How does the op-amp deals with negative inputs?

Question

The diagram shows the internal circuit of a 741 op-amp. Q1 and Q2 are both NPN transistors. So current can only flow through the transistors if a (large enough) positive voltage is supplied to pin 2 or pin 3 (i.e. the input pins). But I learnt that op-amp can also work with negative voltage input. For example, in a voltage follower circuit, a -5V input would produce a -5V output. Can someone explain how the op-amp works with negative voltage input the same way it works with positive voltage input?

Answer 1

It can only work with a negative input if a supply voltage is applied to Vs- that is more negative than the input. In the case of the 741, about -8V supply to be sure of it working with a -5V input.

Then, from the reference point of Vs-, all inputs are positive by 3V or more. The 741 does not have a GND pin- all it cares about is the input and output voltages with respect to Vs- and Vs+.

Answer 2

If the 741 is powered from a single 30V DC supply and the 741 op-amp pin called \$V_s-\$ is connected to 0V, the inputs will have a typical valid range between +2V and +28V.

If you decided to have a power supply of -15V to +15V then you shouldn't be surprised to find that everything just shifts down 15 volts i.e. the typical valid input range is now -13V to +13V.

Does the 741 know the difference - No it doesn't.

If you had two 15V isolated power supplies and stacked them in series and used the mid-point (where the supplies are joined) to make voltage measurements with a DVM, you would measure +15V at the top of the stack and -15V at the bottom of the stack. If you were to make voltage measurements relative to the bottom of the stack, at the mid-point you would measure +15V and at the top you'd measure +30V.

The decision to call one point 0V or a different point 0V is purely arbitrary and one of convenience.