buffer provides sufficient drive for the op amp 10 to power external loads not shown in fig2 a differential gain stage formed by bipolar transistors 22 23 couples to the inverting and non inverting inputs of the op amp 20 respectively the collector of transistor 23 couples to a simple current mirror 24 referenced to v which acts as an active load to transistor 23 current mirror 24 comprises transistor 25 26 27 and resistors 28 29 30 for simplicity the current mirror 24 has a predetermined current transfer ratio of g b defined here as the ratio of the current out of the current mirror 24 from transistor 27 divided by the current into the current mirror 24 from transistor 23 operation of the current mirror 24 is well understood but for purposes here the ratio of the sizes of the transistors 25 27 and of the resistances of resistors 28 30 sets the current transfer ratio g b it is understood that other types of current mirrors may be used such as a compound or wilson mirror having a predetermined current transfer ratio g b resistor 31 acts as a load for the current from the current mirror 24 producing across it the output voltage on the output node out the overall gain both common mode and differential mode is set by the resistance of resistor 31 resistor 32 coupling to the common emitters of the differential pair formed by transistors 22 23 provides for the common mode gain a c in the op amp 20 therefore the approximate differential mode voltage gain of the op amp 20 is and the.