KB6NU's Ham Radio Blog
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2020 Extra Class study guide: E7B - Amplifiers class of operation; vacuum
tube and solid-state circuits; distortion and intermodulation; spurious and
parasitic suppression; microwave amplifiers; switching-type amplifiers
Posted: 18 Feb 2020 06:34 AM PST
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There are several classes of amplifiers, based on their mode of operation.
In a class A amplifier, the transistor is always conducting current.
Because the transistor is always conducting current, the bias of a Class A
common emitter amplifier would normally be set approximately halfway
between saturation and cutoff on the load line.
Where on the load line of a Class A common emitter amplifier would bias
normally be set? (E7B04)
ANSWER: Approximately halfway between saturation and cutoff
In a class B amplifier, there are normally two transistors operating in a
“push-pull” configuration. One transistor turns on during the positive half
of a cycle, while the other turns on during the negative half. One
advantage of using push-pull amplifiers is that they reduce even-order
harmonics.
QUESTION: Which of the following amplifier types reduces even-order
harmonics? (E7B06)
ANSWER: Push-pull
A Class AB amplifier operates over more than 180 degrees but less than 360
degrees of a signal cycle. Class B and Class AB amplifiers are more
efficient than Class A amplifiers.
QUESTION: For what portion of the signal cycle does each active element in
a push-pull Class AB amplifier conduct? (E7B01)
ANSWER: More than 180 degrees but less than 360 degrees
Class C amplifiers conduct over less than 180 degrees of the input signal.
This type of operation distorts the output signal, but it is very
efficient. Up to 90% efficiency is possible.
A Class D amplifier uses switching technology to achieve high efficiency.
They are very efficient because the power transistor is at or near
saturation most of time. To remove switching signal components, the output
of a class D amplifier circuit has a low-pass filter.
QUESTION: What is a Class D amplifier? (E7B02)
ANSWER: A type of amplifier that uses switching technology to achieve high
efficiency
QUESTION: Why are switching amplifiers more efficient than linear
amplifiers? (E7B14)
ANSWER: The power transistor is at saturation or cutoff most of the time
QUESTION: Which of the following components form the output of a class D
amplifier circuit? (E7B03)
ANSWER: A low-pass filter to remove switching signal components
Amplifiers are used in many different applications, and in most signal
quality is very important. Poorly-designed RF power amplifiers, for
example, may emit harmonics or spurious signals, that may cause harmful
interference.
One thing that can be done to prevent unwanted oscillations—that can
generate harmonics or spurious emissions—in an RF power amplifier is to
install parasitic suppressors or neutralize the stage. To neutralize an RF
power amplifier, you feed a portion of the output back to the input, but
shift the phase 180 degrees. Doing this will eliminate the effects of any
parasitic capacitance or inductance that might cause an amplifier to
oscillate.
What can be done to prevent unwanted oscillations in an RF power amplifier?
(E7B05)
ANSWER: Install parasitic suppressors and/or neutralize the stage
How can an RF power amplifier be neutralized? (E7B08)
ANSWER: By feeding a 180-degree out-of-phase portion of the output back to
the input
In order to preserve signal integrity, amplifiers used as the final
amplifier in an amateur radio transceiver, or as an external amplifier, are
Class A or Class AB linear amplifiers. The use of non-linear Class C
amplifiers is not a good choice. The reason for this that a Class C
amplifier used to amplify a single-sideband phone signal can cause signal
distortion and excessive bandwidth.
Which of the following is a likely result when a Class C amplifier is used
to amplify a single-sideband phone signal? (E7B07)
ANSWER: Signal distortion and excessive bandwidth
Although transistorized linear amplifiers are becoming more common, many
high-power amplifiers still use vacuum tubes. These amplifiers require that
the operator tune the output circuit, which is typically a Pi-network
output circuit. To tune this type of output circuit, you adjust the tuning
capacitor for minimum plate current and the loading capacitor for maximum
permissible plate current.
QUESTION: Which of the following describes how the loading and tuning
capacitors are to be adjusted when tuning a vacuum tube RF power amplifier
that employs a Pi-network output circuit? (E7B09)
ANSWER: The tuning capacitor is adjusted for minimum plate current, and the
loading capacitor is adjusted for maximum permissible plate current
The type of circuit shown in Figure E7-1 is a common emitter amplifier. R1
and R2 bias the transistor. This type of bias is called voltage divider
bias. R3 also sets the bias voltages for the transistor. The type of bias
provided by R3 is called self bias.
QUESTION: What type of amplifier circuit is shown in Figure E7-1? (E7B12)
ANSWER: Common emitter
QUESTION: In Figure E7-1, what is the purpose of R1 and R2? (E7B10)
ANSWER: Voltage divider bias
QUESTION: In Figure E7-1, what is the purpose of R3? (E7B11)
ANSWER: Self bias
Another type of amplifier circuit is the emitter follower, or common
collector amplifier. These types of amplifiers are often used as buffer
amplifiers because they have low-impedance outputs that closely follow the
base input voltage.
