Advanced Calculator
Example Data Table
| Case | Input RMS | Rf | Rg | Load | Rails | Expected Use |
|---|---|---|---|---|---|---|
| Small audio driver | 150 mV | 18 kΩ | 2.2 kΩ | 8 Ω | ±18 V | Moderate room listening |
| Bench test stage | 250 mV | 22 kΩ | 2.2 kΩ | 8 Ω | ±24 V | General gain check |
| Higher swing stage | 500 mV | 33 kΩ | 3.3 kΩ | 4 Ω | ±35 V | Power estimate |
Formula Used
Non-inverting gain: Av = 1 + Rf / Rg
Inverting gain magnitude: Av = Rf / Rg
Overall voltage gain: Av(total) = Av(feedback) × driver factor × output factor
Voltage gain in decibels: Gain dB = 20 log10(Av)
Output RMS voltage: Vout = Vin × Av(total)
Maximum unclipped RMS voltage: Vout(max) = (Vcc − headroom) / √2
Load power: Pout = Vout² / Rload
Signal input power: Pin = Vin² / Zin
Power gain: Ap = Pout / Pin
Estimated Class AB DC power: Pdc ≈ (2 × Vcc × Ipeak) / π + 2 × Vcc × Iq
Efficiency: η = Pout / Pdc × 100
Output coupling cutoff: fc = 1 / (2πRloadCout)
How to Use This Calculator
Enter the RMS input signal first. Use millivolts for small audio signals.
Select the feedback topology. Use non-inverting for common voltage amplifiers. Use inverting when the source drives the input resistor.
Enter Rf and Rg values. For custom mode, enter the measured gain instead.
Add driver gain and output stage gain factors. Use values below one when the output stage has loss.
Enter the load, input impedance, supply rails, output headroom, and quiescent current.
Press Calculate. The result appears above the form. Use CSV or PDF buttons to export the same result.
Class AB Amplifier Gain Guide
Overview
A Class AB amplifier sits between Class A and Class B behavior. It keeps both output devices slightly on near zero crossing. This bias lowers crossover distortion. It also keeps efficiency better than a pure Class A stage.
Why Gain Needs Practical Checks
Gain is usually set by feedback. The output transistors often behave like emitter followers or source followers. Their voltage gain is near one, but not exactly one. Driver stages, feedback resistors, load resistance, and supply headroom all affect the real result.
What This Tool Estimates
This calculator combines those practical details. It estimates closed loop voltage gain, gain in decibels, output swing, output power, power gain, efficiency, and heat loss. It also checks clipping risk. The result is useful for pre-design work, lab comparison, and quick education.
Feedback and Signal Setup
Start with the input RMS signal. Then choose the feedback style. For a non-inverting stage, the gain is one plus feedback resistor divided by ground resistor. For an inverting stage, the magnitude is feedback resistor divided by input resistor. A custom mode is also available for measured or simulated gain.
Supply and Clipping
The supply section is important. A Class AB output cannot usually swing to the exact rail. Headroom allows for transistor drops, driver limits, protection resistors, and saturation margin. The calculator limits the output RMS value when the requested swing is larger than the safe unclipped swing.
Efficiency and Heat
Efficiency is estimated from sinusoidal output current. The model uses the common push-pull approximation for dynamic DC power. It also adds idle power from the quiescent current. This makes the estimate more realistic for biased Class AB stages.
Power Gain
Power gain compares output load power with input signal power. It depends on the selected input impedance. Low input impedance raises input power and lowers calculated power gain. High impedance does the opposite.
Thermal Review
Thermal loss is the difference between DC power and delivered output power. Divide it by output device pairs to estimate device stress. This is only a first pass. Final designs still need measurement, safe operating area checks, heatsink design, and distortion testing.
Design Reminder
Use the result as a guide, not a final rating. Real amplifiers include rail sag, speaker impedance changes, temperature drift, device mismatch, and compensation limits. Check results against a schematic, simulation, and bench data before selecting parts or publishing specifications.
FAQs
What is Class AB amplifier gain?
It is the ratio between output signal voltage and input signal voltage. In many Class AB amplifiers, feedback resistors set most of the voltage gain.
Why is output stage gain less than one?
Emitter follower and source follower output stages usually track the driver signal. Device drops, loading, and bias effects can make the gain slightly below one.
What does headroom mean?
Headroom is the voltage the amplifier cannot use near each supply rail. It protects the estimate from assuming an impossible full rail-to-rail output swing.
Why does the result show clipping?
Clipping appears when the calculated output swing is larger than the available unclipped swing from the supply and headroom values.
How is efficiency estimated?
The calculator uses a sinusoidal push-pull power approximation. It also adds idle power from Class AB quiescent current for a more practical estimate.
Can I use this for speaker amplifiers?
Yes, for first-pass estimates. Real speakers have changing impedance, so confirm results with measurements and safe operating area checks.
What is power gain?
Power gain compares output load power with input signal power. It depends on voltage gain, load resistance, and input impedance.
Why include output coupling capacitor?
Some amplifier designs use an output capacitor. The capacitor and load form a high-pass cutoff that affects low-frequency response.