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A Selective RF Low Noise Amplifier

Kin-Keung Lee ([email protected])

Yan Lu ([email protected])

Radio project 2009

Department of Electrical and Information Technology,

Lund University

Supervisor: Göran Jönsson

Report of Radio Project 2009, By K. Lee and Y. Lu

1

Abstract

Low Noise Amplifier plays a key role in the front-end circuit of RF receiver. The usual

requirements are high gain, low noise as well as good input and output matching. In this

project, we provide a selective LNA works in FM broadcast band (88MHz – 108MHz). It

successfully achieves 25dB in-band transducer gain and 18dB mirror frequency rejection.

Table of Contents

Preface .................................................................................................................................... 2

1. Specification .................................................................................................................... 3

2. Circuit Design ................................................................................................................. 3

2.1 System Design ....................................................................................................... 3

2.2 Transistor Design .................................................................................................. 3

2.3 Amplifier Topology ............................................................................................... 4

2.4 Filter Design .......................................................................................................... 5

2.5 DC Biasing Circuit Design.................................................................................... 6

2.6 AC Simulation Result ........................................................................................... 7

2.7 PCB design ............................................................................................................ 7

3. Measurement results........................................................................................................ 8

4. Discussion ..................................................................................................................... 11

5. Conclusion .................................................................................................................... 12

Acknowledgements .............................................................................................................. 13

Reference.............................................................................................................................. 13


2

Preface

In analog super-heterodyne receiver, the selective RF amplifier was usually placed in the

front-end of the circuit, just after the antenna receiving block. The main task of this stage is

to amplifier the received weak radio signals for further processing. Besides that, with an

adjustable band-pass filter working together with the local oscillator, the wanted radio

frequency tone could be easily converted to a fixed IF.

According to the Friis’ Formula, the front-end stages will provide more contributions to the

total system output noise. As a result, the noise figure (NF) of this RF amplifier stage

should be kept as low as possible.

Meanwhile, the frequency rejection capacity is also quite important for analog

super-heterodyne receiver. A normal rejection requirement between signal tone and its

mirror frequency part is around 20dB.


3

1. Specification

– Operating frequency: 88 – 108MHz

– Noise figure: F ≤ Fmin +3dB

– Gain: G ≥ |S21|2

– Image rejection ≥ 20dB

– Vcc: 12V

– Source impedance: 50Ω

– Load impedance: 50Ω

2. Circuit Design

2.1 System Design

In order to achieve good mirror frequency rejection performance, we introduced a LC

band-pass filter after the amplifier. The system is depicted in Figure 1.

Figure 1. System overview

2.2 Transistor Design

BFG520X [1] was chosen in this project because of its good balance between noise and

gain performance (see Figure 2) and it was recommended in this course.

Figure 2. Noise and gain performance of BFG520X

Because most of the parameters in the datasheet were measured under the condition that Ic


4

= 20mA and VCE = 6V, we also used these values in our project to get rid of the parameter

(except S-parameters) extraction.

2.3 Amplifier Topology

Since the gain requirement was not very severe, the single stage common-emitter structure

was adapted. The measured S-parameters at 98MHz were:

S11 = 0.648598∠–52.7598˚ S21 = 31.4684∠143.533˚

S12 = 0.0175707∠66.135˚ S22 = 0.776362∠–30.3839˚

And ∆ = 0.586 and K = 0.289, the transistor was conditional stable and matching networks

were needed to guarantee the stability. In our design, the output was perfectly matched to

the load, so that we could use the input matching network to achieve wanted NF.

Figure 3. Gain, noise, stability circles and reflection coefficients

The MATLAB toolbox DESLIB was used to design the amplifier. The gain, noise and

stability cycles were plotted and we also used them to calculated required reflection

coefficients (see Figure 3). ΓS was selected to 0.447∠–63.4˚ (i.e. ZS = 50 – j50Ω) so that

the input matching network could be utilized by a single capacitor (it also provided the DC


5

coupling function). Because of the perfect matching at the output, an L-network was added

to the output and ΓL =Γout*. The schematic is shown in Figure 4. The values inside the

brackets are the real components values. The capacitor at the output is a variable capacitor

to provide selective filtering function.

Figure 4. Schematic of the amplifier

The simulated transducer gain (with real and calculated values) of the amplifier is shown in

Figure 5.

Figure 5. Transducer gain vs. frequency

2.4 Filter Design

An LC Band-pass filter was added to filter-out the image signal and, at the same time,

minimize the in-band attenuation. The brief specification of the filter was as following:

- In-band frequency: 88 – 108MHz

- In-band attenuation: < 2dB

- Transition band: 11MHz

- Stop-band attenuation: > 20dB

By using the design procedure in [2], the filter order was calculated to be at least 3.5. Since

the matching networks also provided some sort of filtering, we used a third-order


6

band-pass filter. The schematic of the filter is shown in Figure 6. The transfer function of

the filter is shown in Figure 7. With real component values, the in-band attenuation was

about 3 dB and the attenuation at 119MHz was about 10dB.

Figure 6. Schematic of the third-order LC band-pass filter

Figure 7. Transfer function of the LC filter

2.5 DC Biasing Circuit Design

The DC biasing circuit is depicted in Figure 8, the most important advantage is its

insensitivity to temperature and current gain variation [2]. C1 and C2 were 470pF and acted

as decoupling capacitor. The RF choke prevented the AC signal disturb DC biasing circuit.

The calculation of the component values was simple.

