Section 4: Circuit Components Canadian Amateur Radio Basic Qualification (B-004)

4.1 Amplifiers

Covers B-004-001-001 through B-004-001-011

Amplifiers are the workhorses of every radio -- they take feeble signals and make them strong enough to be useful. This section covers what amplifiers do, how their performance is measured, and what happens when they misbehave.

What Is an Amplifier?

An amplifier is a circuit designed to increase the amplitude of a signal. The amount of increase in signal level that an amplifier provides is called gain, and a device that has gain is said to have the property of amplification.

Amplifiers are classified by the type of signal they handle:

• RF amplifier -- amplifies radio-frequency signals. To increase the level of very weak radio signals from an antenna, you would use an RF amplifier.
• Audio amplifier -- amplifies audio-frequency signals. To increase the level of very weak signals from a microphone, you would use an audio amplifier.

Measuring Amplifier Performance

Voltage Gain

Apart from power and current, amplifiers can be specifically designed to increase voltage. A voltage amplifier takes a small input voltage and produces a proportionally larger output voltage.

Gain in Decibels (dB)

Gain is commonly expressed in decibels (dB). A device labelled "Gain = 10 dB" is an amplifier -- the label tells you it makes signals larger, not smaller.

Efficiency

Efficiency is the ratio of useful output power to the DC input power consumed. It tells you how effectively the amplifier converts DC supply power into useful signal output.

Speech Amplifier Frequency Range

The speech amplifier in an amateur radio transceiver is designed specifically for voice frequencies. It typically processes the range 300 Hz to 3,000 Hz (3 kHz), which covers the essential frequencies needed for human speech intelligibility. This is much narrower than full hi-fi audio (20 Hz -- 20 kHz) but is all that is needed for clear voice communication.

Linearity, Distortion, and Feedback

An amplifier should ideally be linear -- meaning the output is a faithful, larger copy of the input. When an amplifier becomes non-linear, the output signal becomes distorted. Distortion generates spurious signals that can interfere with other stations, and is especially problematic with SSB transmissions.

Feedback occurs when some of the output signal is routed back to the input:

• Negative feedback -- reduces gain but improves linearity and stability. This is usually desirable.
• Positive feedback -- increases gain but can lead to instability. Excessive positive feedback causes oscillations to appear -- the amplifier starts generating its own unwanted signal.

  Amplifier with Feedback:

  Input -->[+]---> AMPLIFIER ---+---> Output
             ^                  |
             |   Feedback       |
             +----Network <-----+

  Negative feedback (inverted): Stabilizes, reduces distortion
  Positive feedback (in-phase): Can cause oscillation!

4.2 Diodes

Covers B-004-002-001 through B-004-002-010

Diodes are the simplest semiconductor devices -- they let current flow in one direction and block it in the other. This single trick makes them essential for converting AC to DC, detecting radio signals, protecting circuits, regulating voltage, and even producing light.

What Is a Diode?

A semiconductor diode has two electrodes: the anode (positive side) and the cathode (negative side). Conventional current flows from anode to cathode, but remember that electrons (being negative) actually flow in the opposite direction: from cathode to anode.

  Diode Symbol:              Physical marking:

   Anode    Cathode            +---------------------+
     |     ^  |                |  #####              |
  ---|>|------                 |  Band = Cathode     |
     |  v  |                   +---------------------+
           |
  Current flows --->          The band marks the cathode (-)
  (conventional)

Forward and Reverse Bias

For a diode to conduct, it must be forward biased -- meaning the anode is made more positive than the cathode. When reverse biased (anode negative, cathode positive), virtually no current flows. This one-direction-only property is what makes diodes useful for reverse polarity protection: a diode placed in series with the positive power lead blocks current if the supply is connected backwards.

Rectification and Detection

Because a diode conducts in only one direction, it can convert AC (alternating current) to DC (direct current). This process is called rectification. When alternating current is applied to the anode of a diode, the output at the cathode is pulsating direct current -- the negative half-cycles are blocked, leaving only positive pulses.

  Half-Wave Rectification:

  AC Input:           /\    /\    /\
                     /  \  /  \  /  \
                  --/----\/----\/----\--
                         (sine wave)

                    +-->|--+
  After Diode:      |      |
                    /\    /\    /\
  (Pulsating DC)   /  \  /  \  /  \
                  -/----\-/----\-/----\- (zero line)
                   (negative halves removed)

Diodes are also used in radio receivers for detection (demodulation) -- recovering the information (audio) from a transmitted radio signal. The diode rectifies the RF signal, and filtering extracts the original audio.

