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chellenge questions....

hi everybody

I am facing problems in three circuits

1.     My data acquisition setup. I have made a data acquisition setup to be interfaced with the DAQ Data acquisition card NI 6025 e  .the data acquisition card only takes voltage as input so I used op amp as current to voltage converter. The problem arised may be beacue of the resistor i ahve connected from output to the input

I had an option to connect a resistorhere but the resistor is not doing the required job. The other problems arising because of  this are

a)     The potential difference at the input terminalsof op amp is not zero

b)     Current is going into the output of op amp

I have drawn the circuit in the multisim softwere and I haveattached the file with this for further analysis

2.     i have made a circuit for charging the mobile phone battery. i have made it in proteus and i have attached the picture of it also. i am unable to find the output parameters of it. the battery at its full charged state has the output voltage of 4.2 v . i have selected the output voltage as 4.2 volts. i have the option to use the op amp as comparator at the output to cut off the voltage supply when it reaches 4.2 v. but as for as my knowledge the transistors used in the circuit are used to provide the cut off. i also don't know what value of output current to be selected to charge the mobile battery..

3. i want to design a circuit so that i can switch from the charger to the load. so that my apparatues works with my presence also

please help me in the circuit design

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