431/531 Class Notes 10
7 Comparator Circuits
7.1 Simple Comparator
A comparator can be thought of as a fast, high-gain op-amp which is not used with negative
feedback. This basic idea is shown in Fig. 38. The comparator has large open-loop gain
A
.
The function of a comparator is to decide which of the two inputs has larger voltage. We
have in the limit of very large
A
v
out
=
A
(
v
+
,
v
,
)=
(
+
V
max
v
+
>v
,
,j
V
min
j
v
+
<v
,
where
V
max
and
V
min
are aprroximately the power supply voltages. Therefore, the comparator
converts an analog input signal into an output with two possible states. Hence, this can be
thought of as a 1-bit analog to digital converter (A/D or ADC). The comparator circuit
does not use negative feedback, and so purp osefully violates Golden Rule 1. In fact, as we
shall see below, comparator circuits often employ
positive
feedback to ensure that nothing
intermediate between the two extreme output states is utilized. Finally, without negative
feedback, there is no need to do compensation Thus there is more gain at high frequency,
meaning faster response. Also, the amplier can be optimized for sp eed at the expense of
linearity. Comparators, like op-amps, are readily available as integrated circuit chips, such
as the model 311 (LM311 or LF311) whichwehaveinlab. Table 9.3 (pages 584-5) of the
text lists some of the possibilities on the market.
vout
A
+
-
R
Figure 38: Comparator model.
Wehaveshown explicitly in Fig. 38 the output stage consisting of a transistor with
collector connected to the comparator output. This is the
open collector
output, and is
typical. It is used in the 311 comparators we use in lab. We are obliged to complete
the circuit byproviding a \pull-up" resistor
R
. The transistor emitter is also available as
an external connection. It should be connected to whatever is the lower of the two output
voltage states we require. This is chosen to be ground in the gure. The high-gain dierential
amplier of the comparator has output connected to the base of this transistor. When that
44
Docsity.com
