LTE Band 3 Medium Power Amplifier

Low power RF amplifiers for cellular bands can be obtained as off-the-shelf items without too great a cost. Whereas amplifiers which are capable of producing a few watts or more of RF power tend to become quite expensive. Meanwhile the advent of spectrum licensing schemes which permit operation of medium power systems — such as Ofcom Shared Access in rural designated areas — has meant that affordable medium power RF amplifier solutions are needed.

The objective is to create a low cost RF power amplifier for LTE Band 3 (1800 MHz), which can be used together with another amplifier as a driver stage, between the SDR and the final PA. A target minimum modulated power output of 32dBm (1.6W) has been set, which into an 8dBi gain antenna would result in an EIRP of 40dBM (10W).

Note

Modern wireless systems such as LTE use waveforms which have a high Peak-to-Average Power Ratio (PAPR), which means that a power amplifier back-off of at least 6-7dB is typically required in order to maintain linearity in the PA. This is essential for ensuring both good system performance and wireless regulatory compliance.

Version 0.1

The initial design makes use of a BGY2016 three-stage UHF amplifier module, which is specified to have a gain of at least 28dB and a CW (not modulated) output power of 16W. The reason for selecting this particular device is that, while it is not a current part, it is available in large numbers via sites such as AliExpress and can cost as little as around £8 in single quantity.

Design

B3 MPA v0.1 schematic diagram.

The BGY2016 data sheet provides a reference layout for fabrication using a high performance PCB laminate which is commonly used in professional RF designs. However, since this is quite expensive, the design has been modified to use the OSH Park 4-layer Prototype Service (FR408-HR substrate), which is much lower cost and has previously been used with good results in a number of RF designs.

B3 MPA v0.1 PCB layout.

Warning

The design should not be fabricated using a regular 2 layer PCB service, since this will not provide sufficient impedance control. Other appropriate substrates could be used, however, the microstrip traces would need to be recalculated and modified accordingly.

The PCB design was entered using KiCAD. The project files have been published to the lc-b3-mpa repository. This repository also includes the mechanical design for a CNC machined aluminium palette, which is used to thermally couple the RF module to a heatsink. The palette design also assumes a board fabricated using the OSH Park 4-layer substrate and similarly may need to be modified if some other stack-up were used.

Bill of Materials

B3MPA BOM

Component	Description	Manufacturer	Man. Part	Distributor	Dist. Part
U1	UHF Amplifier	Philips	BGY2016
J1-2	Board edge mount SMA connector	Wurth Elektronik	60312202114514	RS Components	181-0102
C4,8	10pF MLCC capacitor	AVX	06035A100KAT2A	RS Components	135-6311
C3,7	100pF MLCC capacitor	AVX	06031A101JAT2A	RS Components	135-6575
R1-2	10Ω 1W 2512 SMD resistor	KOA	RK73HW3ATTE10R0F	RS Components	123-4469
FB1-2	Grade 4S2 ferrite bead	Ferroxcube	BD3/1/4-4S2	Farnell	242500
C2,6	10nF MLCC capacitor	Kemet	C0603C103J5RACTU	RS Components	801-5316
C1,5	10uF SMD electrolytic capacitor	Panasonic	EEEHA1V100R	RS Components	747-2904
Misc	0.5mm enamelled wire

P1dB Test Results

To test the P1dB compression point of the B3MPA, a continuous wave signal at 1878.4 MHz (centre of the Ofcom Shared Access 1800MHz spectrum allocation) with the power increased from -1 to 18dBm was applied to the input. The output was was connected via a fixed 30dB attenuator to a Rohde & Schwarz ZVL13 equipped with the spectrum analsyer option.

Equipment utilised

Component	Manufacturer	Man. Part	Serial no.
Function generator	Rohde & Schwarz	SMB 100A
Network analyser	Rohde & Schwarz	ZVL 13
30dB attenuator	Inmet	18N50W-30dB	64671
SMA cable from function generator output		FA210A0009153030
SMA cable from amplifier output		FA210A0009153030
SMA cable to network analyser input	HUBER+SUHNER	Sucoflex 104PE	248121 004

The test procedure comprised of the following steps:

1. Set function generator to desired output power within range -1 to 18dBm

2. Turn on function generator output

3. Wait ten seconds for amplifier to settle

4. Make note of output power, supply rail currents and amplifier temperature

5. Turn off function generator output and repeat

For input powers of -1 to 12dBm the internal attenuator on the ZVL was set to 25dB and then increased to 30dB for a 13-18dBm input drive.

Input power (dBm)	Amplifier output power (dBm)	Gain (dBm)	26V current (mA)	5V current (mA)	Temperature (°C)
-1	31.99	32.99	669	62	26
0	33.09	33.09	726	62	26.3
1	34.09	33.09	786	63	26.5
2	35.06	33.06	855	63	26.9
3	36.02	33.02	931	64	27.0
4	36.94	32.94	1014	65	27.2
5	37.97	32.97	1122	67	27.1
6	38.94	32.94	1225	68	27.3
7	39.65	32.65	1335	69	27.4
8	40.38	32.38	1447	71	27.6
9	41.03	32.03	1562	72	27.6
10	41.55	31.55	1677	74	27.7
11	41.96	30.96	1792	76	27.8
12	42.28	30.28	1916	77	28.0
13	42.56	29.56	2048	80	26.1
14	42.74	28.74	2175	81	27.0
15	42.86	27.86	2297	82	27.0
16	42.94	26.94	2401	83	27.2
17	42.99	25.99	2486	83	27.7
18	43.01	25.01	2553	84	27.8

We can see from the above results that the gain drops from ~33dB to ~32dB at an output power of 41dBm, which is 1dB below the specified output (12.5W instead of 16W CW). If we were to then back-off the output power by 7dB for use with an LTE signal, this would give us a maximum useful power output of 34dBm (2.5W). Hence we should be able to meet our target modulated output power with this design.