Second, the Wilson current mirror is susceptible to mismatches in the current gain, formula_30, of its transistors, particularly the match between formula_31 and the current gains of the matched pair Q1 and Q2. Accounting for formula_30 differences among all three transistors, one can show that formula_33 where formula_34 is the Harmonic Mean of the current gains of Q1 and Q2 or formula_35. Beta mismatches of five percent or more are reported to be common, causing an order of magnitude increase in the static error.