An amplifier structure defines the internal topology, biasing scheme, and stage arrangement that dictates how a circuit increases signal power while managing efficiency and distortion. In a real circuit or installation, the chosen structure changes your thermal management requirements, power supply sizing, and the total harmonic distortion (THD) floor of the final build. Makers and students frequently confuse an amplifier's class (the biasing and conduction angle, like Class AB or Class D) with its topology (the physical circuit arrangement, like a push-pull, differential pair, or H-bridge). You must specify both to design a reliable system.
The Core Stages of a Linear Amplifier Structure
When you open up a classic Class AB audio amplifier (like a benchtop power amp or a high-end stereo receiver), the internal structure is almost always divided into three distinct cascaded stages. Understanding this signal chain is critical for troubleshooting oscillation or high-distortion faults.
- 1. Input Stage (Differential Pair): Receives the low-level voltage signal. It provides high input impedance (typically >20kΩ) and rejects common-mode noise. Often built with matched JFETs or BJTs like the 2N5551/2N5401 pair.
- 2. Voltage Amplification Stage (VAS): The muscle of the voltage gain. A single transistor (or cascode pair) converts the differential current into a high-voltage swing. This stage is usually Miller-compensated with a 47pF to 100pF capacitor to prevent high-frequency oscillation.
- 3. Output Stage (Current Buffer): Provides the current gain to drive low-impedance loads (4Ω or 8Ω speakers). It operates as an emitter-follower with near-unity voltage gain but high current capacity, utilizing heavy-duty silicon like the 2SC5200/2SA1943 complementary pair.
If your amplifier has high THD at low volumes, the fault usually lies in the input stage biasing. If it clips asymmetrically or runs away thermally, you look at the VAS current source or the output stage emitter resistors. For a deeper dive into solid-state stage cascading, the All About Circuits semiconductor textbook provides excellent baseline schematics.
Amplifier Topology & Class Comparison Matrix
Selecting the right amplifier structure requires balancing efficiency against linearity. The table below maps the most common structures you will encounter in modern DC/AC and audio designs, detailing their conduction angles and real-world performance metrics.
| Class | Topology Structure | Conduction Angle | Theoretical Max Efficiency | Typical Real-World Efficiency | Primary Use Case |
|---|---|---|---|---|---|
| Class A | Single-ended or Push-Pull | 360° (Full cycle) | 50% (Transformer) / 25% (Resistive) | 15% - 25% | Guitar preamps, high-end headphone amps |
| Class AB | Complementary Push-Pull | 180° to 360° | 78.5% | 50% - 65% | Home stereo receivers, bench power supplies |
| Class D | H-Bridge / Half-Bridge PWM | Switching (0° or 360°) | 100% | 85% - 95% | Active subwoofers, portable Bluetooth speakers |
| Class G/H | Multi-rail Push-Pull | 180° to 360° (Rail switching) | ~85% | 65% - 80% | Pro-audio PA amplifiers, bass shakers |
Worked Example: Thermal Load in Class AB vs. Class D Structures
Let's look at what the amplifier structure actually changes on your workbench: the heatsink mass. Suppose you are designing a 50W RMS amplifier to drive an 8Ω subwoofer. We need to calculate the continuous power dissipation to size the thermal management.
Target Output: 50W RMS into 8Ω.
Required RMS Voltage: V = √(P × R) = √(50 × 8) = 20V RMS (28.3V peak).
Power Supply Rails: ±35V DC (to account for transistor saturation voltage drops).
Scenario A: Class AB Push-Pull Structure
Assuming a realistic continuous sine-wave efficiency of 55% (accounting for VAS quiescent current and saturation losses):
Total Power Drawn (P_in) = 50W / 0.55 = 90.9W.
Power Dissipated as Heat (P_diss) = 90.9W - 50W = 40.9W.
Result: You need a massive extruded aluminum heatsink (roughly 1.5°C/W thermal resistance) and forced-air cooling to keep the 2SC5200 output transistors below their 150°C junction limit.
Scenario B: Class D H-Bridge Structure
Assuming a modern MOSFET H-bridge (like the TPA3255 from Texas Instruments) operating at 90% efficiency:
Total Power Drawn (P_in) = 50W / 0.90 = 55.5W.
Power Dissipated as Heat (P_diss) = 55.5W - 50W = 5.5W.
Result: The IC's exposed thermal pad soldered to a standard 2-layer PCB with copper pours is entirely sufficient. No external heatsink required.
This 35-watt difference in waste heat dictates your enclosure size, power supply transformer VA rating, and overall BOM cost.
Where You Meet Amplifier Structures in Practice
You interact with specific amplifier structures constantly in DIY electronics and home installations, even if you aren't designing them from scratch.
- Active Subwoofer Plates: Almost exclusively Class D H-bridge structures. Because low frequencies don't require high slew rates, the switching noise (300kHz+) is easily filtered out by the speaker's natural inductance and a small LC output filter. Pitfall: If you run unshielded speaker wire over 15 feet with a Class D amp, the wire acts as an antenna, radiating EMI that can interfere with AM radio or Wi-Fi routers.
- Guitar Overdrive Pedals: Often rely on Class A common-emitter structures using JFETs (like the J201) or op-amps pushed into asymmetrical clipping. The inefficiency of Class A is irrelevant here because the total power is under 50mW, and the even-harmonic distortion profile of a single-ended structure is musically desirable.
- RF Transmitters (Ham Radio/IoT): Use Class C or Class E structures. These are highly non-linear (conduction angle < 180°) and would sound terrible for audio, but because they are tuned with LC tank circuits to resonate at a single carrier frequency, they achieve massive efficiency (>80%) for continuous-wave or frequency-modulated signals.
Frequently Asked Questions
Can I parallel Class AB output transistors without emitter resistors?
No. If you parallel BJTs in an output structure without emitter degeneration resistors (typically 0.22Ω, 5W), minor variations in Vbe (base-emitter voltage) will cause one transistor to hog the current. This leads to localized heating, which lowers its Vbe further, causing thermal runaway and a shorted silicon junction. Always use matched pairs and emitter resistors.
Why does my Class D amplifier IC overheat with no speaker connected?
Many Class D structures rely on the speaker load and the output LC filter to provide a stable feedback path and dampen high-frequency ringing. Running a Class D amp into an open circuit can cause the output node to ring at the switching frequency, leading to massive shoot-through currents in the MOSFETs. Always connect the load before applying power, or use a dummy load resistor for bench testing.
How do I measure the dead-time in a Class D H-bridge?
Dead-time is the brief nanosecond window where both high-side and low-side MOSFETs are off to prevent shoot-through. You need an oscilloscope with at least 200MHz bandwidth and high-voltage differential probes. Measure across the low-side gate-to-source and high-side gate-to-source; the dead-time is the gap between the falling edge of one and the rising edge of the other. Typical safe values are 10ns to 40ns, depending on the MOSFET gate charge (Qg). For more on probing techniques, SparkFun's amplifier tutorials offer great baseline safety practices.
