Department of Electrical and
Computer Engineering
Electronics II

ELE 343 LAB 4 Non-ideal Operational Amplifiers

MC1741 Electrical Characteristics
Input Offset Voltage   2 mV
Input Offset Current   20 nA
Input Bias Current   80 nA
Large Signal Voltage Gain   200 V/mV
Common Mode Rejection*
Rs < 10K 		  90 dB
*The common mode rejection ratio of 90 dB is specified here. The common mode rejection ratio is the ratio of the differential gain to the common mode gain. Rs is the resistance of a voltage source applied to the amplifier. It results in a decrease of differential gain due to the voltage divider produced by Rs and rp.

In this lab we study the 741 Opamp and measure;

  • Input offset voltage
  • Input offset current
  • Common mode input range
  • Closed loop frequency response
  • Low frequency open loop gain
    • Input Offset Voltage

    Opamps have an unavoidable built in input offset voltage of the order of a few mV. This voltage causes the opamp output to approach one of the power supply voltages even when the applied input voltage is zero.

    The circuit in Figure 2A can be used to measure the input offset voltage. R2 should be small so that the output voltage is not effected by input bias currents, but large enough so that its loading effect does not influence the measurement.

    Although the input is grounded, the input offset voltage appears like a voltage source connected to the non-inverting input, as shown in figure 2B. The output voltage is,

    Vo = [1 + R2/R1]*Vos

    Figure 2 Circuit A measures input offset voltage
    which appears as a voltage source as shown in circuit B.

    Use a 10K potentiometer to null the offset voltage. Connect the potentiometer wiper to -VEE and the other two terminals to the opamp offset null pins as shown in Figure 3.


    Figure 3   MC1741 Opamp with nulling circuit is show. Null the opamp when it is in the inverting or noninverting circuit.


    Figure 4   The output voltage is
    due to the bias current flowing
    into the non-inverting input.


    Figure 5   With Vcm set equal to zero,
    the output voltage is due to the bias
    current flowing into the inverting input.


    Figure 6   Unity gain buffer can be used to
    measure the common mode range.

    Input Bias Current

    A small current into the inverting and non-inverting inputs is required to bias the internal opamp transistors. Opamps with bipolar input stages require more current than opamps with fet input transistors. The 741 opamp has a bipolar input stage with a typical input bias current of 80 nA. The LF353 opamp with fet input transistors has a typical input bias current of 50 pA. That's 3 orders of magnitude less(60 dB less). Fet bias current is due to protection diode leakage.

    Measure the bias current to the non-inverting input using the circuit in Figure 4. Pick a value of RB to produce an output voltage in the linear range.

    Use the circuit in Figure 5 to measure the bias current to the inverting input. Set Vcm to zero by grounding the non-inverting input.

    Calculate the input offset current.

    Common Mode Input Range

    Opamps only work correctly when the input common mode voltage is within a certain range. Using the circuit in Figure 6, vary the input voltage, Vcm, and plot the output vs the input. When the input is within the input common mode range the output will track the input.

    Frequency Response

    Design an inverting opamp circuit with a gain of 20dB. Take measurements on the circuit to determine the closed loop frequency response. Plot closed loop gain in dB as a function of frequency. Use semilog paper.

    With a one volt AC input signal, measure the frequency where the output is 5, 1, and 0.5V. Use MATLAB to plot the gain in dB as vs the log of the frequency. At these frequencies the output should be dropping at 20dB/dec. The frequency where the gain is one is the gain-bandwidth product. What is the gain-bandwidth product for this opamp?

    Low frequency open loop gain

    High amplifier gain is hard to measure. For a typical gain of 105, a 0.1mV input voltage results in a 10V output.

    The nulling circuit shown in Figure 1 is used to null offset voltages and measure the high DC opamp gain. The circuit measures the low frequency gain of opamp A1. Opamp A2 provides feedback to hold A1 in its linear range.

  • The voltage across R1 is zero assuming no current flows into the opamp.
  • The differential input voltage, Vin = Vout*R2/(R2+R3). Set R2/R3 about 10-5 so if Vout = 10V, Vin = 0.1mV. R3/R2 should be the same order as the opamp gain to reduce Vin and prevent the amplifiers from saturating.
  • Since the amplifier A2 is an integrator, its DC input must be zero. Feedback insures this. The same current flows through R5 and R4. The point where R5 connects to R4 (the input to A2) is at zero volts. The output of amplifier A1 is Vo1 = IR4. Also, Vsrc = -IR5. Therefore Vo1 = -(R4/R5)Vsrc. The overall loop gain can be reduced by making R4 much greater than R5. However, Vo1 should be in the linear range of A1, not at VCC or -VEE.
  • C = 1 to 10nF (Required for stability)
    • Therefore,
    A1 input voltage = Vin = Vdiff1 = Vout*R2/(R2+R3)
    A1 output voltage = Vo1 = -(R4/R5)Vsrc
    Change Vsrc to measure the change in the output voltage of amplifier A1 and measure the change in Vout to determine the change in the differential input voltage. Use these values to calculate the gain of A1.
    GAIN = delta Vo1/delta Vdiff1 = -[(R2+R3)/R2]*(R4/R5)*(deltaVsrc1/deltaVout)
    Working with changes in voltages eliminates the effect of the constant input offset voltage on the measurement.

    Measure the DC amplifier output voltqages to make sure the amplifiers are not saturating.


    Figure 1   Nulling amplifier A2 is used to measure the low frequency gain and input offset voltage. Download the Multisim8 input file. (Right click-->Save Target As...)

    Lab Questions

    1. How does the amplifier you measured compare with the manufacturers spec sheet?
    2. What is the effect of the nulling potentiometer on the bias current measurement?
    3. What is the gain bandwidth product for the amplifier?
    4. What is the effect of the input offset voltage and input offset current on your common mode measurements?
    5. Why not just short the inverting and non-inverting inputs to measure the common mode gain?