3. Analog Radio Circuits on The HamStudy Amateur Extra Class HamBook (2024 - 2028)

3.1 Filters and Matching Networks

Mon, 01 Jan 0001 00:00:00 +0000

Section 3.1: Filters and Matching Networks #

A transmitter’s signal often passes through two networks on its way to the antenna. A low-pass filter keeps harmonics off the air, and a matching network, such as an antenna tuner, gives the transmitter the load it was designed for. Both rely on inductors and capacitors, and one network can often do both jobs.

Matching While Filtering #

A transmitter usually expects a 50 Ω resistance, but an antenna system may present something like $25+j30\ \Omega$ . That load has two problems: 30 Ω of reactance that shouldn’t be there, and 25 Ω of resistance where 50 Ω belongs.

3.2 Amplifier Circuits

Mon, 01 Jan 0001 00:00:00 +0000

Section 3.2: Amplifier Circuits #

An amplifier’s class describes how much of each signal cycle its device conducts. Class A conducts for all of it and is the most linear; Class C conducts for less than half, trading linearity for efficiency. That one choice decides how clean the output is and how hot the heat sink runs. The class comes from the circuit around the device, especially its bias.

3.3 Operational Amplifiers

Mon, 01 Jan 0001 00:00:00 +0000

Section 3.3: Operational Amplifiers #

In the previous section, R3 used negative feedback to hold one transistor’s bias steady. An operational amplifier, or op-amp, puts feedback at the center of the design. On its own, the chip has far more gain than most circuits can use, often 100,000 or more. A few external parts in a feedback path set the working gain instead.

Amplifying a Difference #

An op-amp’s triangle symbol shows two inputs. A positive change at the non-inverting (+) input drives the output positive. A positive change at the inverting (−) input drives the output negative. The amplifier responds to the voltage difference between those inputs. Its stages connect directly, without coupling capacitors, so it can amplify DC as well as changing signals. Its inputs draw almost no current from the signal source, and its output holds its voltage while supplying load current within its rating.

3.4 Regulated Power Supplies

Mon, 01 Jan 0001 00:00:00 +0000

Section 3.4: Regulated Power Supplies #

Run an SSB transmitter from an unregulated supply, and the supply’s output sags on every voice peak, then climbs back when you pause. Filtering smooths the ripple, but a regulator holds the output steady as the input voltage and load current change.

Continuous Control #

Holding an output steady takes a fixed voltage to compare it with and a part that can correct it. The Zener diode from Chapter 1 holds a nearly constant voltage as its current varies. A transistor, conducting more or less as needed, can make the correction.

3.5 Oscillators and Phase Locking

Mon, 01 Jan 0001 00:00:00 +0000

Section 3.5: Oscillators and Phase Locking #

An amplifier that oscillates by accident needs a suppressor. One that oscillates on purpose is an oscillator: an amplifier and a filter in a feedback loop, with the filter picking the frequency. How steady that frequency stays depends mostly on the filter’s resonator. Nearby circuits, vibration, and heat can all pull the frequency.

Quartz Resonance #

Many oscillators use a quartz crystal as their resonator. Quartz is piezoelectric, a property that links its mechanical vibration to the electrical signal.