E1 · Published 1 September 2026 · By The Ham Desk

Mixers, Amplifier Classes and Oscillator Quality

Extra class circuit questions are mostly about consequences: what a mixer does to every signal it sees, why an amplifier class is chosen for a mode rather than for a power level, and what makes one oscillator usable in a receiver and another not.

Band 20 m
Study time 19
Level Core
questions 10

Waveform and antenna diagram for Advanced Circuits and Devices — Advanced Circuits and Devices — waveform overlay and radiation pattern

Pre-flight briefing

The mixer multiplies, and multiplication has side effects

A mixer multiplies two signals and produces the sum and the difference of their frequencies. With a local oscillator at 23.2 MHz and an incoming signal at 14.2 MHz the outputs are 37.4 and 9.0 MHz; the 9 MHz difference becomes the intermediate frequency. Because the arithmetic works both ways, a signal at 32.2 MHz also lands on 9 MHz - the image frequency, which is twice the intermediate frequency away from the wanted signal. That is why a high first IF helps: the further the image sits from the wanted signal, the easier the front-end filter can remove it before it ever reaches the mixer.

A mixer also generates harmonics and intermodulation products, so a strong nearby signal can produce a response on a frequency where nothing is transmitting. Filtering before the mixer, and choosing mixer levels that are not driven into compression, is the standard defence.

Amplifier classes are about the mode

Class A conducts for the full 360 degrees of the cycle, is the most linear and the least efficient - a theoretical ceiling of 50 per cent and in practice 25 to 30 per cent. Class AB conducts for more than half the cycle and is the usual choice for a linear SSB amplifier at roughly 50 to 60 per cent efficiency. Class C conducts for a fraction of the cycle, is highly efficient - 70 to 80 per cent is achievable - and is grossly non-linear, which is fine for CW and FM where the envelope carries no information but fatal for SSB and AM.

Negative feedback trades gain for everything else a transmitter wants: flatter response, wider bandwidth, lower distortion and a more predictable output impedance. Its mirror image, positive feedback, is how an oscillator is made - enough in-phase feedback and the amplifier never stops.

Oscillator quality and heat

Frequency stability is the whole point of an oscillator: drift with temperature, supply voltage and load is what makes a receiver wander. A quartz crystal wins because its Q is on the order of tens of thousands, so the resonant circuit is narrow and stubborn. Phase noise is the frequency-domain view of short-term instability, and it matters because it lets strong nearby signals mix with the local oscillator and dump noise onto the frequency you are trying to hear.

Every one of these stages turns wasted power into heat, and the junction temperature of a transistor - not the case temperature - is what the datasheet limits. A heat sink lowers the thermal resistance from junction to ambient, which is why the same device can be run harder bolted to a large sink than standing free.

Terms you must be able to define
Term	What the examiner wants to hear
Mixer	A non-linear device that multiplies two inputs and outputs their sum and difference frequencies.
Image frequency	The frequency on the other side of the local oscillator, an IF away from the wanted signal, which also converts to the IF.
Class C	An amplifier conducting for less than half a cycle: efficient and non-linear, used for CW and FM.
Q	Quality factor: how narrow and selective a resonant circuit is; crystal Q runs to tens of thousands.
Phase noise	Short-term random frequency instability, seen as noise spreading either side of an oscillator's carrier.

Question deck 10 questions

E1A01 Recall

What does a mixer produce from two input frequencies?

E1A02 Core

A receiver has a 9 MHz intermediate frequency and is tuned to 14.2 MHz. What is the image frequency?

E1A03 Core

Why does a high first intermediate frequency improve image rejection?

E1A04 Core

Which amplifier class is normally used for a linear SSB amplifier?

E1A05 Core

Why is a Class C amplifier unsuitable for SSB?

E1A06 Core

What is the general effect of applying negative feedback to an RF amplifier?

E1A07 Core

Why is a quartz crystal preferred as the frequency-determining element in a stable oscillator?

E1A08 Stretch

What is phase noise in an oscillator, and why does it matter in a receiver?

E1A09 Stretch

What are the two jobs of the pi-network tank circuit at the output of a tube or transistor final amplifier?

E1A10 Stretch

What temperature limit actually constrains how hard a transistor can be driven?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

E1A01 Explanation 1

Key A — Their sum and their difference

A mixer is a multiplier, and multiplying two sine waves produces components at the sum and the difference. Either output can be selected as the intermediate frequency; the other is filtered out.

Rule Fundamentals - mixer operation

E1A02 Explanation 2

Key D — 32.2 MHz

The local oscillator sits at 14.2 + 9 = 23.2 MHz, and a signal at 23.2 + 9 = 32.2 MHz also converts to 9 MHz. In general the image is twice the IF away from the wanted signal.

Rule Fundamentals - image response

E1A03 Explanation 3

Key C — The image frequency is further from the wanted signal, so the front-end filter attenuates it more

Image separation equals twice the IF. If the first IF is high, the image falls well outside the front-end passband and is attenuated before it reaches the mixer; low first IFs need more careful filtering.

Rule Fundamentals - receiver architecture

E1A04 Explanation 4

Key B — Class AB

Class AB is linear enough to reproduce an SSB envelope while being considerably more efficient than Class A. Class C and the switching classes are non-linear and would distort an amplitude-varying signal.

Rule 47 CFR §97.307(b) - emission quality

E1A05 Explanation 5

Key A — It is non-linear, so the envelope of an SSB signal would be distorted

Class C conducts in short pulses, which is harmless when the signal is a constant-envelope CW or FM carrier and ruinous when the amplitude itself carries the voice. Intermodulation from a Class C SSB stage splatters across the band.

Rule 47 CFR §97.307(b) - spurious emission limits

E1A06 Explanation 6

Key D — Gain falls, and bandwidth, linearity and stability improve

Negative feedback returns part of the output out of phase, reducing gain while flattening response, widening bandwidth and reducing distortion. Push the feedback far enough in phase rather than out of phase and you have built an oscillator.

Rule Fundamentals - feedback

E1A07 Explanation 7

Key C — Its very high Q makes the resonant frequency insensitive to circuit and temperature changes

Crystal Q values are in the thousands to tens of thousands, against a few hundred for an LC tank, and a high-Q resonance moves very little when stray capacitance or temperature shifts. That is why crystal-controlled and synthesised references hold frequency far better than free-running LC oscillators.

Rule 47 CFR §97.307 - frequency stability expectations

E1A08 Explanation 8

Key B — Random short-term frequency instability that lets strong nearby signals mix down as noise on the wanted frequency

Phase noise spreads the oscillator's energy either side of its nominal frequency. In a receiver that means a strong signal a few kilohertz away can mix with the local oscillator and deposit noise right where you are listening - the mechanism called reciprocal mixing.

Rule Fundamentals - oscillator performance

E1A09 Explanation 9

Key A — Impedance matching and harmonic attenuation

The network transforms the low plate or collector impedance up to the 50 ohm feed line and, because it is resonant, rolls off the harmonics that the non-linear final stage generates. This is why the output network is tuned at all rather than simply coupled.

Rule 47 CFR §97.307(d) - harmonic suppression

E1A10 Explanation 10

Key A — The junction temperature of the semiconductor die

Datasheets specify a maximum junction temperature, typically 150 to 200 degrees Celsius, and thermal resistance from junction to case. The heat sink matters because it lowers the total thermal resistance to ambient, which is what lets more dissipation occur without exceeding that junction limit.

Rule Fundamentals - thermal design

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.