Base current compensation in a PLL charge pump circuit

摘要

Lateral PNP current switching transistor Q11 has a low current gain so that a significant proportion of the current 15 applied to its emitter is lost in its base circuit. To compensate for this current loss, a current Icomp is added to 15. This compensating current is derived from transistor Q6 via mirror circuits 7 and 8. Q6 conducts the base current from PNP transistors Q4 and Q5, which are similar in construction to Q11 and carry a similar current. The matching of Isource and Isink is improved.