E7B: Amplifiers

Amplifiers are the core of RF signal chains, and their design involves critical trade-offs between linearity, efficiency, bandwidth, and stability. The class of operation determines how much of the input cycle the active element conducts, which directly affects distortion, efficiency, and suitability for different modulation modes.

This lesson covers amplifier classes A through D, push-pull operation, operating points, grounded-grid amplifiers, emitter followers, parasitic suppression, intermodulation distortion, and the circuit shown in Figure E7-1.

Amplifier Classes

Amplifier class defines the portion of the input signal cycle during which the active element (transistor or tube) conducts current:

The Class A common emitter amplifier is biased so its operating point sits approximately halfway between saturation and cutoff on the load line. This central biasing allows the full input signal swing to be reproduced at the output without clipping in either direction.

Using a Class C amplifier to amplify an SSB signal produces signal distortion and excessive bandwidth, because the Class C amplifier only conducts for a fraction of the input cycle and will not faithfully reproduce amplitude variations in the SSB signal.

Push-Pull and Class AB

Push-pull amplifiers use two active elements — one conducting during the positive half-cycle, the other during the negative half-cycle. This topology cancels even-order harmonics and improves efficiency over single-ended Class A designs.

In a Class AB push-pull amplifier, each active element conducts for more than 180° but less than 360° of the input signal cycle. This slight overlap prevents crossover distortion that occurs in pure Class B designs, where each device turns on exactly at zero crossing.

Switching Amplifiers

A Class D amplifier uses switching technology to achieve high efficiency. Instead of operating in a linear region, the active devices are switched fully on (saturation) or fully off (cutoff). This eliminates the power dissipated in the linear region of the device.

Switching amplifiers are more efficient than linear amplifiers because the switching device is at saturation or cutoff most of the time — neither state dissipates significant power. The output of an RF switching amplifier requires a filter to remove harmonic content generated by the switching process, restoring a clean sinusoidal output.

Common Amplifier Topologies

Different transistor circuit configurations offer different combinations of gain, impedance, and phase relationships:

• Common emitter: High voltage and current gain; signal is inverted (180° phase shift). The most common RF amplifier configuration.
• Common collector (emitter follower): Unity voltage gain; input and output signals are in-phase (no phase inversion); high input impedance, low output impedance. Used as a buffer or impedance transformer.
• Common base (grounded-grid in tubes): Low input impedance; good high-frequency performance; no phase inversion. The grounded-grid amplifier is used in VHF/UHF applications where low input impedance and high stability are needed.

Distortion and Parasitic Suppression

RF power amplifiers can develop unwanted oscillations due to parasitic feedback paths — stray capacitance and inductance inside the transistor and on the PCB can create unintended feedback loops. To prevent this, parasitic suppressors (small resistors or ferrite beads) are added in series with gate or base leads, and the stage can be neutralized by feeding back a signal that cancels the parasitic feedback.

When two signals mix in a nonlinear amplifier, intermodulation distortion products are created — new frequencies that are sums and differences of the input frequencies and their harmonics. Third-order intermodulation products fall close to the desired signals and are the most troublesome.

Figure E7-1: Common Emitter Amplifier

Figure E7-1 shows a typical common emitter amplifier circuit with three labeled resistors. Three exam questions ask about the function of each resistor.

• R1 and R2: These form a voltage divider bias network. They are connected from the supply rail to ground, with the transistor base connected at the midpoint. This sets a stable DC base voltage regardless of transistor beta variations.
• R3: This is the emitter resistor, providing self bias (also called emitter degeneration). The emitter current flowing through R3 develops a voltage that opposes changes in base current, stabilizing the operating point against temperature drift and device variations.
• The circuit type: the transistor base receives the input, the collector drives the output, and the emitter is the common point — making this a common emitter amplifier.

E7B Practice Questions