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Gain Compression

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Gain compression occurs when an amplifier's input power is increased to such an extent that it reduces its gain and results in a nonlinear increase in output power.

  • Gain compression can lead to distortion in an amplified signal.

  • Understanding and managing gain compression is crucial for achieving desired performance and maintaining signal integrity in electronic systems.

Power transfer curves for an amplifier with a gain of 3 (4.77 dB). Gain compression is the difference between the green curve and the red curve.

Power transfer curves for an amplifier with a gain of 3 (4.77 dB). Gain compression is the difference between the green curve and the red curve.
(Source: Mr. PIM at English Wikipedia)

Gain compression occurs when an amplifier's input power is increased to such an extent that it reduces its gain and results in a nonlinear increase in output power. Consequently, the system's amplification ability decreases as the input signal becomes stronger. Gain compression can lead to distortion in the amplified signal.

Different factors can result in gain compression. Here are the most common:

Common Factors Contributing to Gain Compression

Device Saturation

Increasing input signals may cause an active device, such as a transistor, to reach saturation, which limits its ability to amplify further.

Power Supply Limitation

Gain compression may occur when the input signal approaches or exceeds the power supply's limits, such as a maximum voltage or current output.

Mismatched Impedances

Mismatched impedances between different stages of an amplifier can contribute to gain compression and affect overall performance.

Overdriving

Amplification can suffer from gain compression and degradation of signal quality if the amplifier is pushed beyond its specified limits.

Dynamic Range Limitations

Gain compression may occur at low and high signal levels if an amplifier has a limited dynamic range.

Nonlinearities in Components

A strong input signal may cause specific amplifier components to behave nonlinearly, resulting in gain compression.

Device Aging

As devices age, their linearity can be affected, resulting in gain compression.

Temperature Effects

Temperature changes may cause gain compression.

Component Variability

There can be varying degrees of gain compression between components with slightly different characteristics.

Frequency Dependencies

At different frequencies, some amplifiers may exhibit different gain compression characteristics.

Gain compression is an essential consideration in several applications, including wireless communication systems, audio amplification systems, and radar systems. Engineers must design amplifiers with minimal gain compression to ensure the system can handle a wide range of input signals without degradation.

Methods for Measuring Gain Compression

Measuring gain compression in amplifiers involves determining how the gain of the amplifier changes as a function of the input signal level. Various methods and metrics can be used to quantify gain compression, including: 

  • 1 dB compression point represents the input signal level at which the gain of the amplifier decreases by 1 decibel (dB) from its small-signal gain. Manufacturers often specify this measurement in datasheets.

  • Output intercept point (OIP) measures the point at which the output signal intercepts the linear extrapolation of the amplifier's gain. The third-order output intercept point (OIP3) is commonly used and provides information about the third-order distortion products.

  • Third-order intercept point (IP3) is a metric used to evaluate the linearity of an amplifier by measuring the point at which the third-order intermodulation distortion equals the fundamental signal level. A higher IP3 indicates better linearity and less gain compression.

  • Harmonic distortion measurements can provide insights into the non-linear behavior of the amplifier as it approaches compression.

  • Load-pull measurements involve varying the impedance at the output of the amplifier while measuring its performance. This can help determine how the gain and compression characteristics change under different load conditions.

  • Sweeping input power involves sweeping the input power over a range of values and measuring the corresponding output power. Plotting the output power versus input power can reveal the compression characteristics of the amplifier.

  • Intermodulation distortion measurements examine the levels of intermodulation products generated by the amplifier at different input power levels, which provides information about its compression behavior.

  • Dynamic range measurements assess the amplifier's dynamic range, which is the range between the smallest and largest input signals without significant distortion. These measures can indirectly reveal information about gain compression.

  • Small-signal gain vs. input power curve provides a visual representation of how the gain changes as the input power increases by plotting the small-signal gain of the amplifier against input power. 

When measuring gain compression, consider the application's specific requirements and the amplifier's intended use. Measurement techniques may vary depending on frequency, signal type, and the desired level of precision in characterizing gain compression behavior.

Gain Compression and PCB Design

Understanding and managing gain compression is crucial for achieving desired performance and maintaining signal integrity in electronic systems. Critical aspects of gain compression in PCB design are detailed in the table below:

Gain Compression Considerations in PCB Design

Amplifier Design

✔ Gain compression characteristics of amplifiers must be considered during the PCB design phase.

✔ Select amplifiers with suitable compression points and ensure the overall system performance meets the specifications.

Component Selection

✔ Manufacturers often provide specifications, such as compression points.

✔ Choose signal processing components that meet requirements for linearity and distortion.

Dynamic Range

✔ Gain compression affects a system’s dynamic range, which is the difference between the smallest and largest detectable signals without distortion.

✔ Consider the dynamic range requirements of the application and choose components, including amplifiers, that provide adequate performance across a range of signal amplitudes.

Signal Integrity

✔ Gain compression can impact signal integrity, especially in high-frequency applications. 

✔ Consider the potential effects on signal quality and take measures to mitigate any adverse consequences.

PCB designers must carefully consider the characteristics of amplifiers and other components to ensure that the overall system meets the desired performance criteria with minimal distortion and optimal signal integrity.

Manage Gain Compression With Cadence Software 

PCB design balances cost and parameters (gain compression, bandwidth, signal-to-noise ratio, etc.), and designers need to consider multiple components and topologies to discover the best performance that aligns with project requirements. Cadence’s PCB Design and Analysis Software suite provides design teams with tools and component models to accelerate product development while leaving no stone unturned. When it’s time to switch to board design, OrCAD PCB Designer integrates seamlessly with simulation results.

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