Protecting your power amplifier stage with analog switches


As the story of “The Hare and the Tortoise” taught us, sometimes it pays to be steady and calculated. With growing demand from consumers for higher bandwidth and speeds for their wireless data, the pressure is on semiconductor manufacturers to design systems that meet these requirements – much like how the hare focuses on being the fastest to reach the finish line. However, as the tortoise shows, it is just as important to be steady in this pursuit by ensuring that systems are rugged and reliable.

Because communications equipment, such as radio units and active antennas, is primarily based outdoors, it’s critical that internal components operate reliably regardless of environmental factors. Analogous to Aesop’s fable, systems must be high performing (like the hare), while being rugged (like the shell of the tortoise) to protect internal circuitry from external fault conditions. One way to ensure protection is to use an analog multiplexer, also known as a “mux,” to protect the internal power amplifier (PA) stage.

Why the PA stage?

Amplifier integrated circuits (ICs) use electric power from a power supply to increase the power of an input signal. By using an amplifier, you can produce a strong output signal from a weak input signal. For example, PAs are used to drive the loads of output devices, such as headphones, speakers, servos and radio frequency (RF) transmitters.

In the case of RF transmitters, RF PAs amplify low-level RF signals in massive multiple-input multiple-output (MIMO) antenna systems. Traditional massive MIMOs contain eight transmitter and eight receiver (8T8R) RF channels to amplify their antenna signal. In contrast, modern 5G systems will have up to 64T64R channels that increase download/upload data rates and throughput. Having this many channels in one remote radio unit requires protecting each channel from external fault conditions. A simple and cost-effective way to protect a system from these fault conditions is to use a 2-to-1 analog switch per channel, as shown in Figure 1.

PA stage protection per channel using an analog switchAs you can see from Figure 1, there are multiple PA stages based on the number of transmit and receive channels in the radio unit. Getting these PAs to function correctly requires applying a bias voltage (V-BIAS) to the gate of each FET. Unfortunately, V-BIAS is susceptible to external fault conditions such as overcurrent, overvoltage or overtemperature events that can exceed nominal safe values. In such cases, a field-programmable gate array or microcontroller detects the fault condition and immediately sends a select logic signal to the mux, disconnecting the V-BIAS signal path. Without the V-BIAS signal, the PA stage turns off, protecting the channel from the fault condition. Ultimately, the 2-to-1 analog switch turns the PA stage off in the event of a fault while providing a safe path to ground for the low-level RF signal (RF-IN). Analog switches, such as one-channel, 2:1 general-purpose analog multiplexers with 1.8-V logic control like the TMUX1219 or TMUX1247, can safely perform this function while operating at temperatures up to 125°C. Additionally, they can be directly controlled by 1.8-V field-programmable gate arrays or microcontrollers (MCUs) without the need for a level shifter due to their 1.8-V logic support. Read the application note, “Simplifying Design with 1.8 V logic Muxes and Switches” to learn more about the 1.8-V logic of these devices.

Protecting remote radio unit RF channels is critical because a fault event in one of these channels can significantly damage a system. With up to 64 channels per unit, this level of protection is critical to designing a high-bandwidth, high-speed system that has reliable performance. So if you keep the tortoise’s mindset when considering reliability and protection, you will remain in the race to meet the needs of next-generation networks.