Thermocouple Amplifer

Q1 and Q2 form a current mirror circuit. The current at the collector of Q1 is set by R1 and the supply voltage. The voltage across R1 is the supply voltage minus about 0.7 V.

If Q1 and Q2 are matched then the current at the collector of Q2 is the same as at the collector of Q1.

Thermocouples can supply many milliamps of current even though the voltage they generate is small.

This allows thermocouple T1 to add or subtract (according to temperature) voltage going into the base of Q2 without any loading effect.

Q2 responds exponentially (via its collector current) to the change in input voltage causing the voltage across R2 to change.

The load resistor R2 could be replace with a relay, for example, for temperature control applications. 

You should be careful not to run too much current through the transistors to avoid internal thermal effects, though heat sinking may help.