Equation (4) substantially underestimates the differences between the input and output currents that are generally found in this circuit for three reasons. First, the emitter-collector voltages of the inner current mirror formed by Q1 and Q2 are not the same. Transistor Q2 is diode connected and has formula_28, which is typically on the order of 0.6 to 0.7 volts. The collector emitter voltage of Q1 is higher by the base-emitter voltage of Q3 and therefore is about twice the value across Q2. The Early effect (base-width modulation) in Q1 will force its collector current to be slightly higher than that of Q2. This problem can be essentially eliminated by the addition of a fourth transistor, shown as Q4 in the improved Wilson current mirror of Fig. 4a. Q4 is diode connected in series with the collector of Q1, lowering its collector voltage until it is approximately equal to formula_29 for Q2.