Designing a negative feedback circuit in an audio amplifier is a common practice to improve performance, stability, and reduce distortion. Negative feedback works by comparing the output of the amplifier with the input signal and then applying a portion of the error signal (the difference between the two) back to the input in the opposite phase. This helps to reduce the overall gain and linearize the amplifier's response.
Here's a general guideline to design a negative feedback circuit in an audio amplifier:
Select the operational amplifier (op-amp): Choose a suitable op-amp that meets the requirements of your audio amplifier application, such as bandwidth, voltage range, and input impedance.
Determine the desired gain: Decide on the desired gain for your audio amplifier. The gain is the ratio of output voltage to input voltage. For example, if you want a gain of 10, the output voltage will be ten times the input voltage.
Calculate the feedback resistor (Rf) and input resistor (Ri): Once you have the desired gain (A) and the input resistor (Ri), you can calculate the feedback resistor (Rf) using the formula: Rf = Ri * (A - 1). The negative feedback circuit should be connected as shown below:
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+----------------+
Input ---->| |
| Op-Amp |----> Output
| |
GND ----->| | |
+-------|--------+
|
Rf
|
|
Ri
|
V-
Choose capacitor values (optional): In audio amplifiers, you might need to add coupling capacitors at the input and output to block DC components. These capacitors should be chosen based on the low-frequency response requirements and should not affect the desired audio frequency range.
Compensate for high-frequency roll-off: The feedback loop can introduce a high-frequency roll-off, causing a decrease in bandwidth. You can compensate for this by adding a compensation capacitor in parallel with the feedback resistor (Rf) and possibly adjusting other component values.
Consider stability: The negative feedback circuit should be designed to maintain stability and avoid oscillations. Depending on the application, you might need to include additional compensation components, such as a series resistor and capacitor in the feedback loop or a phase lead network.
Simulate and test: Before building the circuit, it's a good practice to simulate it using SPICE simulation software or equivalent tools. Simulations can help you verify the stability, frequency response, and other performance parameters of your audio amplifier.
Remember that the actual design specifics can vary based on the specific requirements of your audio amplifier and the op-amp you are using. Additionally, for more complex amplifier designs, you might want to consider other factors like power supply regulation, thermal considerations, and protection circuitry.