The BTech AMP-U25D is an affordable amplifier for increasing the power output of both lower-power analog and digital communications. It offers good performance and reasonable efficiency (40-50%). A surprising and exciting result is that the amplifier outputs 25w with a 1w input, which enables the amplifier to amplify common 1w packet radio modules.
Starting this spring (roughly March 2018), BTech (a US company that’s a major importer of Baofeng radios) began shipping a power amplifier for 2m, 1.25m, and 70cm Handheld Transceivers (HT’s). The ostensible purpose of this amplifier is to allow an HT to function as a substitute for a mobile rig. To this effect, the amplifier provides features making it behave like a mobile rig, such as providing a microphone port and having a 4W speaker on the unit. The overall product line purports to take a 2-6w signal from an HT and amplify it to 30-40w. All of the amplifiers are designed for constant-envelope modulation schemes like analog FM and digital FSK.
There are five different variants of the BTech AMP series. Different models support different key frequency bands — 2m, 1.25m, and 70cm. Moreover, the 2m and 70cm amplifiers come in two versions, one version designed for non-TDMA frequency-based modulation standards, and another amplifier designed to support Time Division Multiple Access (TDMA)-based standards like DMR. The TDMA-friendly amplifiers are designed to rapidly switch on and off in response to the fast on/off cycling inherent to TDMA.
While these amplifiers are marketed to boost the transmit power of HT’s and make them work like a mobile rig, they also have a lot of potential for boosting the power of different IoT-oriented packet radio devices. Generally, low bitrate packet radio devices operating in the 70cm band use some sort of frequency-modulated, constant-envelope scheme like FSK or LoRa. As a result, they should be well-suited for amplification with these amplifiers. IoT packet radio systems also need to be able to switch quickly from transmitting a packet to receiving packets. As a result, the TDMA-friendly amplifier is likely the best choice for amplifying these transceivers.
Reviewed in this post is one model of the BTech AMP series, the AMP-U25D. The AMP-U25D is the 70cm model that supports TDMA. This particular model was chosen because I intend to use this amplifier to boost the transmit power of a LoRa transceiver running at 433MHz.
First, let’s start with the unboxing! First, I tried ordering it from Amazon. The first time, they sent me the 2m version, and then, when I tried to reorder the amplifier, the page no longer existed. After that, I successfully ordered it off of BTech’s website for $104.89.
There’s definitely some ambiguity as to what the actual power output of this amplifier is, as the different unboxing photos show. Besides that, the materials supplied are fairly straightforward. There’s the amplifier, at push-to-talk speaker/microphone, and some cables.
Measuring Power Output
Next, I attempt to measure the output power of the amplifier. With the medium power setting of the Baofeng F8HP HT, it’s apparently outputting around 35w.
At the F8HP’s low output (roughly 1W), the amplifier outputs approximately 25w. This is actually a really exciting result, as there are many packet radio modules made by companies such as HopeRF that can output at a maximum of 1w. As a result, this amplifier can be used by 1w RF modules to boost their output to 25w.
Next, it’s time to measure the power consumption of the amplifier. For all of these tests, the amplifier is being fed 13.8V. For the low power result (roughly 1w in, roughly 25w out), the amplifier pulls 4.45A of current, or 61.41w. For the medium power result (roughly 4w in, roughly 35w out), the amplifier consumes 5.31A, or 73.28w. Efficiency-wise, the amplifier is approximately 41% efficient when 1w is fed into it, and approximately 48% efficient when 4w is fed into it. This result suggests that sub-2w inputs take the amplifier out of saturation and into a more linear region. Potentially-interesting future work would be to study this amplifier’s output when fed with sub-1w inputs, and also to characterize the amplifier’s linearity when sub-2w inputs are fed into it.
Using the AMP-U25D with Packet Radio
Fed with a 1w input, the AMP-U25D was used to set the ground-based LoRa distance record, with the AMP-U25D being used to amplify the base station’s power output. The photo on the left shows the amplifier setup inside the case. The photo on the right shows the skin temperature after a full day of transmitting while sitting on the top of the roof of the patio of my house during an Albuquerque, NM summer afternoon/evening.