Service manual

STP 11-25R13-SM-TG
Q - 41
Figure Q-41. Voltage Limiter
j. The differential comparator (Figure Q-42) is used to compare two voltage sources and produce
a difference output.
Figure Q-42. Differential Comparator
Q-10. Special Integrated Circuits (ICs).
In older equipment, a special circuit might have taken up an entire circuit board. Now these special
circuits are housed within one IC chip. It is important to know the functions of these chips in order to
know how the external components help perform these functions. We will discuss only three of the
special ICs: memory, monostable multivibrators, and oscillators.
a. Memory. Most digital circuits require the storage of data in one way or another. Some means
of storing this data must be accomplished. There are basically two classes of memory, large scale and
small scale. Large-scale memory devices are tape, disc, etc. This material will cover only the small-
scale class of memory.
(1) In small-scale memory, there are several common devices. The flip-flop (FF) is the most
common device with memory capabilities. When large amounts of data need storage space, FFs
become too cumbersome to work with.
(2) When this happens, a device such as random access memory (RAM) can be used. This is
usually a one-chip device that can store thousands of bits of data. The RAM has the capability of
temporary storage of data. Information can be put in and read out of any location in a random order
according to the use and need of the stored information.
(3) Another type of small-scale memory is the read only memory (ROM). This type of device is
different from the RAM in the fact that the data stored is permanent. In RAM, data can be changed very
easily, while in ROM, the data is either factory-written into the circuits of the ROM or can be programmed
by the user. ROM data can only be read in normal use.
b. Monostables and Oscillators. A very common type of integrated circuit is the multivibrator (MV)
and oscillator-type IC. These circuits are usually set up to operate with the addition of external timing
components. So as is the case of a monostable multivibrator (MSMV), the same chip may be used to
give different results with the use of different external components. Oscillator chips normally require the
addition of an external crystal to establish the running frequency. Some have internal circuits for
connecting to automatic frequency control (AFC) circuitry.