isn't enough. As you surely know from the theory of the emitter follower, then U1 must be about 2,7V or more.
Let's assume your application needs that 741 must be fully operational even in case the +input is connected to ground or it gets a signal voltage which happens now and then be zero voltage above your ground potential. To keep 741 operational there's no other possiblity than to connect VEE line to negative voltage and it should be at least 2,7V below zero. If you must put negative signal voltages in the +input, then even more room is needed. -3V to +input forces to use VEE=-5,7V
If you want to get negative voltages or even zero from the output, you also need negative VEE.
Be informed that you can connect VEE to the ground, but then all input voltages must be far enough from zero. If the 2V minimum UE is true, then U1 must be at least 2,7V
The 2V limit is my coarse simplification to keep the numbers short, you can read from the datasheet how much lower VEE must be than the input.
ADD: You can easily find simple opamps (say 3704) that are advertised to work from a single positive supply. If you look at their interior schematics, you see that they have just mirror PNP input stages. There the voltage reserve is a must between the input and VCC line. Dual supply is still needed, if you want to have both + and - input or output voltages.
Rail to rail opamps use input circuits that do not need that voltage room between rail and input. They have fet outputs which can pull the output to both rails if needed.