Switchless Bi-Directional Amplifier

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Proceedings of Asia-Pacific Microwave Conference 2006

Copyright 2006 IEICE

Switchless Bi-directional Amplifier

Chi Sun YU, Ka Tsun MOK, Wing Shing CHAN and Sai Wing LEUNG

Department of Electronic Engineering, City University of Hong Kong, Kowloon,Hong Kong, SAR PRC

Tel: +852-27844304, Fax: +852-27844712, E-mail: [email protected],

[email protected]

Abstract Traditional radio frequency bi-directional amplifiers rely on switches with theirassociated increase in cost, size and loss to achievetime division duplexing. A switchless bi-directionalamplifier is proposed whereby the power amplifier(PA) and Low noise amplifier (LNA) are connecteddirectly in an anti-parallel configuration.Experimental results show that there is no gaindegradation when compared with the individualpower amplifier and low noise amplifier.

Index Terms Bi-directional, Low NoiseAmplifier, Power Amplifier, Switchless, TimeDivision Duplexing

I. INTRODUCTION

Radio transceiver chips designed for standard

wireless communication applications are

becoming more integrated. In some applications

[1], the whole of the RF front end, including the

RF filter and T/R switch, is integrated into the

transceiver chip which only leaves a single RF I/O

pin for the antenna connection. This pin is used

for both transmission and reception under time

division duplex, TDD. In this case, if we want to

add an external power amplifier to increase the

transmitter output power or add an external low

noise amplifier to enhance receiver sensitivity, a

bi-directional transmit/receive module is typically

used [2, 3]. This configuration normally uses two

single pole double throw (SPDT) RF switches to

toggle between the transmitting and receiving

paths; two SPDT switches make the configuration

more expensive and also lowers the performanceby increasing the insertion loss. If these RF

switches can be eliminated and yet still provide bi-

directional amplification, it will be an ideal low

cost and simple solution for portable wireless

communication applications.

II. PROPOSED IDEA

In this paper, a switchless bi-directional

amplifier is proposed for a highly integrated radio

transceiver front-end whereby the power amplifier

(PA) and Low noise amplifier (LNA) isconnecting directly in anti-parallel as shown in

Fig. 1. The direct connection has been proven to

work well [4] but had limitations in the required

ON/OFF impedance, which the work here will

address.

In this configuration, one end of the bi-

directional amplifier module is connected to the

antenna and another end is connected to the RF

I/O of the radio transceiver chip. When

transmitting, the control signal TX_EN is used to

enable the power amplifier for operation while the

RX_EN is used to disable the low noise amplifier.

The transmission signal from the RF I/O will be

amplified by the power amplifier and then fed to

the antenna for radiation. When receiving, control

signals TX_EN and RX_EN will be toggled to

disable the power amplifier and to enable the low

noise amplifier. Received signal from the antenna

will be amplified by the low noise amplifier and

then fed to the RF I/O.

Fig. 1. Proposed bi-directional amplifier concept

For this concept to work the OFF state

impedance for both the power amplifier and the

low noise amplifier must be high (ideally an open

circuit). Since the OFF state input and output

impedance of the power amplifier and low noise

amplifier is not an ideal open circuit, the poweramplifier and low noise amplifier will load each

other during transmission and reception


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respectively. The power amplifier maximum

output power, power-amplifier/low-noise-

amplifier gain and low noise amplifier noise figure

will correspondingly be affected. In order to

minimize this effect, four transmission line lengths

(TL1, TL2, TL3 and TL4) were carefully selected

at both the input and output of the power amplifierand low noise amplifier as shown in Fig. 2.

Fig. 2. Proposed bi-directional amplifier with optimized

transmission lines

Four transmission lines are used to transform

the input and output impedance of the power

amplifier and low noise amplifier to a higher

impedance value during its OFF state, and remains

matched during its ON state. This is because the

input and output impedance of the low noise

amplifier and power amplifier are designed to be

close to the transmission line characteristic

impedance Z0, the four transmission lines will not

have much effect on the input and output

matching conditions during the ON state.

