2Diffseries -250GM Class G Power Amp Mosfet Output Stage
This is a high-end classical voltage amplifier, featuring:
• Fully symmetrical differential input stage
• Cascoded and compensated VAS
• Moderate global feedback
• MOSFET output stage
• Class G dual-rail supply (±22 V / ±36 V)
It is a technically sophisticated design aimed at high power, high efficiency, and excellent stability with real loudspeaker loads.
⸻
1. Input Stage and RF Filtering
• Input coupling capacitor: 1 µF
• Series resistor: 1 kΩ
• RF filter: 47 kΩ + 220 pF
Functions:
• DC blocking
• RF suppression
• High input impedance
This is a textbook reference-grade input configuration.
⸻
2. Complementary Dual Differential Stage
• NPN and PNP differential pairs
• 100 Ω emitter resistors
• Current mirrors
• 10 V Zener voltage references
Functions:
• Compare input signal with global feedback
• Cancel DC offset
• High common-mode rejection
• Perfect signal symmetry
Key benefit:
Even-order distortion is naturally canceled before feedback is applied.
⸻
3. Current Sources and Voltage References
• 10 V and 22 V Zeners
• Biasing resistors
• Transistor-based current sources
Functions:
• Stable operating currents
• Isolation from rail variations
• Excellent thermal stability
This is essential for well-behaved Class G operation.
⸻
4. Differential-to-Single-Ended Conversion
• Balanced intermediate stage
• Active loads
• Preserves symmetry
Function:
• Deliver a clean, well-defined signal to the VAS
⸻
5. Cascoded VAS and Compensation
• Dedicated VAS transistors
• 22 V Zener cascode
• 50 pF Miller capacitor
• 10 pF local compensation capacitor
Functions:
• Provide the main voltage gain
• Ensure loop stability
• Define the slew rate
Benefits:
• High speed
• Low distortion
• Excellent phase margin
⸻
6. Global Feedback Network
• 33 kΩ feedback resistor
• Return point at the differential input
Functions:
• Set overall voltage gain
• Reduce THD
• Lower output impedance
Feedback is moderate, not excessive.
⸻
7. MOSFET Driver Stage
• Dedicated bipolar driver transistors
• 100 Ω gate resistors
• 120 Ω source/emitter resistors
• Protection diodes
Functions:
• Rapidly charge and discharge MOSFET gates
• Isolate the VAS from heavy capacitive loads
• Prevent high-frequency oscillations
This stage is critical for MOSFET reliability.
⸻
8. Class G Rail Switching (Dual Supply)
This is one of the defining features of the design:
• Low rails: ±22 V
• High rails: ±36 V
• Diodes and transistors detect signal level
• Automatic switching to high rails only when required
Functions:
• Reduce power dissipation at moderate levels
• Preserve high peak power capability
• Improve thermal efficiency
Importantly:
• The audio signal itself is never switched
• Only the supply rails change
• No audible switching artifacts are introduced
⸻
9. MOSFET Output Stage
• Parallel power MOSFETs
• 220 mΩ source resistors
• Source-follower configuration
Functions:
• Deliver high output current to the load
• Ensure current sharing
• Provide inherent short-term protection
MOSFET benefits:
• Positive thermal coefficient
• Excellent stability
• Smooth crossover behavior
These traits make MOSFETs ideal for Class G designs.
⸻
10. Output Network and Load Stability
• 3 µH output inductor
• 10 Ω damping resistor
• Zobel network: 10 Ω + 100 nF
Functions:
• Stability with reactive loudspeakers
• Immunity to long speaker cables
• High-frequency control
⸻
Architectural Advantages
Compared to conventional Class AB
✔ Much lower heat dissipation
✔ Higher peak power capability
✔ Smaller heatsinks
✔ Improved long-term reliability
Compared to Class D
✔ Fully analog signal path
✔ No LC output filter in the audio band
✔ Predictable behavior with any load
Benefits of MOSFET Outputs
✔ Natural thermal stability
✔ Reduced risk of catastrophic failure
✔ Smooth, controlled sonic character
⸻
Final Summary
This amplifier is:
• A Class G power amplifier
• With MOSFET output devices
• A refined, classical high-performance architecture
• Thermally efficient and extremely stable
• Capable of delivering high real-world power
This is the kind of topology commonly found in:
• Serious professional audio amplifiers
• High-power hi-fi systems
SIMULATION RESULTS :
Power Output 250 Wrms @ 4 Ohms
Frequency response 8.5Hz - 40KHz (-1dB)
Input sensitivity 540mV