Special Diode Types

Zener Diodes

The primary purpose of a Zener diode is to regulate or maintain a constant voltage. It is designed to operate in reverse breakdown at a specific, predictable voltage.

  Zener Diode Symbol:        Voltage Regulation:

   Anode    Cathode            Vin --+-- R --+-- Vout (regulated)
     |    /^  |                      |       |
  ---|>|/-----                       |    +--+--+
     |  v  |                         |    | DZ  | (Zener)
                                     |    +--+--+
  Note bent bar on cathode           v       v
  (distinguishes from regular)       GND     GND

LEDs (Light-Emitting Diodes)

A light-emitting diode (LED) is a semiconductor device that glows different colours depending on its chemical composition. When forward biased, it emits light instead of just heat.

4.3 Bipolar Transistors (BJT)

Covers B-004-003-001 through B-004-003-011

The bipolar junction transistor (BJT) is the most basic semiconductor amplifying device. A tiny current at its input controls a much larger current at its output -- making it the foundation of amplification, switching, and signal processing in radio circuits.

What Is a Bipolar Transistor?

A bipolar transistor (BJT) is the most basic semiconductor component used to amplify signals. It can amplify small signals using low voltages. A semiconductor device labelled as a "general purpose audio NPN device" is a bipolar transistor. The most basic semiconductor component used to amplify is simply called a transistor.

When a low-level signal is placed at the input and a higher level signal is produced at the output, this effect is called amplification.

The Three Electrodes

A bipolar transistor has three electrodes: Collector, Emitter, and Base.

• Base -- the control electrode. A small base current controls the much larger collector current. When the BJT is used as a switch, the base controls its state.
• Collector -- collects the main current flow. Connected to the load.
• Emitter -- emits charge carriers into the base-collector junction.

NPN and PNP Types

There are two basic types of bipolar transistors: NPN and PNP. They differ in the polarity of their power supply connections. You cannot directly substitute a PNP transistor for an NPN transistor because the polarities are reversed -- an NPN needs positive voltage on the collector, while a PNP needs negative voltage.

  NPN Transistor:              PNP Transistor:

       Collector (C)                Collector (C)
          |                            |
          |                            |
  Base ---|                    Base ---|
  (B)     |>  (arrow             (B)   |<  (arrow
          |    points                  |    points
          |    OUT)                    |    IN)
          |                            |
       Emitter (E)                  Emitter (E)

  Mnemonic:                    Mnemonic:
  NPN = "Not Pointing iN"     PNP = "Points iN Permanently"
  (arrow points away           (arrow points toward
   from base)                   the base)

Switching: Saturation and Cutoff

A transistor can also be used as a switch. It alternates between two states:

• Saturation -- transistor fully ON; maximum current flows (like a closed switch).
• Cutoff -- transistor fully OFF; no current flows (like an open switch).

When a transistor is alternately driven between saturation and cutoff, it behaves like a switch.

Failure from Excessive Heat

The operating condition most likely to cause a transistor to fail is excessive heat. Heat increases leakage current, which creates more heat -- a destructive cycle called thermal runaway. Always ensure adequate heat sinking for power transistors.

4.4 Field-Effect Transistors (FET)

Covers B-004-004-001 through B-004-004-011

Field-effect transistors do the same job as BJTs -- amplifying and switching -- but they work in a fundamentally different way. Instead of being controlled by current, they are controlled by voltage, giving them extremely high input impedance.

What Is a FET?

A field-effect transistor (FET) is a semiconductor device with three electrodes: a gate, a drain, and a source. Unlike a BJT, which is current-controlled, a FET is voltage-controlled. The FET is considered a high impedance device because the gate never conducts current -- it uses high resistance semiconductors. The input impedance can be millions of ohms.

The Three Electrodes

• Gate -- the control electrode. A voltage on the gate controls the resistance of the channel. The gate never conducts current, which is why FETs have high impedance.
• Source -- where charge carriers enter the channel.
• Drain -- where charge carriers leave the channel.

The source and drain are connected to the ends of the channel.

  N-Channel JFET Symbol:        P-Channel JFET Symbol:

       Drain (D)                      Drain (D)
          |                              |
          |                              |
  Gate ---|                      Gate ---|
  (G)   <-|                     (G)   ->|
          |                              |
          |                              |
       Source (S)                      Source (S)

  Arrow points IN               Arrow points OUT
  (toward channel)               (away from channel)

N-Channel and P-Channel Types

When considering the material between source and drain, the two basic types of field-effect transistors are N-channel and P-channel. These correspond to NPN and PNP in BJT terminology.

