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.
- Knowing
the non-inverting input, Vp, is at the same potential as the inverting
input, Vm, (called the common-mode voltage, VCM) what are the maximum
and minimum allowable common-mode voltages?
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.
- What is a good estimate for the op-amp's open-loop gain?
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.
- What is a good estimate for the offset voltage? For worst case design what value would you use?
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.
- What is the common-mode voltage, VCM? Does VCM change? Why or why not?
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
- What is the ideal closed-loop gain?
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
- 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?
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:
- What is the maximum allowable input signal amplitude? Why?
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.
- What is the maximum allowable input signal if the magnitude of the gain is increased to 10? Why?
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.
- 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?
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.
- 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.
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:
- What is the input offset current? What does this term describe?
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.
- Note that if the output voltage is precisely the same as VCM then the op-amp has no offset voltage (this is very possible).
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:
- To measure small offset voltages increase the gain by increasing RF to 100k or larger. Explain what is going on.
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.