First, if we assume · and · (where β = 120 according to the

datasheet). Then, Ω=++

−= 273

BDC

CECCC

III

VVR .

We can further assume V42 =⋅= BD VV , then,

- Ω=+

−= k034.11

BD

DCCB

II

VVR


7

- Ω== k19.22

D

DB

I

VR

and

- Ω=−

= k98.113

B

BDB

I

VVR

The real values of RC, RB1, RB2 and RB are 270Ω, 1kΩ, 2.2kΩ and 12kΩ respectively.

Figure 8. Schematic of DC biasing circuit

2.6 AC Simulation Result

After the design of the amplifier and filter were done, an AC simulation was done using

Agilent ADS. The result is shown in Figure 9. The maximum gain was 29.5dB and the

image rejection was 19.9dB.

Figure 9. AC simulation result

2.7 PCB design

The PCB layout is shown in Figure 10. The vias were used to make connections between

the top and bottom planes


8

Figure 10. PCB layout

3. Measurement results

From the measured S-parameters (see Figure 12) of the whole system, we could find that

the transducer gain (S21) had a flat performance around 25dB during the FM radio

bandwidth, which was larger than |S21|2 and full filled the specification. The peak frequency

was around 99MHz, not far from what we expected. The feedback (S12) was kept as low as

-35dB for the entire measured frequency range (50-150MHz). However, there was an

unstable region at the input. It may be because ΓL (was set to Γout* in the design) was too

close to the output stable circle.

The 3dB bandwidth for this amplifier was from 87MHz to 111MHz, with a peak value of

25dB at 98MHz. The transducer gain was flat during the FM band.

Figure 11. Measured transducer gain


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Figure 12. Resultant Smith Chart

By turning the variable capacitor, we successfully moved the peak frequency to 92MHz,

with a 3dB bandwidth form 84 to 102MHz.

Figure 13. Mirror frequency rejection

The task of mirror frequency rejection was achieved by a 3rd order Butterworth band-pass

filter. It can be seen in Figure 13, an 18dB mirror frequency rejection was achieved the


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designed center frequency 98MHz. Actually, there were trade-offs between circuit

complexity and performance. If using high Q value components or hiring higher order

filter, we could get an even better mirror frequency rejection performance.

The 1dB compressing point was measured using VNA. The worst output compression

point in-band was 2.4dBm, and that will make an input limit at around -23dBm during this

power supply option.

Figure 14. Result of 1dB compression point measurement

The 3rd order interception point was measured by Spectrum Analyzer. The trend was also

plotted based on the measured results. We can speculate a 3rd order interception point for

98MHz is around +16dBm.

Figure 15. Result of IP3 measurement

-60

-50

-40

-30

-20

-10

0

10

20

30

-80 -60 -40 -20 0

Ou

tpu

tt P

ow

er

(dB

m)

Input Power (dBm)

IP3 Measurements

Fundumental

3-rd Harmonic


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The noise performance is shown in Figure 16. The blue and red curves represent the gain

and the NF of the amplifier respectively. We can find the NF was between 2 to 3dB for the

whole bandwidth, which fulfills the specification. The peaks in the noise curve were due to

outside-world radio interference. They could be eliminated if the measurement was carried

out in a shield room.

Figure 16. Result of noise measurement

4. Discussion

To perform frequency selection is a challenge in this design. We had to make trade-offs

between circuit complexity and performance. Meanwhile, with the mirror frequency

rejection filter, implementing the frequency selection by a LC tank became even more

difficult. Because the frequency response of band-pass filter strongly affected the turning

circuit. Actually, turning the shunt capacitor in the filter was more effective according to

the MATLAB simulation, the transfer function is shown in Figure 17, however, such big

variable capacitors were not available in our lab.

Another trick came from the bias circuit. Because the current gain of the transistor was

different to the expected, the value of RB2 was increased to 2kΩ.


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Figure 17. Transfer function of the filter under different shunt capacitance

Finally, because of the on-board parasitic capacitance, the operating frequency range was

different from our calculated value. As a result, we changed the shunt capacitor in the

band-pass filter to 180pF (originally 330pF).

The final schematic is shown in Figure 18.

Figure 18. Schematic of the whole circuit

5. Conclusion

We successfully designed a Selective Low Noise RF amplifier in this project. Most of the

specifications are met. The 3dB bandwidth of this amplifier covers the whole FM radio

frequency range; in-band transducer gain is around 25dB, the mirror frequency rejection is

18dB. In addition, by turning the capacitor, this circuit also shows a frequency selective


13

characteristic. The measured 1dB compressing point and IP3 point are around normal

values and will not be a problem for FM receiver.

The future work may include a better band-pass filter which could combine output

matching and frequency selection together. That will also save components.

Acknowledgements

We would like to express our great thanks to Göran Jönsson, Dept. of Electrical and

Information Technology of Lund University. He gave us many pieces of precious advice

during the whole work and always be patient to our mistakes. We also like to thank Lars

Hedenstjerna, who made this so beautiful PCB for us. Special thanks to Joakim Ericsson,

Wang Jing and Anders Dahlström from Sony Ericsson for giving us technical opinions and

sharing their industrial experience with us throughout the course.

Reference

[1] “BFG520X Product Specification”,

Website: http://www.nxp.com/acrobat/datasheets/BFG520XR_N_4.pdf

[2] L. Sundström, G. Jönsson and H. Börjeson, “Radio Electronics”, 2004

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