Bias and Current Control

In a FET, the gate voltage controls how wide the conductive channel is. Decreasing the reverse bias on the gate widens the channel, allowing more current to flow. So the circuit parameter change that causes current to increase is: the reverse bias is decreased.

BJT-to-FET Electrode Correspondence

This is a critical exam topic. The electrodes of BJTs and FETs have direct functional equivalents:

4.5 Vacuum Tubes

Covers B-004-005-001 through B-004-005-009

Vacuum tubes were the original amplifying devices -- predating transistors by decades. While largely replaced by semiconductors, tubes are still used in high-power amateur radio amplifiers. Understanding their electrodes is important because the exam tests the correspondence between tubes, FETs, and BJTs.

What Is a Vacuum Tube?

Inside the envelope of a triode tube is a vacuum. Electrons are emitted from a heated cathode, controlled by a grid, and collected by a plate.

The Triode

A vacuum tube with a cathode, a single grid, and a plate is called a triode (tri = three electrodes). Its three electrodes each have a specific role:

• Cathode -- the heated element that emits electrons (electron source).
• Grid -- the control element. Varying the voltage on the grid controls electron flow.
• Plate (anode) -- collects the electrons. Operated at the highest positive voltage.

The output current flows through the cathode and plate.

  Triode Vacuum Tube Symbol:

             Plate (P)
               |
               |
       +-------+-------+
       |   - - - - -   |   Grid (G) --- (dashed line)
       |               |
       |   ^ ^ ^ ^ ^   |   Cathode (K) --- (with heater below)
       +-------+-------+
               |
           +---+---+
           |Heater |         Electron flow: Cathode --> Plate
           +-------+         (electrons boil off heated cathode)

  Plate    = highest positive voltage (attracts electrons)
  Grid     = control element (small voltage controls large current)
  Cathode  = emits electrons when heated

Tube vs. Transistor Comparison

The semiconductor device most similar to a triode vacuum tube is the field-effect transistor, because both are voltage-controlled devices. A feature common to both triode tubes and transistors is that both can amplify signals -- this is their primary purpose.

Tubes are still used in some amateur radio applications because they can handle higher power than many transistors. High-power amplifiers (1 kW+) often use vacuum tubes.

4.6 Resistor Colour Code

Covers B-004-006-001 through B-004-006-011

Resistors are too small for printed labels, so they use coloured bands to encode their value and tolerance. Learning to read these bands is a practical skill that the exam tests in several ways.

Reading a 4-Band Resistor

On a 4-band resistor, the first three colour bands indicate the value of the resistor in ohms, and the fourth band specifies the tolerance (how far the actual value might deviate from the marked value). Here is how each band works:

Colour Code Table

Visual Colour Band Example

  Example: Red, Violet, Yellow, Gold

  +------------------------------------------------+
  |  ||       ||       ||                     ||    |
  |  || RED   || VIOL  || YEL                 ||GOLD|
  |  || (2)   || (7)   || (x10,000)           ||(5%)|
  |  ||       ||       ||                     ||    |
  +------------------------------------------------+
   Body colour (tan/beige)        Gap before tolerance band

  Reading:
  Band 1 (Red)    = 2            (first digit)
  Band 2 (Violet) = 7            (second digit)
  Band 3 (Yellow) = x 10,000     (multiplier)
  Band 4 (Gold)   = +/- 5%       (tolerance)

  Value = 27 x 10,000 = 270,000 ohms = 270 kilohms +/- 5%

Which Band Differentiates Similar Values?

Consider two resistors: 33 ohms and 39 ohms. Both start with "3" (orange) as the first band. The second band differentiates them: 33 has orange-orange, while 39 has orange-white.

Now consider 120 ohms vs. 1200 ohms. Both start with brown-red (12). The third band (multiplier) differentiates them: 120 = brown-red-brown (×10), while 1200 = brown-red-red (×100).

Tolerance and Precision

The tolerance tells you the range of actual values a resistor might have. A 100-ohm resistor with 10% tolerance can be anywhere from 90 ohms to 110 ohms:

Lower tolerance = higher precision. A 0.1% resistor is far more precise than a 20% resistor. Among the common tolerances (0.1%, 5%, 10%, 20%), 0.1% is the highest precision and 20% is the lowest precision.

Temperature Effects

Resistance changes with temperature. How much it changes depends on the resistor's temperature coefficient, which is specified as a given fraction per degree Celsius. A resistor rating specified this way is called the temperature coefficient.

Complete Component Correspondence Table

This is one of the most heavily tested topics in the B-004 section. The exam asks you to map electrodes across all three amplifying device families -- in any direction.

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