negative inputs of the transconductor gm should be interchanged etc the same as above to adjust the voltage vr by means of a variable resistor control circuit 7 controlling the resistance of a variable resistor r 4 is only an illustrative embodiment to show the concept the spirit is to provide a circuit for adjusting the voltage vr according to a comparison between slope directions of current and power when the slope directions are opposite to each other the circuit decreases the voltage vr when the slope directions are the same to each other the circuit increases the voltage vr any variation under this spirit falls within the scope of the present invention there are many ways to embody the current sensing circuit 8 one of which is shown in fig1 the circuit shown in fig1 senses the current iin and transfers it to a voltage signal to be sent to the direction comparison circuit 60 again this embodiment is for illustration not for limitation a power meter 40 is used in the embodiments of fig1 11 and 14 from a first sight the use of a power meter complicates the circuit because a power meter needs to measure and calculate product of current and voltage values actually under the concept of the present invention it does not require an accurate measurement of power and thus it does not require a sophisticated power meter what is required is only to know the direction of changes of the output power of the primary photovoltaic device group 21 therefore it is sufficient to use a.