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?