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I am trying to design a digitally controlled current source but I have doubts regarding whether the driving of the Mosfets in the Blue box is correct or not , I understand that for example when the output of U3 is postive then the P-channel Mosfet will be the one that conducts and need negative Gate Voltage and vise versa so that I didn't add the Voltage on the Emitter to the driving Gate voltage , but I am not sure that is correct and should work. I will post the schematic; it will be helpful if some one tell me whether that part am confused about is correct or not, also if any one have any concern about the functionality should work correct or not.

The Step size is programmable also not only the value of the current.

Digitally Controlled Current Source Schematic

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  • \$\begingroup\$ I am having a little hard time following this schematic. But you say that you are making a current source. So the Mosfets in the blue box, are they supposed to control the current through some load? The only current flowing through the Mosfets are coming from U3. \$\endgroup\$ Commented Jul 9 at 16:10
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    \$\begingroup\$ No the mosfets in the blue box acts as a bidirectional analog switches , Q3 and Q4 will provide the resistance (in both directions) needed to produce a gain on U3 such that a scaled output of U2 is equal a scaled version of U3 output , U8 provides whatever voltage on Q3 and Q4 to accomplish this , the step value is controlled by choosing which scale is in action to be used through the Decimal encoder and a driving circuit \$\endgroup\$ Commented Jul 9 at 16:27

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The way you have wired the MOSFETs can never work. Look at the symbols and note the parasitic diodes representing the body diode within all commercial 3 pin devices. This will cause diode conduction irrespective of the gate signals or polarity: -

enter image description here

In effect, Q13 and Q14 might as well be anti-parallel diodes. Ditto the other MOSFETs. Have you tried using a simulator to see this for yourself?

Maybe you should use series source-to-source N channel MOSFETs? This is the tried and tested way of making solid-state switches.

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  • \$\begingroup\$ Will the solution you are suggesting bidirectional ? \$\endgroup\$ Commented Jul 9 at 16:36
  • \$\begingroup\$ I have done some additional research and yes I understand now your answer , that makes use of the diode also , should I edit my question to post new correct schematic also if any one needs it so the correct version is there ? \$\endgroup\$ Commented Jul 9 at 17:35
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    \$\begingroup\$ @AbdAllahTalaat no, we don't like questions that are modified after the answer has been given unless it's absolutely clear that you have made an edit such as.... Question edited to show the type of circuit implied by andy in his answer. Then, there is no ambiguity. Or, you can leave a link to an image that I can check. \$\endgroup\$ Commented Jul 9 at 19:13
  • \$\begingroup\$ Thanks too much @Andy \$\endgroup\$ Commented Jul 9 at 22:01

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