Explain amplifier classes, why switching amplifiers are efficient, and read the bias parts in a common-emitter circuit.
12 pool questions1 on the exam
Syllabus: Amplifiers: class of operation; vacuum tube and solid-state circuits; distortion and intermodulation; spurious and parasitic suppression; switching-type amplifiers
Loading lesson…
Go deeper
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Bias decides where the input signal sits on the device's curve. Ride high and it conducts all the time (linear, wasteful). Ride low and it conducts only in bursts (efficient, distorted).
Slide the bias down: the device conducts for less of the cycle. Efficiency rises, linearity falls.
Class
Conducts for
Trade-off
A
all 360° (Q-point halfway between cutoff and saturation)
most linear, least efficient
AB
more than 180°, less than 360°
in between
B
180°
efficient, needs push-pull
C
less than 180°
most efficient, very nonlinear
Push-pull Class AB: two devices share the cycle, each conducting more than 180° so the handoff is smooth. Class C chops the waveform, so it is fine for FM or CW but distorts SSB: distortion products and extra bandwidth.
RememberSmaller conduction angle: more efficiency, less linearity. SSB needs linear.
Switching amplifiers waste almost no heat
A Class D (switching) amplifier drives the device hard on and hard off. Heat is voltage times current, and a switch never has both at once.
Heat is voltage times current. A switch never has both at once.
The price is a square-edged output rich in harmonics, so it needs a filter to remove them.
A filter at the output of a switching amplifier removes the harmonics.
RememberSwitching = saturation or cutoff = high efficiency. Always follow it with a harmonic filter.
Reading Figure E7-1: what each part does
Find the transistor, then the two resistors on its base: they are a voltage divider setting the DC operating point. The resistor in the emitter leg stabilises it (self bias). Capacitors only pass the signal in and out.
Pick a part. Teal marks the part being explained.
RememberR1 and R2 = voltage-divider bias. R3 = self bias. Input at base, output at collector = common emitter.
Common emitter, follower, grounded grid
Find the leg the signal does not use. That is the "common" one. Tubes follow the same pattern: grounded grid is the common-base layout.
Name the leg that is shared by input and output. That is the 'common' one.
Stray inductance and capacitance can feed some output back to the input and start an unwanted (parasitic) oscillation. The cure is not tuning harder: add parasitic suppressors, and/or neutralize the stage by feeding back a cancelling signal.