Analog Integrated Circuit Design Laboratory
Laboratory Report 3: Op-amps I, basic topologies, finite gain, and offset.
EMAIL: mesah1@unlv.nevada.edu
02-19-2020
What is the common-mode voltage, VCM? Does VCM change? Why or why not?
The common mode voltage or VCM can be considered as the reference voltage on which op amp output signals ride. Generally, VCM can be found by taking the mean of VCC+ and VCC-.
What is the ideal closed-loop gain?
The ideal closed loop gain for an inverting op amp topology can be described by the equation bellow.
What is the output swing and what is it centered around?
What happens if the input isn't centered around around VCM, that is, 2.5 V?
Adding a DC offset to the input signal adds to the net offset of the op amp.
What is the maximum allowable input signal amplitude?
The maximum allowable input signal should have an amplitude less than 2.5V due to the values of VCM, VCC+, and VCC-.
What is the point of the 0.01 uF capacitors from VCC and VCM to ground?
Are these values critical or could 0.1 uF, 1,000 pF, 1 uF, etc. capacitors be used?
C1 and C2 are decupling capacitors. Decoupling capacitors are used to even voltage out in the event of slight variance in the voltage from the power supply. Decoupling capacitors do not need to have specific capacitance values. Therefore, capacitance values such as 0.1 µF, 1000 pF, and 1 µF could all be used. When a larger decoupling capacitor is used; the startup time is longer. However, when a smaller decoupling capacitor is used; the startup time is shorter.
The data sheet shows that this op-amp has an input bias current that flows out of the op-amp's inputs of typically 20 nA.
This current flows out of both the non-inverting and inverting inputs through the resistors connected to these inputs.
Show how the operation of the circuit can be effected if, for example, R1 and R2, are much larger. Explain what is going on.
According to the LM324 datasheet, the op amp has an input bias current that flows in or out of the op amp’s input terminals. The flow of the input bias currents can be seen in Figure 13. This bias current is usually flows out of the op amp’s input terminals at around 20 nA. This current flows though the electrical devices connected to the input terminals. While considering the input bias current of an op amp, one might wonder how this current would effect R1 and R2. If R1 and R2 are larger the voltage drops over these resistances due to the input bias current will be larger. If these voltage drops (over R1 and R2) are too large, VCM and VM will differ. Likewise, by increasing, RF and RI, the voltage drops across these resistors become larger, due to Ohm’s Law. Also resulting in VCM and VM to differ. Thereby, adding to the net offset voltage. Offset voltage, due to bias currents, result in input offset current.
Hand Calculations to Detail Circuit's Operation:
Our Vcc+ = 5V and Vcc- = 0V. Therefore, our common mode voltage is 2.5V.
The equation above show us that there will be no gain on our output. However, the op-amp will invert the signal by 180˚.
Simulation:
Experimental Results:
Input vs. Output
Hand Calculations to Detail Circuit's Operation:
Vcm = 2.56V and Vout = 2.52V. Therefore, the offset voltage = 2.0mV.
Simulation:
Experimental Results:
|
|
Measured Offset Voltage |
|
LM324 |
2.0mV |
|
LM741 |
1.17mV |
|
LM348 |
1.06mV |
|
UA741 |
0.31mV |