What Is High Gain and How Does It Impact Amplifier Performance?

High Gain RF Amplifier

RF sensing and communication applications often involve very weak signal levels. It is often necessary to use RF Amplifiers to increase the gain, or signal level, of these signals in order to digitize them or otherwise condition these signals to usable levels. However, there are a few trade-offs to consider when increasing a signal’s power level, especially at very high gain factors.

As with all performance parameters and signal quality factors, attempting to improve one factor/parameter tends to require sacrificing another, or several. For instance, the gain bandwidth product (GBP) is a figure of merit that compares the open-loop gain of an amplifier with the available bandwidth at that gain level. With many amplifiers, increasing the gain lowers the available bandwidth, while attempting to achieve a wide-bandwidth necessitates using lower gain. Bandwidth in this case is defined as the frequency range over which the amplifier gain is relatively flat.

There are also other considerations when attempting to optimize an amplifier for gain. These include noise and stability. Typically, higher gain amplifiers also introduce higher levels of added-noise figure. Even with very low added noise figure amplifiers, the noise of the signals will ultimately also be amplified just as much as the original signal. This is why filters are often used at various stages in signal conditioning to limit the amount of out-of-band noise energy. However, it is often very difficult to remove the in-band noise that is added by increasing the gain of a signal.

With stability, the higher gain amplifier typologies must be carefully designed to avoid becoming unstable over the specified input signal energy, loading, and frequency range. This includes accounting for dynamic factors over the entire specified range of the amplifier’s operation, such as temperature and other environmental factors. As this requires accounting for the behavior of the load, high gain/RF Power Amplifiers are often tested using load-pull and other methods that vary the load behavior to properly qualify an amplifier design.

Though many RF amplifiers are designed with a fixed gain profile, some are designed to vary the gain over a range. These amplifiers are known as RF Variable Gain Amplifiers (VGAs), and exhibit the same trade-offs and fixed gain amplifiers, but also present more nuanced performance. VGAs are often used in test and measurement applications or automated testing. However, there are many types of integrated VGAs used for radio and radar designs that may benefit from operating at varying gain levels either at the signal input stage or output stage.