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Lecture 10: Amplifiers and Comparators
Today, we will
 Learn how to design op-amp circuits to perform a task
 Piece together basic op-amp circuits and adjust
resistances
 D/A converter
 Investigate another digital application of the op-amp: the
comparator
 1-bit A/D converter
V0
V+
+
V

Designing Op-Amp Circuits
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You can design a new op-amp circuit by
connecting our basic op-amp circuits together
and selecting resistor values.
Even if there is an element (or another circuit)
attached to the output of an op-amp circuit, the
op-amp circuit behaves the same.
Break the desired task into smaller pieces which
are easily done with one op-amp circuit, then
connect the circuits together.
Example
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Design a circuit whose output is:
1
2
4
VOUT  V0  V1  V2
5
5
5
Number Representation
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A computer represents the number “1” (logic 1)
by some positive voltage; usually 3 V to 5 V.
The number “0” is represented by 0 V.
A number in the computer is stored in binary, or
base two representation.
Each binary digit (bit) is represented by a
voltage at a separate point.
V2
Memory Chip
V1
V0
3-bit memory
Number Representation
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In the circuit below, V0 could represent the
“ones” place, V1 the “twos” place, and V2 the
“fours” place.
One can have either a 1 or a 0 in each place.
So, the 3-bit memory can store numbers 0
through 7.
The number in each place is represented by a
voltage, 0 V for 0 and, say, 5 V for 1.
V2
Memory Chip
V1
V0
3-bit memory
Digital to Analog (D/A) Conversion
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The op-amp circuit that we just designed
converts the digital number representation in
our memory chip to an analog representation.
It takes a number currently represented by three
voltages with place values, and reinterprets the
number so that “1” is represented by 1 V, “6” is
represented by 6 V, etc. The circuit:
 Divides
each input voltage by 5 so each will have the
value 0 V or 1 V
 Multiplies by the place value that number represents
 Adds up the numbers
D/A Conversion
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Your stereo speaker has cones in it that vibrate
to make the sound. An analog voltage causes
the cones to vibrate.
The D/A converter helps translate digitally stored
music into an analog voltage for the speakers.
Digital music (CD, MP3) provides a number
indicating the sound amplitude at each sample
time. These numbers get translated into analog
voltage by the D/A converter.
The more bits used to store each sample, the
more audio levels represented (better quality)
A/D Conversion, Signal Degradation
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Naturally, we want to be able to go in the other direction as
well, and convert analog representation to digital.
This is useful not only in audio and data acquisition, but
within digital computation as well.
As a digital signal propagates, it is degraded by natural
resistance and capacitance in circuits.
Pretty soon, the signal is not only 0 or 1 most of the time, but
has in-between (nonsense) voltages too.
5V
5V
0V
0V
Always 0 or 1
Degraded Signal
Comparator
The degraded signal can be “cleaned up”, transformed into
a signal which is nearly always 0 or 1, using a comparator.
To make a comparator,
VIN
 We set the high rail on
the op-amp to the logic 1
voltage, and the low rail to
VTHR
logic 0 (0 V).
 We set the threshold voltage
VTHR to be around halfway
between logic 0 and logic 1.
+

V0
Using the Rails
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Note that in the linear region, VO = A (VIN – VTHR).
Since A is large, the amplifier will hit the top rail when VIN is
just a little above VTHR.
It will hit the low rail when VIN is just a little lower than VTHR.
Only a small range of VIN will leave it in the linear region.
The comparator “decides” whether VIN is logic 0 or logic 1.
VIN
+

V0
V0
Slope is A
VTHR
V  V
Example
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Suppose we have a comparator with:
 Logic
1 voltage = 5 V
 Logic 0 voltage = 0 V
 Threshold voltage = 2 V
 Ri = ∞, RO = 0 Ω, A = 1000
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For the input signal VIN(t) = 5 – e-2t V,
sketch VO(t) for t between 0 and 3 seconds.
2-Bit A/D Converter (with rounding)
V0
V1