Objective: The purpose of this lab is to build simple
voltage comparators that signal when an input voltage is greater
than or less than a reference value. These comparators are
constructed from op-amps without negative feedback such that they
saturate near their positive or negative supply voltages depending
upon the difference between the voltages applied at the inverting
and non-inverting inputs.
Pre-lab:
Recall the operating equation for an op-amp is v_{out} =
A(v_{+} - v_{-}) where v_{out} is the output
voltage, v_{+} is the voltage at the non-inverting input,
and v_{-} is the voltage at the inverting input. What is the
rated (large signal voltage) gain A of the LM741 and LF356 op-amps
in V/V?
Given the op-amp gain A, what would one expect v_{out}
to be if v_{+} > v_{-} (taking saturation into
account)?
What would one expect v_{out} to be if v_{+} <
v_{-} (taking saturation into account)?
A typical LED allows current to flow in only one direction (from
anode to cathode - see Figure 1 below), has a voltage drop of around
2V when forward biased, and requires approximately 10mA of forward
bias current to emit light (less current and it will be dim, more
current risks damage). Consider the op-amp circuit of Figure 2 in
the lab. If the output of the op-amp is saturated near either its
positive supply voltage (+15V) or negative supply voltage (-15V),
what resistor value should be chosen for R_{lim} such that
the current through the LED is near 10mA? Hint: does
R_{lim} = (14V - 2V)/10mA make sense?. Note the +-14V
output voltage swing is given in the op-amp's data sheet when supply
voltages of +-15V are used.
When v_{in} = v_{+} > v_{-} =
v_{ref} in Figure 2, which LED (red or green) would you
expect to be on?
When v_{in} = v_{+} < v_{-} =
v_{ref} in Figure 2, which LED (red or green) would you
expect to be on?
Determine the resistor values (R_{1}, R_{2},
R_{3}, R_{p}) needed for part two of the lab based
upon the criteria given there and Figure 3.
Laboratory Procedure: The comparator circuits shown below
produce a positive output voltage when the input voltage is greater
than a reference voltage and a negative output voltage when the input
voltage is less than a reference voltage. These positive and negative
output voltages are used to drive green and red light emitting diodes
(LEDs) to signal the result of the comparison.
Build the circuit shown in Figure 2 using a 356 or 741 op-amp, a
one turn pot to vary the input voltage v_{in} around 5V, 5V
for the reference voltage v_{ref}, and a resistor value for
R_{lim} near that determined in the prelab. Use a green LED
to signal an input voltage greater than the reference and a red LED
to signal an input voltage less than the reference voltage.
A realistic scenerio is the need to monitor a battery's voltage
and display its state. A two-stage 15V battery voltage monitor
as shown in Figure 3 can be constructed using the concept of the
op-amp comparator discussed above and shown in Figure 2.
Consider the circuit shown in Figure 3, and find values of
R_{1}, R_{2}, and R_{3} such that the
reference voltages are set to v_{1} = 14V and
v_{2} = 13V noting the high input impedance of the
op-amp will have little effect on the resistor chain.
A one-turn potentiometer (here in the simulated battery) can be
difficult to tune to desired values of resistance, and
correspondingly voltage. Therefore, a resistor is often placed
series with it such that the combined resistor and potentiometer
can more easily be "tuned" to desired values. Assuming a
10kΩ potentiometer is used in the simulated battery, find a
value for the resistor R_{p} in series such that the
intermediate voltage v_{p} = 12V when 15V is connected
to the top of the potentiometer as shown in Figure 3.
Build the circuit in Figure 3 using the simulated battery's
voltage as the input v_{in} to the comparators.
Use the potentiometer to vary the input voltage simulating a
discharging battery. In the beginning (battery voltage > 14V)
the LEDs should read green-green. When the voltage is between
13V and 14V the LEDs should read red-green and when the voltage
falls below 13V the LEDs should read red-red.
Note: The op-amp switching time from one rail to the other
can be reduced by choosing an op-amp with a higher slew rate and by
placing diodes around the op-amp such that the op-amp never enters
saturation.