How to Measure High Power Amplifier Gain

Calculate Amplifier Gain

High-power amplifiers are constructed for linear undistorted gain at high power output levels. They may include several internal signal amplifiers called operational amplifiers, or opamps, in their circuitry. The gain of the amplifier is a function of its opamps.

Things You'll Need

• circuit diagram of amplifier
• ohmmeter

Instructions

1. Identify the opamps in the circuit diagram. They are represented by triangles tilted 90 degrees to the right with the input coming into the base and output coming out of the apex. If multiple opamps are present in the circuit, the output of the first one will become the input for the next one.

2. Set up the gain equation for the first opamp in the amplifier circuit. There will be one input resistor and one feedback resistor connecting the output back into the input. The gain (G) is calculated as negative one times the resistance of the feedback resistor (Rf) divided by the input resistor (Ri). In equation form, it is: G = - Rf/Ri.

3. Measure the resistance of the input and feedback resistor. Since most resistors have 1 percent to 5 percent error, use an ohmmeter to get the true values of the resistors on your opamp. Insert these values into the equation for the opamp's gain. For example, if the feedback resistor is 995 ohms and the input resistor is 99 ohms, the gain is G = -Rf/Ri = -10.05.

4. Repeat the gain calculation for each opamp in the amplifier. Then multiply the gain of each opamp to solve for the amplifier's gain. For example, if one opamp of gain -10.05 feeds into an opamp of gain -9.95, the total gain for the amplier is 99.9975.

Tips & Warnings

• The opamp calculations in this article consider the simplest and most common configuation, which is a linear inverting opamp. If the resistor network around the opamps in your circuit is different than a resistor (or resistors) in series with the input and a resistor (or resistors) connected to the input resistor, then the opamp calculations in this article do not apply.

References

• All About Circuits: Amplifier Gain