DIY 300 Watt FM Broadband Amplifier with SD2932 or SD2942

Date:2014/5/26 17:18:35 Hits:

This broadband power amplifier operating over the frequency range 88 - 108 MHz using the new STMicroelectronics RF MOSFET transistor SD2932.

Amplifier design

Input matching network

Typical input gate-to-gate impedance of SD2932 at 100 MHz is Zin = Rs + jXs = 2 - 2.6 j, and can also be expressed as the combination of parallel resistance and reactance using the following formulas:

Equation 1

Equation 2

Therefore, in order to achieve good input matching performances over the frequency range 88-108 MHz the unbalanced 50 Ω is to be transformed into an impedance with a value as close as possible to Rp of 5.38 Ω.

From the circuit schematic given in Figure 3 , we can see that the input matching network is based on a two section balun (1:1 balun in cascade with a 9:1 balun transformer) which transforms the unbalanced 50 Ω to a balanced 5.56 Ω (2 x 2.78 Ω / 9:1 ratio). The first section, a 5" long - 50 Ω coaxial cable and the second section, a two 3.9" long - 25 Ω flexible coaxial cables with ferrite core NEOSIDE, are connected as described: a 10 nH inductor (L1) is connected between the two gates to compensate SD2932 input parallel reactance Xp.

Input matching network tuning

Figure 1. Input Impedance of 1:1 balun in cascade with 4:1 balun

Figure 2. Input Impedance of 1:1 balun in cascade with 9:1 balun

SD2932 input matching network was tuned in order to achieve the best compromise in terms of power gain (Gp) and input return loss (Rtl) over the frequency range 88 - 108 MHz. Best results were achieved by adding a 10 pF chip capacitor (C1) between RFIN and the 1 nF blocking capacitor (C2).

Output matching network

The output impedance of each side is a combination of the output capacitance Coss (195 pF) and the optimum load resistance which can be determined as follows:

Equation 3

The total optimum load, seen by SD2932 (drain to drain), is 2 x 6.02 = 12.04 Ω. Therefore, a simple two section balun (1:1 balun in cascade with a 4:1 balun transformer) is used to transform the unbalanced 50 Ω to a balanced 12.5 Ω (2 x 6.25 Ω) which is very near to the total optimum load resistance.

The first section, a 5" long - 50 Ω flexible coaxial cable, and the second section, two 5" long - 25 Ω flexible coaxial cables, are connected as described in Figure 3.

To compensate for the output capacitance Coss of SD2932 , a 40 nH inductor (L2) is connected between the two drains. This LC network (L2 & Coss) is a high pass filter with a resonance frequency calculated at 10 % below the minimum operating frequency:

Equation 4