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circuit design problem

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Hi

i have desined a R-2R ladder network(attached it pic below) to convert digital to analog and i used a op amp to amplify the output of R-2R ladder network to somelevel. The problem here is i'm not able to generate ouput when i connect the same circuit in general purpose board and it's giving exact ouput in multisim someone pls help me what could be the problem.

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Solution
Accepted by dhanu99

The common mode input voltage range for the 741 op amp is specfied as +/-12 V with a +/-15 V power supply. Because of the design of the input circuit that range will not change much with the power supply voltage. The voltage at the non-inverting input in your circuit is ~0.6 V which is well outside the common mode input range.

 

If you need to use a single power supply, then look for an amplifier with the negative supply voltage included in the input common mode range.

 

Consider the output voltage range also. For the 741 the output range is +/-12 V minimum and +/-14 V typical which means that your circuit cannot guarantee an output voltage lower than 3 V regardless of the digital input.

 

The R11 R12 bias circuit does not do much because the R-2R network has a much lower impedance. R11 is in parallel with the output impedance of the network (R).  The contribution of the 12 V bias is about 70 mV.  The input bias current must flow into the op amp inputs becaues they are the bases of NPN transistors. If the output of the R-2R network is within the input common mode range sufficient bias current will flow through the network (80 uV error typical).  If the output is not within the input common mode range the circuit will not work so the impedance will not matter.

 

Lynn

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