Explain amplifier classes and efficiency, how oscillators and neutralizing work, and what basic logic circuits do.
Syllabus: Digital circuits; amplifiers and oscillators
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Bias sets where the signal sits against cutoff. Class A conducts all 360° of the cycle. B conducts half (180°). AB is between. C conducts for less than half.
Less conduction means less idle waste, so efficiency rises: A, AB, B, C. But the output also stops copying the input. A linear amplifier keeps the output waveform shaped like the input. Class C is not linear, so it suits only constant-amplitude signals such as FM, not AM or SSB.
RememberClass A: 100% conduction, most linear. Class C: most efficient, FM only.
Efficiency compares the useful output (RF) with what you paid for (DC input). The difference is lost as heat.
RememberEfficiency = RF output power ÷ DC input power.
Feed part of an amplifier's output back to its input through a filter. If the loop adds back as much as it loses, the signal sustains itself. The filter decides which frequency survives.
RememberSine wave oscillator = filter + amplifier in a feedback loop.
The tank circuit is an inductor and capacitor. It is the filter in the loop, so its values set the frequency. Larger L or C means lower frequency.
RememberLC oscillator frequency = set by the inductance and capacitance of the tank.
Inside real amplifying parts, a little output leaks back to the input through internal capacitance. In phase, it turns the amplifier into an unwanted oscillator. Neutralizing feeds back an equal, out-of-phase signal to cancel it.
RememberNeutralizing eliminates self-oscillation.
A two-input AND gate is high only when both inputs are high. NAND is its opposite. OR and NOR act on either input. Try each one.
Memory hookAND = both. OR = either. The N in NAND and NOR flips the output.
A counter with n bits has 2ⁿ states: 3 bits give 2³ = 8 (000 to 111), then it wraps. A shift register is a clocked array of circuits that moves data one step along the array each clock pulse.
Remember3-bit counter = 8 states. Shift register = clocked array that passes data in steps.