Watch for the IIP3 vs. OIP3 points in this amplifier circuit
Without power amplifiers, many of the power electronics we use daily would not be possible. These amplifier circuits are responsible for amplifying wireless signals and other analog signals to a useful level, making them invaluable in many applications. When it comes to ensuring signal integrity of frequency-modulated signals, what can a circuit or board design do to prevent harmonic distortion, clipping, and intermodulation?
Unfortunately, you can’t do anything to change the design of a COTS active power amplifier once it is placed in your board. However, understanding the IIP3 vs. OIP3 points in a power amplifier will help you determine the upper limit of input power and what to expect on the device output. Once compared with your design requirements, you can determine whether an alternative component is desired, or whether you can increase the signal to a useful level.
Power Amplifiers and Nonlinear Saturation
Power amplifiers are often run near the saturation regime in order to provide maximum power output. These amplifiers are constructed from power transistors, which are designed to run at high voltage/high current and operate at relatively high temperatures if needed. In order to get the maximum output current from the device, the transistor is typically run near the saturation input voltage for an input signal.
The input signal can have some applied DC bias, or it can simply be a strong analog signal. While we like to think of amplifiers and transistors as being perfectly linear devices, they are slightly nonlinear, even when run below saturation. As has been discussed in several places on this blog, harmonic generation occurs when you input an oscillating signal into a nonlinear component. In other words, the fundamental harmonic being input into the transistor circuit will produce multiple higher order harmonics at the output. Each harmonic frequency will be an integer multiple of the input frequency (weighted by the relative sensitivity at each frequency within the component’s bandwidth). This is shown in the image below.
Harmonic generation in a power amplifier
Here, the output signal contains harmonics that are some integer multiple of the input frequency. When a signal with multiple frequency components (e.g., an FM signal) is input into a power amplifier, the nonlinear nature of the component creates intermodulation products through frequency mixing, which produces intermodulation distortion in the output signal. In the time domain, intermodulation distortion is difficult to distinguish from other sources of signal distortion. However, intermodulation products can be easily seen in the frequency domain.
The frequencies of intermodulation products are equal to sums and differences of the different frequency components in the input signal. It is quite easy to see this effect in the graphic below. The odd-order difference frequencies are the most important. In particular, the 3rd-order intermodulation products (shown in green, labelled IM3) will lie closest to the desired carrier frequencies f1 and f2.
Intermodulation product generation in a power amplifier.
The goal in designing a signal chain with a power amplifier is to minimize distortion on the output due to harmonic generation and intermodulation products. The higher order harmonics can be easily filtered, but the intermodulation products are much more difficult and will limit the input signal level.
What is IIP3 vs. OIP3?
These two terms refer to the input and output signal levels, respectively, where the 3rd order intermodulation products have the same output level as the desired input signal. The output signal level for the fundamental frequencies and the IM3 frequencies can be plotted on an output vs. input graph, as shown below. The IIP3 vs. OIP3 points are plotted on this graph.
If you approximate the output vs. input curve as a Taylor series, you’ll find that the IM3 frequencies increase at 3x the rate of the fundamental frequencies. The output vs. input curves (log-log scale) for the fundamental and IM3 frequencies will intersect at 1 dB above the saturation level, also called the 1 dB compression point.
Intermodulation product generation in a power amplifier.
There are some important points to take from this graph:
At low input levels, the output will be nearly free of intermodulation distortion. This is because the higher order intermodulation products are very weak compared to the fundamental frequency band.
Eventually, as the input signal level increases, compression will cause the fundamental frequency to stop increasing, while the IM3 products will continue increasing.
The 3rd order intercept point cannot be observed directly. Instead, it can only be determined through interpolation from measurements of the IM3 product signal levels and the fundamental frequency signal level.
If you know the IIP3 vs. OIP3 point on an output vs. input graph for your power amplifier, you can determine the appropriate input signal level to use. The 3rd order intercept point tells you the level of nonlinearity in the amplifier. A larger number 3rd order intercept point means the IM3 products are weaker, thus there will be less distortion on the output from the amplifier. The peak of the input signal level should not exceed the 1 dB compression point.
Active vs. Passive Intermodulation
Note that we’ve discussed intermodulation produced by an active circuit, but the same effects can occur in passive components at very high frequencies. This type of intermodulation distortion, known as passive intermodulation, becomes problematic at microwave and mmWave frequencies and is a well-known signal integrity problem in telecom base stations, passive RF amplifiers, and in shoddy components used in RF systems.
The points discussed here also apply to passive intermodulation. In 5G test systems, the familiar 3rd-order passive intermodulation products are known to significantly decrease download speeds, even when these intermodulation products are more than 100 dB below the desired carrier frequencies. The causes of passive intermodulation are still an active area of research, and there are many opportunities for innovators to develop unique solutions to these problems.
If you have access to the right PCB layout and design software, you can design and analyze all aspects of your analog system, including IIP3 vs. OIP3. Allegro PCB Designer and Cadence’s full suite of design tools are ideal for creating a layout for a new device, simulating the device’s behavior, and preparing for manufacturing. No other product provides these capabilities in a single platform.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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