Reading
• LAB 8 Lecture
• Malvino, Ch. 20.1-20.4, Ch. 21
• Hayes and Horowitz, Class 10, Lab 10, and Ch. 4 worked examples on pp. 227ff
Notice
• Use AD711 instead of LF411s here
• Don’t forget to include commentary, explanations, calculations, and justifications for the
data you are collecting
• Also, try to apply the same standard of reporting with electronic data. E.g. if you wouldn’t
include graphs or pictures of real data of something (like a constant voltage), then don’t
include a screenshot of a constant voltage! There’s no more information in it than just stating
the number, and it makes reports hard to read
1 Comparators
a) Connect an AD711 op-amp as a comparator (Fig. 1).
b) Drive the input with a sine wave and observe the output. You are using the high open-loop
gain of the op-amp and swinging between positive and negative saturation.
c) Op-amps that are intended to be used as an amplifier have a slow response time, which is
caused by the internal compensating capacitor used to avoid high-frequency oscillations.
Measure this limited response time.
d) Connect the LM311 as a comparator and observe its improved performance.
TIP for d) LM311 is not in the standard Analog Devices library. You’ll have to add it yourself.
Add LM311 with the .asy and .lib files given.
The LM311 is an op-amp that is designed to be used as a comparator. Unlike most
op-amps, it has a faster response, and you can change the voltage levels of the output. With
this op-amp, you get -12 V if Vin < Vref and +12 V otherwise. The open-collector output of
the 311 allows you to change these output levels independently of the power supply that
runs the op-amp. Pin 1 comes from the emitter of the IC’s output transistor. It is not
connected inside the chip. Whatever voltage level you connect this pin to becomes the
output if Vin > Vref. Often this pin is connected to ground. Similarly pin 7 goes to the output
transistor’s collector. This collector is not connected in the chip either – that’s why it is called
an open collector. You connect this pin to the voltage that you want when Vin < Vref, for
example 5 V. Open collector outputs are commonly used if logic voltage levels representing
“true” or “false” are different in one part of the circuit from another part.
2 Schmitt trigger
a) Because of its fast response, the 311 comparator can “chatter” if Vin hovers indecisively
near the comparison point. In this case, the output will swing erratically between positive and
negative output levels as the input drifts up and down by very small amounts. Observe and
discuss.
Tip for a) To see this, repeat the previous circuit, but this time using a noisy voltage source
provided. This uses the arbitrary voltage source component, bv, which is mathematically
programmable. I’ve added noise using the rand command, which generates a uniformly
distributed random number between 0 and 1, which I’ve shifted to be centered about zero
and then scaled with an amplitude. The rand command looks up random numbers from a
preset algorithmic list, so that rand(1) always produces the same number, rand(2) a different
number, and so on. Only integer arguments are used, so I multiplied the small (µs) time
steps by 10^6 to generate noise with a 1 MHz bandwidth. Note that if you make this number
too large, it bogs down the calculation, forcing LTspice to make very small time steps. Feel
free to play with it, but you may need to use the halt command in the Simulate menu.
b) Calculate the trip points of the circuit in Fig. 3.
Tip for b) A Schmitt trigger uses positive feedback to reinforce the comparator’s decision.
Immediately after the input voltage crosses the trip point (threshold level), the trip point
changes so that the input must significantly retrace its path in order to reverse the decision.
This means that there are two trip points: one for rising input signals, and another, lower
one, for falling input signals.
c) Build it and see if it operates like it should.
d) Put a sine wave on the input and note the “hysteresis.” Also note that the triggering stops
for sine waves below certain amplitudes.
Tip for d) One nice way of displaying this is to export the data and plot Vin and Vout
parametrically.
3 RC relaxation oscillator
Tip for Qs below) The next circuit shows a square wave generator called a relaxation
oscillator. What you do here is remove the input to the Schmitt trigger and reconnect it to
one end of a capacitor. The other end of the capacitor is grounded, and the capacitor is
allowed to charge through a resistor coming from the output of the comparator. After the
capacitor charges to the upper trip point, the comparator output goes low to ground. Now the
capacitor will discharge until it reaches the lower trip point. The comparator output goes high
again and the charging cycle continues. The output oscillates between high and low with a
frequency determined by the RC time constant of the resistor and capacitor.
a) Predict by manually calculating the frequency of the circuit
b) Build it and measure the frequency
c) Measure the peak-to-peak voltage of the output and the voltage of the low level,
d) observe the inverting point X and explain its behaviour, and
e) try to change the frequency by changing the capacitor and resistor combination.
4 Wien bridge oscillator
Tip for Qs below) Generating a sine wave is more difficult than generating a square wave.
The Wien bridge circuit uses the parallel-series, a lead-lag circuit to maximize positive
feedback for its critical frequency, fc = 1/2πRC. The tungsten lamp in the negative feedback
loop limits oscillations when they grow past a certain limit. As the current through the lamp
increases at higher output voltages, its resistance increases and cuts down the gain of the
amplifier. Thus a stable frequency is maintained by the lead-lag circuit in the positive
feedback loop, and a stable amplitude is established by the self-regulating effect of the
tungsten lamp in the negative feedback.
The lamp is a really nifty way of getting the feedback right, but the simulation is a bit
challenging in LTspice, since one needs to get the lamp resistance and power rating very
close, and even then it requires a few seconds to stabilize – long for a high-frequency
simulation. It’s ok try it, but I suggest replacing the lamp with a resistor.
a) Calculate what resistor will give stable oscillations, rather than let the lamp auto-level it
b) Build the circuit in Fig. 5 and check if its frequency is 1/2πRC
c) When you first turn it on you’ll see the amplitude grow larger until the negative feedback
increases after the lamp warms up. When you poke the non-inverting input with your finger
the output will wobble. If you sweep the scope slowly and poke the non-inverting input, you
can see the envelope of the oscillation bob up and down. Can you explain this?
Tip for c) In real life, this oscillator gets seeded by input offset currents and random noise
and pickup. Those don’t work as well in SPICE, so instead, you may need to kickstart the
oscillations. If you add a pulsed voltage source to the inverting input via a 100kΩ resistor
(large enough not to perturb the existing circuit), then the feedback will get seeded by the
pulse and oscillations proceed. Use a voltage source, in pulsed mode, with a single short
pulse.
d) You can use this oscillator for radio frequencies. Using R = 470 Ω, C = 220 pF in the
lead-lag circuit will give 1/2πRC = 1.5 MHz. When I tried it, though, I got a frequency of
about 500 kHz. The shift is probably because of stray capacitances and inductances in the
circuit. Experiment yourself to get a frequency in the AM radio band. We also found that
increasing the 560 Ω feedback resistor to 1 kΩ improved the output amplitude. This way you
can use the oscillator for the carrier frequency of the AM transmitter of the FET experiment.
Tip for d) This is harder than just changing the components. You may want to approach the
frequency incrementally to watch what the circuit is doing. In fact, you may need a different
op-amp! When scrolling through the list, look for the phrase “high speed.”
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