Designing a negative feedback circuit in an audio amplifier is a common technique used to improve the overall performance and reduce distortion. Negative feedback helps stabilize the amplifier's gain, lower its output impedance, and improve linearity. Here's a step-by-step guide on designing a simple negative feedback circuit for an audio amplifier:
Select the amplifier type: First, choose the type of amplifier you want to use (e.g., common-emitter for a transistor-based amplifier or operational amplifier - op-amp for an integrated circuit-based amplifier). For simplicity, let's consider an op-amp-based audio amplifier.
Choose the feedback network topology: The most common feedback network used in audio amplifiers is the voltage-divider feedback network. It involves connecting a resistor in series with the output and a resistor of lower value in parallel with the feedback path.
Calculate the desired gain (Av): Determine the desired overall voltage gain of your amplifier. The overall gain is a combination of the amplifier's open-loop gain (Aol) and the negative feedback network's attenuation (Af). The formula for overall voltage gain (Av) is Av = Aol / (1 + Aol * Af).
Determine the feedback ratio (Af): Calculate the feedback ratio required to achieve the desired overall gain. Rearrange the formula from step 3 to calculate the feedback ratio: Af = (Aol / Av) - 1.
Select feedback resistors: Once you know the feedback ratio (Af), you can choose appropriate feedback resistors. The parallel feedback resistor (Rf_parallel) can be calculated as Rf_parallel = R_output / Af, where R_output is the output resistor in series with the load.
Calculate the gain-setting resistor: To determine the series feedback resistor (Rf_series), you can use the relationship Rf_series = Rf_parallel * (1 / Af - 1).
Stability considerations: While negative feedback improves stability, it's essential to avoid introducing instability in the circuit. Ensure that the amplifier's open-loop gain (Aol) and bandwidth are sufficient for the desired overall gain. Additionally, consider using a compensation capacitor if necessary to avoid high-frequency instability.
Component tolerances: Take into account component tolerances and select resistors that are readily available in standard values or use trimmer potentiometers to fine-tune the gain if required.
Sensitivity to load: Check how the amplifier behaves under different loads to ensure it remains stable and does not oscillate or distort.
Simulation and Testing: Before implementing the circuit, it's good practice to simulate the design using circuit simulation software to verify its behavior. After simulation, build a prototype and thoroughly test it with real-world audio signals to validate its performance.
Remember that designing audio amplifiers involves some complexities, and the above steps serve as a basic guide. Depending on the complexity of your amplifier, you may need to consider additional factors such as frequency compensation, noise, and thermal considerations. Always be cautious when working with electrical circuits and follow safety guidelines.