Figure 1

ELE541 Old Exam Nulling Amp Problem.

The nulling amplifier A2 is used to measure the open loop gain of about 1e5 of the DUT, A1 shown in Figure 1.
  • Specify resistor values.
    Answer

    Let R1=R2 to minimize the effects of bias currents.

    Let R4=R5, then Vsrc = the negative of the opamp output voltage.

    Assuming zero current into the opamp, V- = 0
    and the differential input voltage equals V+

    V+ = [R2/(R2+R3)]Vout     V+ also equals Opamp output voltage divided by the gain.
    Assuming voltages of the order of one volt,
    1+R3/R2 = Gain
    Therefore R3 = R2*1e5

    If R2 =100 ohms, R3 = 10MEG.
    Let R1=R2 = 100 Ohms
    R4=R5 = 10K to minimize opamp loading.