Bruno 1

Amplifier Design Project

Abstract:

The goal of this project was to design, simulate, and test a high gain 300 MHz amplifier in order to put theoretical RF knowledge into practice. This amplifier was originally designed for a piece of HP equipment; it preceded a band pass filter and came after a cascade of mixers. The amplifier was necessary due to the losses from the mixer. The amplifier is far enough down the cascade chain that the noise figure does not need to be considered. Also, since filters are particular about the source and load terminations the amplifier must serve as an impedance buffer between the mixers and the band pass filter. For the project stability, gain, and an L-section matching network were all calculated by hand and verified by MATLAB. The theoretical gain of the amplifier was 20.9 dB. The amplifier was then assembled and tested. The overall project was successful. The amplifier had a gain of 20.90 dB at 300 MHz, which is equal to the expected value.

System Specifications:

Transistor: BFR-91 NPN bipolar transistorOperating Frequency: 300 MHzVcc: 15 VIc: 20 mAVCE: 5 VBeta: 40Characteristic Impedance: 50 Ω

Scattering Parameters at 300 MHz:

S11 0.140∠-141.2S21 9.820∠95.9S12 0.040∠71.4S22 0.440∠-28.8

System Design:

Using the µ test, the BFR-91 transistor was deemed to be unconditionally stable at 300 MHz, µ = 1.12 > 1. Stability of the amplifier was also checked using MATLAB. Figure 1 shows that the amplifier is unconditionally stable because both the load and source stability circles lie completely outside of the Smith chart.

Bruno 2

Figure 1

Using the unilateral assumption (¿ S12∨¿0) the amplifier was designed with 0 dB of input section gain and maximum output section gain. The unilateral assumption can be made because S12 is sufficiently small, furthermore the maximum input and output gain was calculated using MATLAB. The max input section gain is 0.086 dB, this is so small it is not worth designing a matching network to maximize it. The maximum output section gain is 0.935 dB, sufficiently large to justify developing a matching network in order to maximize it. By fixing the normalized gain factors gS and gL to yield these gains, the constant gain circles represent both Γ S and Γ L, this information is graphed in figure 2.

Bruno 3

Figure 2

With this information the matching network can be developed. It will consist of a series capacitor and shunt inductor. This topology is superior to a series inductor and shunt capacitor because the capacitor acts as a DC block and the inductor will act as a RF choke along with providing the necessary impedance network to match the load.

Figure 3

The amplifier topology in figure 3 was chosen so the BFR-91’s emitter could be connected directly to the ground plain. By doing this the need for an emitter bypass capacitor could be eliminated. During the construction of this amplifier it will be important to keep the emitter lead as short as possible.

Bruno 4

Figure 4

Figure 4 is the measured value of the S21 parameter of the constructed amplifier at 300 MHz. From the measurement the magnitude of the S21 parameter is 20.90 dB, this represents the transducer gain of the network. This is in line with the theoretical gain of 20.9 dB.

Abstract:

Calculations:

µ Test∆=S22 S11−S12S21=0.337∠−16.8

μ=1−|S11|

2

¿ S22−∆S11¿ ∨+¿ S12S21∨¿>1 ¿

μ=1.12>1

Input Section Gain

Gs=1−|Γs|

2

|1−S11Γs|2=1

Gsmax=1

1−|S11|2 =1.02

Output Section Gain

GL=GLmax=1

1−|S22|2 =1.24

Bruno 5

Constant Gain Circle Parameters

gS=GS

GSmax=0.98

gL=GLGLmax

=1

CS=gSS11

¿

1−( 1−gS )|S11|2 =0.137∠141.2

RS=√( 1−gS )(1−|S11|

2)

1−( 1−gS )|S11|2 =0.137

CL=gL S22

¿

1−(1−gL )|S22|2 =0.44∠28.8

RL=√ (1−gL) (1−|S22|

2)

1−(1−gL )|S22|2 =0

Matching Network Parameters

LS=Z0

(−b ) 2πf=37.4nH

CS=1

(−x )Z0 2πf=8.84 pF

Bruno 6

Further Figures Used:

Finalized and Constructed 300 MHz Amplifier With output matching network

L section matching network

Γ L* = S22 = 0.44∠‒28.8°

z L* = 1.9−j1.0b = ‒0.71

x = ‒1.2

Bruno 7