Finally, the collector current in a bipolar transistor for low and moderate emitter currents conforms closely to the relation formula_36 where formula_37 is the thermal voltage and formula_38 is a constant dependent on temperature, doping concentrations, and collector-emitter voltage. Matched currents in transistors Q1 and Q2 depend on conformity to the same equation but observed mismatches in formula_38 are geometry dependent and range from formula_40 percent. Such differences between Q1 and Q2 lead directly to static errors of the same percentage for the entire mirror. Careful layout and transistor design must be used to minimize this source of error. For example, Q1 and Q2 may each be implemented as a pair of paralleled transistors arranged as a cross-coupled quad in a common-centric layout to reduce effects of local gradients in current gain. If the mirror is to be used at a fixed bias level, matching resistors in the emitters of this pair can transfer some of the matching problem from the transistors to those resistors.