QUESTION: Which of the following describes an emitter follower (or common
collector) amplifier? (E7B13)
ANSWER: An amplifier with a low impedance output that follows the base
input voltage
Thermal runaway is one problem that can occur if a transistor amplifier is
not designed correctly. What happens is that when the ambient temperature
increases, the leakage current of the transistor increases, causing an
increase in the collector-to-emitter current. This increases the power
dissipation, further increasing the junction temperature, which increases
yet again the leakage current. One way to prevent thermal runaway in a
bipolar transistor amplifier is to use a resistor in series with the
emitter. This resistor keeps the collector-to-emitter current under control.
QUESTION: What is one way to prevent thermal runaway in a bipolar
transistor amplifier? (E7B15)
ANSWER: Use a resistor in series with the emitter
RF power amplifiers often generate unwanted signals via a process called
intermodulation. Strong signals external to the transmitter combine with
the signal being generated. These intermodulation products can cause the
transmitter to output spurious signals. Odd-order, rather than even-order,
intermodulation distortion products are of concern in linear power
amplifiers because they are relatively close in frequency to the desired
signal.
QUESTION: What is the effect of intermodulation products in a linear power
amplifier? (E7B16)
ANSWER: Transmission of spurious signals
QUESTION: Why are odd-order rather than even-order intermodulation
distortion products of concern in linear power amplifiers? (E7B17)
ANSWER: Because they are relatively close in frequency to the desired signal
One type of amplifier that is often used as a power amplifier is the
grounded-grid amplifier. Grounded-grid amplifiers are relatively easy to
build, are very stable in operation, and have a low input impedance. This
is a useful characteristic because the output impedance of most amateur
radio transmitters, which are used to drive these amplifiers, is 50 ohms.
QUESTION: What is a characteristic of a grounded-grid amplifier? (E7B18)
ANSWER: Low input impedance
The post 2020 Extra Class study guide: E7B – Amplifiers class of operation;
vacuum tube and solid-state circuits; distortion and intermodulation;
spurious and parasitic suppression; microwave amplifiers; switching-type
amplifiers appeared first on KB6NUs Ham Radio Blog.
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2020 Extra Class study guide: E7A - Digital circuits: digital circuit
principles and logic circuits; classes of logic elements; positive and
negative logic; frequency dividers; truth tables
Posted: 17 Feb 2020 12:39 PM PST
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Digital circuits are used for a variety of functions in modern amateur
radio equipment. Unlike analog circuits, the output voltage of an ideal
digital circuit can only be one of two values. One of these
voltages—normally a positive voltage—represents a digital 1. The other
value—normally near 0 V—represents a digital 0. This type of logic is
called positive logic. On the other hand, some digital logic systems use a
low voltage to represent a digital 1 and a high voltage to represent a
digital 0. This type of logic is called negative logic.
QUESTION: What type of logic defines 1 as a high voltage? (E7A11)
ANSWER: Positive Logic
The microcomputers that control today’s transceivers are very complex
digital circuits. These complex digital circuits are made by combining many
smaller building blocks called logic gates. These gates perform basic
digital logic functions.
Table E7-1. This two-input NAND truth table show the output (Q) for each
combination of inputs (A,B).
One of the most basic digital circuits is the NAND gate. The output of a
NAND gate is a logic 0 when all of its inputs are a logic 1. Truth tables
describe how logic gates work. That is to say that they show what the logic
gate output is for each combination of inputs. Table E7-1 shows a truth
table that describes the operation of a two-input NAND gate. A and B are
the two inputs; Q is the output.
QUESTION: What logical operation does a NAND gate perform? (E7A07)
ANSWER: It produces logic 0 at its output only when all inputs are logic 1
QUESTION: What is a truth table? (E7A10)
ANSWER: A list of inputs and corresponding outputs for a digital device
Table E7-2. Two-input OR gate.
Other types of gates perform different logical functions. OR gates, for
example, output a logic 1 if any or all of its inputs are a logic 1. Table
E7-2 shows a truth table that describes the logical operation of a
two-input OR gate.
QUESTION: What logical operation does an OR gate perform? (E7A08)
ANSWER: It produces logic 1 at its output if any or all inputs are logic 1
Table E7-3. Two input XNOR.
A NOR gate produces exactly the opposite output of an OR gate. That is to
say that the output is a logic 0 if any or all of the inputs are a logic 1.
An exclusive NOR (XNOR) gate is similar to the NOR gate, except that its
output is a logic 0 when its inputs are not all the same value. If all the
inputs are a logic 1 or a logic 0, the XNOR gate will output a logic 1 as
shown in Table E7-3, which is a truth table for a two-input XNOR gate.
QUESTION: What logical operation is performed by an exclusive NOR gate?
