Lab3 - EE 420L 

Authored by Allan Pineda
pineda3@unlv.nevada.edu
February 15, 2017   
Lab Description: Op-amos I, basic topologies, finite gain, and offset


This lab will utilize the LM324 op-amp (LM324.pdf).
Review the data sheet for this op-amp.
For the following questions and experiments assume VCC+ = +5V and VCC- = 0V.

Answer:
    The maximum allowable common-mode voltages is rated as Vcc-1.5 and a minimum of 0V at 25 degree celsius. Since we assume that our Vcc+ is 5V and Vcc- is 0V, our maximum Vcm is 5V - 1.5V = 3.5V. This result can be verify using the LM324 data sheet below.




Answer:
    Ideally, we want to obtain an infinite gain. We need to bring the Vp and Vm equal to each other to obtain an infinite gain. But for this specific op-amp's , a good estimate gain is about 100db at Vcc+ = 5V. The data sheet below Figure_1  (a ) shows variety of gain that changes with respect to Vcc+. In Figure_1 (b), shows the open loop frequency response gain per db. It is clearly stated that the gian for this op-amp's is 100db at frquency around 10Hz.

Figure_1: 
           
                              (a)                                                                                (b)

Figure 2:

Below is the open loop gain for the op-amp used in this experiment. It stated that at room temperature, the open loop gain is about 100db.



Answer:
    A good estimate offset voltage value for this design is 2mV with maximum 5V at 25 degree celsius. For worst case design, a 9mV offset will be used to account for high temperature variations and provide a large range. To support this estimation. See Figure_3 below.

Figure_3:


Build, and test, the following circuit. Note that a precise value for the 5k resistors isn't important. You can use 4.7k or a 5.1k resistors.


Answer:
    The common-mode voltage of the op-amp's is approximately about 2.5V with a supply voltage of 5V. See Figure_4. This can be verify by using voltage divider calculation. Due to a voltage divider, the VCM value will not change for as long as the supply voltage constant. See Figure_5 for hand calculation.

Figure_4:


Figure_5

Answer:
    The close loop gain is -1 since we have a inverting op-amp's circuits. This indicate that the result is a unity gain with 180 degrees out of phase with the input. See Figure _6 for verification.

Figure_6


Answer:
    The output swing is the range of voltage that an op-amp's physically provide at its output. The positive output swing is 2.5V + 0.1V = 2.6V and the negative output swing is 2.5V - 0.1V = 2.4V. The output swing is centered at around 2.5V. Since the op-amp have an output offset, we need to supply offset voltage from function generator to cancel the output offset voltage of  the op-amp. Without cancelling the output offset voltage, the output of the op-amps may be in saturation or clipped. In case the VCM is not centered or set to 2.5V, the output wave will be centered in different values. Since the op-amp used has a 5V+ and 0V-, output swing cannot pass this range or the output wave will clip as well. See Figure_7 below.

Figure_7:


Answer:
    The maximum allowable input signal amplitude is between +5V and 0V. If one passed this range, the wave signal output will start clipping. Since the offset is 2.5 voltage, the op-amps maximum allowable voltage is calculated as +5V – 2.5V =+2.5V and 0V – 2.5V = -2.5V (Theoretical).  In the Figure_7 above one can observe when the offset voltage supplied by the generator is being decrease and passed the 1.70V mark, the wave output start to saturate or clipped and when increasing the offset voltage pass the 3.80V mark, the wave output swing start to clipped. This shows that the maximum allowable voltage is approximately 5V and 0V.

Answer:
    When the gain is increased by 10, the output signal is 10 times larger than the input.  Thus, the maximum allowable input signal amplitude is 10 times smaller than (+/-) 2.5V or (+/-) 250mV. See Figure_8.


Answer:
    The capacitors serve as a decoupling capacitor in the circuit. It does not affect the value of VCM. The reason we have capacitors in the circuit is to stabilize and reduce any noise that may occur in the circuit. But in general since the capacitor act like an open circuit in DC; and short in AC, it will not affect the circuit circulation. Answer:
    When resistor R1 and R2 have larger value, the VCM may significantly change its value since VCM is also dependent on the bias current. Change in VCM value will result in saturation or clip in the output wave signal. For example, assume that we have 150M ohms resistor for both R1 and R2 and while ignoring the 5V VCC, these two resistor will be connected in parallel with current of 20nA.  We see in the calculation below that the resulting VCM is change to 4V since the voltage drop will added to the value of VCM. As a result of these change, the same problem will arise discuss earlier. See figure_9 for hand calculation.

Figure_9:


Answer:
    Input offset current is the difference between the biasing current of the op-amp terminals. Just like offset voltage, the input offset current must be taken into account in using op-amp. One may use biasing current technique to balance the offset current and will not have an effect in output of the circuit




Explain how the following circuit can be used to measure the op-amp's offset voltage.
Answer:
    Since both terminals (inverting and non-inverting) are VCM, this means that there is no offset voltage and offset current in the circuit. To verify this assumption, one can measure the offset voltage by measuring the differential between the output and VCM and by dividing it through the gain of 20V/V. See Figure_10 for hand calculation. Increasing Rf to 100k means increasing the gain to 100V/V, which can help us to see the small offset value.

Figure_10:

Answer:

    There are four different op-amp was use to measure their respective offset voltage. Each will have different offset voltage because it is really hard to manufacture exact the same chips. Since the oscilloscope cannot measure a very small difference in measurement, a multi-meter was used to measure the offset voltage as shown in Figure_11. The LM339AN has the smallest offset Voltage of the all four.

Figure_11:

LM324

 

LF351N

       

LM339AN

     

LM348N

   

Conclusion:

            This laboratory exercise presented valuable information in using an op-amp. The basic operational such as common mode voltage, close loop gain, output swing and maximum allowable input signal was carefully observed as well as the data sheet of LM324 op-amp.