Department of Physics, Stanford University
Physics 105, Analog Electronics (Pam)
Lab 8.12 DRAFT
Page 1
Lab 8: Positive Feedback: Oscillators and Comparators
(this lab will count as half of a lab)
Read:
Meyer, Chapter 6, Section 6.8
PRELAB -Practice:
The comparator circuit in Figure 3 has a pull up resistor on the output so that
Vout = +15 V (Q is off) when Vin < 0 ( “input low”), and
Vout = 0
(Q is on) when Vin > 0 . (“input high”)
Design the output circuit so that instead, the output state for a “high” input is -15 volts,
and the output for a “low” input state is 0V.
Part 3.1
Predict the thresholds and output voltages of the Schmitt trigger comparator in Figure 5.
Part 3.2
Now connect the “Ground” pin #1, the emitter of the output transistor, to -15V instead of
ground. Predict the threshold and output voltages again.
Comparator Background:
A comparator is a specialized version of the op amp, where the output is either “high” (binary 1) or “low”
(binary 0) depending on the relative size of the inputs. You have already constructed a kind of comparator
in Part 1 of Lab 7: this first opamp circuit performed as a simple “polarity detector”. However, in Lab 7
you used negative feedback and threw away most of the opamp’s gain to achieve good linearity. For
switching, one always uses positive feedback in the Schmitt trigger configuration to achieve stability
during the switching operation. Always use positive feedback—a Schmitt trigger—with a comparator.
Comparators are much better suited to fast switching demands than linear opamps such as the 411. The
311 is one such device, but in addition to its faster response time, be aware of the following significant
features. The 311 has (a) a different pinout than the 411, and (b) an “open collector” output.
Warning: do NOT think of the 311 comparator as an opamp. The output behaves very differently
from that of an opamp, and the opamp “golden rules” do not apply: the output only switches on/off, it
does not amplify.
The open collector output means there is a transistor connected to the output and to a “ground pin”—an
NPN in the drawing below—but it provides no signal output. It is up to you, the user, to provide an
external voltage source with two voltage states. The comparator’s output simply switches your external
source between two voltages.
The output algorithm is: V- is the input to be compared to a reference at V+. When the input is higher
than the reference, V- > V+ , the output transistor is turned on, i.e., a signal is applied to its base and it
operates as a switch with VCE = 0. When V- < V+, the output transistor is turned off. This open collector is
a useful output mode for many integrated circuits with 2-state outputs because it allows the user to control
the voltage levels representing “high” and “low” for the IC. These are typically chosen to be ground (0
volts) and +5 or +3.3 volts for standard digital logic.
Department of Physics, Stanford University
Physics 105, Analog Electronics (Pam)
Lab 8.12 DRAFT
Page 2
Open collector outputs require a “pull up” resistor at the collector to link the output to the upper voltage
level. In Figure 2, the 1 k is the pull up. Some IC’s have pull ups built into the chip; others require a
user-provided pull up resistor. The figures below contrast the output stages of the ‘311 and ‘411
Figure 1
Figure 2
LAB
Part 1 Opamp Comparator
LF411
7
in
10k
2
-
3
6
out
+
4
Drive the opamp of Figure 3 with a 100kHz sine wave, and look at the output
on the scope.
1. Compare the output amplitude with the “Output Voltage Swing”
on the LF411 spec sheet:
2. Submit the output waveform, and explain why it’s not so square.
+15V
-15V
Part 2 Comparator with no feedback
Figure 3
Build the same circuit (Figure 4) with the LF311 comparator; note that the
pinout is different from the LF411 opamp, and that it requires a 1k “pullup”
resistor at the output.
+15
8
+15
3
2
1k
-
311
+
7
out
1
1. Drive the circuit with a 100kHz sine wave, and submit the
output waveform. Compare the waveform, in shape and
amplitude, with that of Part1.
2. Drive the circuit with a low frequency sine wave until you see
output oscillations. Submit the output waveform.
10k
4
in
-15
Figure 4
Department of Physics, Stanford University
Physics 105, Analog Electronics (Pam)
Lab 8.12 DRAFT
Page 3
+15
Part 3 Positive Feedback: Comparator with Schmitt Trigger
10k
3
2
4.7k
-
311
+
7
out
1
Add positive feedback in the form of a Schmitt trigger to the 311 circuit as
in Figure 5. Drive with a low-frequency sine wave, and look at the output
and both inputs on the scope.
in
100k
1. Look at both inputs on the scope, and measure the two
thresholds. Compare with Prelab calculations.
2. Submit waveforms for input (-), reference (+), and output
pins.
Now connect the “ground” pin, Pin 1, to -15V
3. Repeat steps 1 and 2 above.
10k
Figure 5