(E7A09)
ANSWER: It produces logic 0 at its output if only one input is logic 1
Flip-flops are circuits that are made from combinations of logic gates. By
“latching” the state of an input at a particular time, a flip-flop can be
said to have memory. A D flip-flop, and its truth table is shown in the
figure below.
As shown, the output changes only on the rising edge of the clock (CLK)
signal. That is to say, when the signal goes from 0 to 1. If D = 1, Q = 1.
If D = 0, then Q = 0. The other output, denoted by a bar over the Q, is the
inverse of Q.
When a D flip-flop is connected as shown in the figure—with the inverted
output connected to the D input—a flip-flop can divide the frequency of a
pulse train by 2. You can connect the Q output to a second flip-flop to
divide the frequency even further. Consequently, 2 flip-flops are required
to divide a signal frequency by 4. By connecting a number of flip-flops
together, and resetting the circuit once ten pulses have been input, you
can build a decade counter.
QUESTION: Which of the following can divide the frequency of a pulse train
by 2? (E7A03)
ANSWER: A flip-flop
QUESTION: How many flip-flops are required to divide a signal frequency by
4? (E7A04)
ANSWER: 2
QUESTION: What is the function of a decade counter? (E7A02)
ANSWER: It produces one output pulse for every 10 input pulses
A flip-flop is a bistable circuit. That means its output is stable in
either state—1 or 0. Some circuits—called monostable circuits—are stable in
only one state, but not the other. They will switch from one state to the
other, but then return to the original state. One such monostable circuit
is the monostable multivibrator. A trigger pulse causes the monostable
vibrator to switch from a 1 to a 0 (or vice versa), but after a set time,
the output of the monostable vibrator will return to its original state. An
astable multivibrator is not stable in either state. Its output
continuously alternates between two states without an external clock. In
other words, it is an oscillator.
QUESTION: Which circuit is bistable? (E7A01)
ANSWER: A flip-flop
QUESTION: What is a characteristic of a monostable multivibrator? (E7A06)
ANSWER: It switches momentarily to the opposite binary state and then
returns to its original state after a set time
QUESTION: Which of the following is a circuit that continuously alternates
between two states without an external clock? (E7A05)
ANSWER: Astable multivibrator
The post 2020 Extra Class study guide: E7A Digital circuits: digital
circuit principles and logic circuits; classes of logic elements; positive
and negative logic; frequency dividers; truth tables appeared first on
KB6NUs Ham Radio Blog.
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Operating notes: Slowing down nets QSOs, 80m afternoon
propagation, "adelante"
Posted: 17 Feb 2020 12:18 PM PST
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When I call CQ, I generally set my CW speed between 21 and 25 wpm. If I
dont get a bite at that speed, I start cranking it down. Often, at a slower
speed, Ill finally get a reply.*Im always surprised at this, as I will
gladly slow down for anyone answering my CQ. So, if you hear me calling, no
matter what speed Im sending at, please give me a call at what ever speed
youre comfortable with. I want to talk to you more than I want to have a
high-speed QSO.
On a related note, Ill often find someone calling CQ very close to my
frequency, but at a slower speed. This happened to me just the other night.
I cranked down my speed and replied to them, and we had a nice QSO. I wish
they had just called me in the first place.
Late-morning 80m propagation
Were often taught that 80m is not an appropriate band to use during the
day. There are even two questions in the General Class question pool that
cover this:
QUESTION: Which ionospheric layer is the most absorbent of long skip
signals during daylight hours on frequencies below 10 MHz? (G3C11)
ANSWER: The D layer
QUESTION: Why is long-distance communication on the 40-meter, 60-meter,
80-meter, and 160-meter bands more difficult during the day? (G3C05)
ANSWER: The D layer absorbs signals at these frequencies during daylight
hour
Well, a couple of days ago, I wandered down to the shack late in the
morning, and turned on the radio. The radio was set to 80m, as I had been
operating that band the night before. Lo and behold, W9KY was calling CQ on
the exact frequency. I answered him, and we had a nice QSO.
It wasnt much DX—QRZ.Com says W9KY is 217 miles away—but I was surprised
that we were able to have such a nice QSO during the late morning hours. I
guess that from now on, Ill have a look around 80m should my radio come up
on that band after turning it on, no matter what time of day it is.
¡Adelante!
This belongs in the you learn something new every day file. The other night
I was listening to phone QSOs on 40m, and for some reason, there seemed to
be a lot of Spanish-languages conversations going on. Since I know a little
Spanish, I decided to listen in on one of them.
At the end of their transmissions, the operators would say, ¡Adelante! In
my mind, that always meant something like, Come on. Lets go! or something
similar. One of the definitions on Merriam Websters Spanish Central website
is, come in! So, I guess this makes sense.
¡Adelante OMs!
The post Operating notes: Slowing down nets QSOs, 80m afternoon
propagation, adelante appeared first on KB6NUs Ham Radio Blog.