Model 8640B TM 9-4935-601-14-7&P
8-64. If the counter has a count enable (CE) input, it
must be held high for successive T inputs to cause the
counter to increment (or count). When the counter
reaches the nine count, a high at CE causes a terminal-
count or carry (in this case, a high) to appear at the
carry (TC) output.
8-65. A low on the clear (CLR) input clears all outputs
to a low independent of any other input conditions.
8-66. Programmable Up/Down Counter. The
programmable up/down counter, shown in Figure 8-13,
operates similarly to the programmable counter (which
could be called a programmable up counter). The
up/down counter has two trigger or clock inputs, count
up (CU) and count. down (CD). A low-to-high transition
of either count input (while the other count input is held
high) increments the count by one. If both CU and CD
are high, the count does not increment.
8-67. The counter's outputs (Q
A
, Q
B
, Q
C
, and Q
D
) can
be set to any count from zero to fifteen by entering the
count at the data inputs (D
A
, D
B
, D
C
, and D
D
) while the
load input (LD) is held low. Then the count can be
incremented up or down by activating either the CU or
CD input.
8-68. The borrow (BRW) output is low whenever the Q
outputs are at BCD zero (0000). The carry (CRY) output
is low whenever the Q outputs are at BCD nine (1001).
The master clear input (CLR) overrides all other input
conditions and forces the Q outputs to BCD zero.
8-69. Linear Integrated Circuits
8-70. Operational Amplifier. Figure 8-14 shows a
typical operational amplifier. Circuit A is a noninverting
buffer amplifier with a gain of 1. Circuit B is a non-
inverting amplifier with gain determined by the
impedance of R1 and R2. Circuit C is an inverting
amplifier with gain determined by R2 and R1. Circuit D
shows typical circuit connections and parameters. It is
assumed that the amplifier has high gain, low output
impedance, and high input impedance.
8-71. An operational amplifier can be characterized as
an ideal voltage amplifier having low output impedance,
high input impedance, and very high gain. Also the
output voltage is proportional to the difference in the
voltages applied to the two input terminals. In use, the
amplifier output drives the input voltage difference close
to zero through a negative feedback path.
8-72. When troubleshooting an operational amplifier,
measure the voltages at the two inputs with no signal
applied; the difference between these voltages should
be less than 10 mV. A difference voltage much greater
than 10 mV indicates trouble in the amplifier or its
external circuitry. Usually this difference will be several
volts and one of the inputs will be very close to an
applied circuit operating voltage (for example, +20 V, -
12 V).
8-73. Next, check the amplifier's output voltage. It will
probably also be close to one of the applied circuit
potentials: ground, +20 V, -12 V, etc. Check to see that
the output conforms to the inputs. For example, if the
inverting input is positive, the output should be negative;
if the non-inverting input is positive, the output should
be positive. If the output conforms to the inputs, check
the amplifier's external circuitry. If the amplifier's output
does not conform to its inputs, it is probably defective.
8-74. Comparator. Comparators are used as sense
amplifiers, pulse height discriminators, and voltage
comparators. A voltage reference is connected to one
of the amplifier's inputs as shown in Figure 8-15. When
the input signal voltage crosses the reference, the
output goes positive; the output remains positive until
the signal re-crosses the reference.
8-11
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