Interference-free RF amplification with multilayer ferrite inductors

The ubiquity of wireless communications is driving the need for general-purpose RF amplifiers that can be used in many categories, from 5G systems to numerous wireless IoT applications. When designing an RF amplifier, the first step is to define important parameters such as the target frequency and gain. By selecting the optimal passive components, the device’s transmission characteristics can be enhanced and improved. A well-designed layout that can transmit RF signals and the DC current to power the amplifier stage in a single transmission line without interference further improves RF performance.

A key component for this interference-free transmission of mixed signals (RF and DC) is the inductor for decoupling the RF and DC supplies. Next to a standard inductor, it’s also possible to use a multilayer ferrite inductor. An evaluation board of an RF amplifier was realized and measured to compare and evaluate the two alternatives.

RF amplifiers

To perform the measurement, two block amplifiers with similar gain, ADL5544 and HMC311ST89, both from Analog Devices, were selected. The ADL5544 is an unbalanced RF/IF amplifier with a gain of 17 dB and wideband operation from 30 MHz to 6 GHz. The IC has an integrated independent bias control circuit.

The board configuration can be adjusted to achieve the best performance in the particular frequency band. The recommended AC coupling capacitors C1 and C2 are between the values of 100 nF and 100 pF. For decoupling the RF signal, three different inductance values – 12 nH, 100 nH and 1000 nH – are recommended for the inductance L1, depending on the frequency band. This is normal in the design phase and gives the user the flexibility to determine the frequency spectrum and modify components accordingly.

The HMC311ST89 is a single-ended block amplifier. Similar to the ADL5544, it provides up to 16 dB of gain over a frequency range of DC to 6 GHz. However, this IC doesn’t have an integrated bias control circuit, so an external series resistor is required. For a more stable current source, a small, low-current, low-cost linear LED driver can be used. The BCR402W LED driver from Infineon, for example, offers better current regulation with very low voltage drop, unlike a series resistor.

The AC coupling capacitors C1 and C2 are all specified at 100 pF, but in the frequency range around 50 MHz, the target value is 0.01 μF, according to the datasheet. For inductor L1, in contrast, a series of different values, ranging from 3.3-270 nH, is recommended depending on the frequency.

Inductors

Since the two RF amplifiers have similar characteristics, the inductance is selected to cover a frequency spectrum as wide as possible in both circuits. The RF signal blocking inductor, also called an RF choke, is used to decouple high-frequency signals by providing a high impedance while allowing DC current to pass. Thus, the choice of choke depends on the amplifier’s operating frequency range. This means that chokes with different values are designed to perform their best decoupling in a given frequency range. The disadvantage is that if the design specifications change, a new inductor must be selected. This is where multilayer ferrites come in as an unconventional alternative to inductors, allowing universal use at a lower cost.

Multilayer ferrites are passive components with high attenuation over a wide frequency range. Their primary function is to reduce high-frequency noise in signal branches by acting as an impedance (resistor) in the high-frequency range, allowing DC signals to pass and ‘filtering out’ AC signals. Since they’re usually specified with impedance values, the inductance must first be calculated to assign it to the recommended application. This inductance can be estimated by taking the impedance at a specific frequency and dividing it by 2π times that frequency.

For air core inductors, factors such as saturation due to DC bias have no effect on maintaining inductance. In contrast, DC bias has a large effect on multilayer ferrites and the current flowing through the component changes the inductance, causing the impedance curve to change in the lower frequency range (< 100 MHz) and the self-resonant frequency to shift to higher frequencies at the same time. Therefore, a ballpark figure of the impedance as a function of the operating current should be considered to approximate the correct inductance in the application.

Comparison

Taking the RF amplifier evaluation board, several standard inductors and multilayer ferrites have been investigated for use as chokes. The WE-KI and the WE-KI HC are wire-wound ceramic inductors ideal for RF applications. The manufacturer’s recommended values of 56 nH and 100 nH form the basis for comparison with the alternative multilayer ferrites WE-CBF, WE-CBF HF and WE-TMSB.

The investigation shows that with the multilayer ferrite, a broader frequency spectrum and higher stability due to lower reflections, ie better matching, could be achieved. More specifically, the ferrite WE-CBF HF had a high and stable impedance and therefore a high decoupling with low peak resonances while the standard choke WE-KI had a lower bandwidth and visible resonances throughout. Comparing the two RF amplifiers, the HMC311ST89 with external bias was better than the ADL5544 with integrated bias. The goal to achieve a broader usable frequency spectrum application has been achieved.