Therefore, whilst Z1 and Z2 are transformed to

high impedances, Z3 and Z4 remain close to Z0

when transmitting; Z1 and Z2 remain close to Zo

while Z3 and Z4 are transformed to high

impedances when receiving.

III. EXPERIMENTAL VERIFICATION

A circuit was designed for the 2.4GHz ISM

band to verify this idea and is shown in Fig. 3. An

Infineon BFP650 NPN RF transistor was used for

the transmit power amplifier while the Infineon

BFP620 NPN RF transistor was used for the

receive low noise amplifier. The BFP620transistor is in the common emitter configuration

and biased with a supply voltage, VCC1, of 1.5V

and a collector current of 5mA. R1 is used to set

the bias current, L1 is the choke, C1 and C2 are

the DC blocking capacitors and L2 is for output

matching. RX_EN was 1.5V to enable the low

noise amplifier while TX_EN was 0V to disable

the power amplifier during reception. The BFP650

transistor is also in the common emitterconfiguration but biased with a supply voltage

VCC2 of 2.0V and a collector current of 50mA.

R2 is used to set the bias current, L3 is the choke,

C3 and C5 are the DC blocking capacitors and

capacitor C4 is used for input matching, TX_EN

is 2.0V to enable the power amplifier while

RX_EN is 1.0V to disable the low noise

amplifier during transmission. The negative

control voltage was used here is to avoid the low

noise amplifier being turned ON by the power

amplifier at the higher output power level.

Fig. 3. Circuit diagram of proposed bi-directional

amplifier

The BFP650 power amplifier and BFP620 low

noise amplifier stages were designed separately at

2.442GHz with the input and output impedance of

both power amplifier and low noise amplifier

matched close to 50 using lumped components

in its ON state. The input impedance, output

impedance and transducer power gain GT of the

individual BFP650 power amplifier and BFP620

low noise amplifier were measured in its ON and

OFF states and are recorded below in TABLE I,


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TABLE III

INPUT IMPEDANCE,OUTPUT IMPEDANCE AND

TRANSDUCER POWER GAIN

OF BI-DIRECTIONAL AMPLIFIER

Bi-directional amplifierTransmission

Bi-directionalamplifier Reception

Z6 = 62.6+j32.2 Z6 = 118.6+j21.9 GT= 14.0dB GT= 14.4dB

Z5 = 49.1+j52.4 Z5 = 87.5+j7.0

where the notation of the input and output

impedance refers to figure 2. The input power ofthe BFP650 power amplifier was set to 0dBm

while for BFP620 low noise amplifier it was set

to 20dBm. The noise figure of the BFP620 low

noise amplifier was measured to be 1.43dB during

its ON state using a noise figure meter.

The four transmission line lengths (TL1, TL2,

TL3 and TL4) were calculated to provide phase

shifts to transform the OFF impedances to a

higher value. Transmission lines were

implemented using semi-rigid cables with outer

diameter of 2.2mm and inner conductor diameterof 0.65mm. The length of these four transmission

lines were calculated to be TL1=12mm,

TL2=26.4mm, TL3=16.8mm and TL4=43mm

respectively, and the measured input impedance,

output impedance and transducer power gain GT

of the power amplifier and low noise amplifier

after insertion of these transmission lines are

shown in TABLE II.

After insertion of these transmission lines, the

impedance of both the power amplifier and low

noise amplifier increased significantly in the OFF

state. Subsequently, a bi-directional amplifier was

constructed using these transmission line lengths

as shown in figure 3. Since the bi-directional

amplifier is targeted for portable wireless

applications, the gain and power level of the

power amplifier were adjusted to the proper levels

suited for typical 2.4GHz portable wireless

applications. The input power to the power

amplifier was set to 0dBm and the input power

level to the low noise amplifier was set to -20dBm.

Control signals TX_EN and RX_EN were used to

control the power amplifier and low noise

amplifier operation respectively. Then, the inputimpedance, output impedance and transducer

power gain GT of the bi-directional amplifier

were measured under its transmission andreception state respectively as shown in TABLE

III.

