FAQ
TL;DR: For a given load, you choose the output behavior (voltage- or current-source), and Ohm’s law gives the rest; e.g., 100 W into 8 Ω ≈ 28.3 Vrms. “You can not set both the voltage and current.” [Elektroda, Olin Lathrop, post #21659320]
Why it matters: This FAQ helps newcomers design and size amplifier outputs safely and correctly for real loads in audio and other applications.
Quick Facts
- Ohm’s law and power: V = I·R, P = V·I, P = V²/R, P = I²·R. [Elektroda, Alec, post #21659326]
- You pick the V–I relationship; the load sets the operating point. [Elektroda, Olin Lathrop, post #21659320]
- Example: 100 W into 8 Ω needs ≈28.3 Vrms and ≈3.54 Arms (≈40 Vpk). [Elektroda, Alec, post #21659326]
- “Load” means the device being powered; it can be resistive, inductive, or capacitive (complex impedance). [Elektroda, Olin Lathrop, post #21659324]
- Start by defining the application (audio, RF, etc.) and studying targeted app notes. [Elektroda, Alec, post #21659331]
Can I set both voltage and current at my amplifier’s output?
No. You choose how voltage and current relate; the load fixes the actual operating point on that curve. “You can not set both the voltage and current.” [Elektroda, Olin Lathrop, post #21659320]
What exactly is a “load” in a circuit?
The load is whatever your circuit powers. It is not just a resistor. It can include resistance, inductance, and capacitance, forming a complex impedance that varies with conditions. [Elektroda, Olin Lathrop, post #21659324]
How do I calculate output voltage and current for a known power and resistance?
Use Ohm’s law and power equations: P = V²/R or P = I²·R. Then V = √(P·R) and I = √(P/R). Example: 100 W into 8 Ω → 28.3 Vrms and 3.54 Arms. [Elektroda, Alec, post #21659326]
What output do I need to drive a 100 W, 8 Ω speaker?
Target about 28.3 Vrms and 3.54 Arms into 8 Ω. Allow headroom for peaks, so design for about 40 Vpk swing per channel in a typical audio amp stage. [Elektroda, Alec, post #21659326]
Which should I prioritize when designing—voltage or current?
Pick the output characteristic that suits the load. You control the V–I relationship; the load determines the exact voltage and current at runtime. [Elektroda, Olin Lathrop, post #21659320]
Are “power rating” and “load” the same thing?
No. The load is the device you drive. Its power rating is how much power it can safely dissipate. Loads can be complex, not purely resistive. [Elektroda, Olin Lathrop, post #21659324]
Where should I start if I want a 1000 W amplifier?
Start by defining the application (audio vs. RF) and studying reliable textbooks and application notes. Scope the problem before picking a topology. [Elektroda, Alec, post #21659331]
Does power factor matter for amplifier loads?
Yes, if the load has reactive components. Nonresistive loads shift current relative to voltage, changing the apparent vs. real power the stage must handle. [Elektroda, David Figueroa, post #21659321]
How do reactive loads affect design choices?
Inductive or capacitive loads change phase and impedance with frequency. Your chosen V–I behavior interacts with that, moving the operating point under signal. [Elektroda, Olin Lathrop, post #21659324]
Give me a quick 3‑step method to size my amp output for a resistive speaker.
- Identify speaker impedance (e.g., 8 Ω) and target power (e.g., 100 W).
- Compute Vrms = √(P·R) and Irms = √(P/R).
- Add voltage and current headroom for peaks and losses. [Elektroda, Alec, post #21659326]
What about a 100 W, 4 Ω speaker—what numbers should I expect?
Vrms = √(100×4) = 20 Vrms. Irms = √(100/4) = 5 Arms. Ensure your supply and devices safely deliver these with margin. [Elektroda, Alec, post #21659326]
What was the original application discussed in the thread?
The OP clarified the target as an audio system driving a 100 W loudspeaker, not RF or other uses. [Elektroda, Praveen Kumar Avala, post #21659333]
Do I need to share a schematic to get useful help?
Yes. Clear problem statements and schematics improve guidance. Words alone are ambiguous, especially across languages. Provide diagrams and specs. [Elektroda, Alec, post #21659328]
What failure can occur if the load is too low (near short)?
Current rises as I = V/R. Extremely low R causes large current, overheating devices and tripping protection or destroying parts. Design current limits. [Elektroda, Alec, post #21659326]
Can I stabilize the output with diodes or clamps?
Clamps can bound voltage, but they do not set both voltage and current. They must be rated for the expected power and load behavior. [Elektroda, David Figueroa, post #21659321]
