*3.2.1 Presentation du transistor*

The transistor amplifier are very recurrent microwave amplifiers their characteristics depend on the properties of the Transistor for the design in our work we looked at the FET transistor sp\_aiiAF035P1\_00–19941209 manufactured by Alpha Industries so we will first illustrate the characteristics of our transistor namely the input and output reflection coefficients S11, S22 the transmission coefficient S21 which represents the transmission from the input to the output or the own gain of the transistor and of the future amplifier S12, represents the isolation or the reverse transmission from the output to the input the drain source voltage Vds = 5 V and the current Ids = 70 mA. We will therefore present and illustrate the S-parameters listed below in **Figure 5**.

The S-parameters of the transistor obtained are as follows: S11 = �1.067 dB, S22 = �0.219 dB, S21 = �11.164 dB, S12 = � 35.289 dB. We realize that the inherent gain of the transistor is very low, it is imperative for us to improve this gain as well as the characteristics of the amplifier in order to produce an amplifier that meets the constraints imposed by 5G technology.

#### *3.2.2 Study of the stability of the transistor*

Considering K as the stability factor, the expression of the stability factor is given by the following formula:

*Co-Design Block PA (Power Amplifier)-Antenna for 5G Application at 28 GHz Frequency Band DOI: http://dx.doi.org/10.5772/intechopen.98653*

#### **Figure 5.**

*Schematic of the transistor and simulation of S-parameters.*

$$K = \frac{\mathbf{1} - |\mathbf{S}\_{11}|^2 - |\mathbf{S}\_{22}|^2 + |\boldsymbol{\Delta}|^2}{2|\mathbf{S}\_{12}\mathbf{S}\_{21}|}\tag{11}$$

and if j j Δ <1 and if K > 1 then the transistor is unconditionally stable [9] Δ: The determinant of the matrix S

$$
\Delta = \mathbf{S}\_{11}\mathbf{S}\_{22} - \mathbf{S}\_{21}\mathbf{S}\_{12} \tag{12}
$$

The Stability constant K of the transistor and mod\_delta Δ are respectively 1.267 and 0.861 whose transistor is unconditionally stable.

#### *3.2.3 Architecture of a microwave amplifier*

The architecture of a Microwave Amplifier is shown as follows (**Figure 6**).

### *3.2.4 Modeling of the microwave amplifier*

For the design of the amplifier, we will first start with impedance matching. The adaptation with Stub has been opted for by this document. The structure of the stub is shown in **Figure 7** below.

**Figure 8** below shows the S-parameters of the Stub.

The validation of our Stub is confirmed by an impedance adaptation to 50 ohms at the input and at the output the table below summarizes the result obtained (**Table 2**).

The above table presents the results of synthesis of the S-parameters of the stub at the resonant frequency of the transistor. From these results, it can be seen that:

**Figure 7.** *Stub structure.*

**Figure 8.** *Stub S-parameters.*