From the measurement results, the gain of the

bi-directional amplifier was 14.0dB during

transmission and 14.4dB during reception. Since

the input power during transmission was 0dBm

and the gain measured was 14.0dB, the output

power level of this bi-directional amplifier was

14.0dBm, which fulfills the typical Bluetooth

Class 1 output power requirement. The noise

figure was measured to be 2.3dB and giving a

0.87dB degradation, which is better than those

achieved using low cost PIN diode switches [5, 6]

when compared to at 2.442GHz.

In particular, there is no gain degradation during

the transmit state when comparing with the

individual power amplifier; in fact, the gain

increased by 0.3dB during the receive state when

compared with the individual low noise amplifier.

IV. CONCLUSION

In this paper, a switchless bi-directional

amplifier is proposed and examined at 2.442GHz.

14.0dB gain was achieved under transmission

with 14.0dBm output power while 14.4dB gainwas achieved under reception, which is suitable

TABLE II

INPUT IMPEDANCE,OUTPUT IMPEDANCE AND TRANSDUCER POWER GAIN OF

POWER AMPLIFIER AND LOW NOISE AMPLIFIER WITH THE FOUR TRANSMISSION LINES

PA ON state PA OFF state LNA ON state LNA OFF state

Z4= 46.5-j22.2 Z4

= 778.6-j7.5 Z1

= 47.2-j19.2 Z1

= 425.3+j405.3

GT = 13.9dB GT = -15.8dB GT = 14.4dB GT = -14.5dB

Z3= 55.9-j5.3 Z3

= 525.7-j252.2 Z2

= 63.3-j26.0 Z2

= 476.8+j189.4

TABLE I

INPUT IMPEDANCE,OUTPUT IMPEDANCE AND TRANSDUCER POWER GAIN OF

INDIVIDUAL POWER AMPLIFIER AND LOW NOISE AMPLIFIER

PA ON state PA OFF state LNA ON state LNA OFF state

Z4 = 53.5 j13.6 Z4 = 113.8+j282.6 Z1 = 83.6 j0.2 Z1 = 10.2+j55.9

GT = 14.0dB GT = -15.8dB GT = 14.1dB GT = -14.5dBZ3 = 45.9 + j11.4 Z3 = 6.9+j32.9 Z2 = 33.3 + j10.8 Z2 = 6.4-j10.4


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for Bluetooth Class 1 applications. There is no

gain degradation during both transmit and receive

states. Although the noise figure was degraded by

0.87dB in the receive state, it is similar to that

obtained with a switch and is acceptable for

Bluetooth applications. The concept behind the

switchless bidirectional amplifier has been provento work and the elimination of two SPDT RF

switches is ideally suitable for low cost

applications.

ACKNOWLEDGEMENT

The authors would like to acknowledge the City

University of Hong Kong [research grant number

7001780] for the financial support of the work

presented here. We would like to acknowledge

Agilent Technologies for the use of ADS software.

REFERENCES

[1] Zeevo ZV4002 Product Brief, Single ChipBluetoothTM Solution for Embedded Applications.

[2] W.R. Wissenman, L.C. Witkowski, G..E. Brehm,R.P. Coats, D.D. Heston, R.D. Hudgens, R.E.Lehmann, H.M. Macksey and Q.Q. Tserng, X-

Band GaAs Single-Chip T/R Radar Module,Microwave Journal, September 1987, pp.167-172.[3] Zeevo ZV4301/4002 Class-1 Design Guide APP-

1035, version 3.0 23Sep2004.[4] T.K. Lee, W.S. Chan and T.Y.M. Siu, Power

amplifier/low noise amplifier RF switch, IEEElectronics Letters, Vol. 36, No. 24, 23rdNovember 2000, pp.1983-1984.

[5] An SPDT PIN Diode T/R Switch for PCNApplications, Agilent Technologies ApplicationNote 1067.

[6] DECT(1.9GHz) Transmit-Receive PIN-DiodeSwitch, Infineon Application Note No. 007.

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Switchless Bi-Directional Amplifier