working out Vout for an Opamp circuit that has a common base npn transistor instead of a resistor?
You know the voltage at pin 2 is 0V so you can can calculate the current through R1 (assuming the op-amp is in balance and is stable). 

So you also know the collector current of Q1, and the base voltage. As G36 indicates you can ignore the base current and treat the transistor as an (ideal) diode with \$\eta\$ = 1.  

You will notice that less-than-useful things will happen if Vin &lt; 0V.