Indian Journal of Radio & Space Physic Vol. 32, June 2003, pp. 172-176

Design and fabrication of a low noise amplifier for a microwave receiver

M K Raina & S S Inamdar

Department o f Electro nics & Communicati on, Bharati Vidyapeeth Co ll ege o f Enginee ring, A-4 Paschim Vihar, New Delhi I 10 063

and

Sushil Raina

Microwave Tube Research and Development Center, DRDO, Bangalore 560 013

Received 12 July 2002: revised 17 Janua ry 2003; accepted 7 March 2003

A low noise amplifier employed in the microwave/mm-wave radiometers is used to amplify a signal to overcome the inherent drift in the synchronous detector to achieve a higher degree of stability. A low noise amplifier designed and fabricated is described. The amplifier has been designed for a gain of 60 dB with noise less than 10 mY peak-to-peak, having an output impedance of !00 n and an input impedance of 1.5 Mn. The supply used for design consideration was 24 V DC.

1 Introduction Microwave/rum-wave radiometric systems record

very weak signals from the atmosphere and, therefore, need high gain receivers. The power required to operate a conventional pen recorder is of the order of 10-3 W. The change in power due to a radio source in the aerial beam may be as small as 10- 17 W. To record such weak signals the gain 1-

3 of the receiver should be 1014 or 140 dB.

In radiometric systems the total gain of the receiver is usually provided, before the detection, by the pre­amplifier and the I.F. amplifier and after the detection by a low noise amplifier. In this paper the design and fabrication of a low noise amplifier is described using Smart network CAD software.

2 Design considerations The design of any circuit will depend upon the

characteristics of the transistors and diodes. The development of the circuit was based on the availability of suitable transistors and bias evaluated on the basis of the characteristics of a particular transistor in use, which are provided in the manufacturers manual4

-6

. The specified collector-to­base breakdown voltage had to be taken care of to see that the collector is not connected to a source higher than the maximum voltage rating. In order to understand the principle of the bias scheme, consider Fig. 1 where one section of the circuit under design

has been shown. A supply voltage of 24 V is pro­vided. For a minimum distortion, the collector Q­point voltage should be 12 V [(24+2)=12 V]. The collector current Cic) at Q-point was found to be 8.16/lA, the base current 0.16/lA and emitter current 8.321J.A. The collector current of 8.16 llA was obtained by drawing the characteri stic curves of the transistor and choosing the Q-point near the middle of the DC load line. This value was the best that could be conceived.

After having arbitrarily selected the operating point (Q) we have to calculate the value of resistance R6, R7

and R8• The voltage drop across R6 and R7 should be such that the quiescent collector voltage is approximately half the supply voltage. The intercept made by the DC load line on the ic axis is 16.32 IJ.A. The value of Vcci(R6+R?)=16.32 11A; R6+R1=Vccl 16.32 llA = 24/16.32 llA =1.470 x 106 Q which has been split into R6=1 MQ and R7 =470 kQ.

The value of R8 is a trade-off between stability and gain. With an emitter voltage of 3.9 V and emitter current of 8.32 IJ.A, the value of the resistance R8 ""'

470kn. The value of R5 is dependent upon trade-offs

between the value of R8, current gain, stability and desired input impedance. As a basic rule, R5 should never be greater than 20 times of R8• A higher value of R5 will increase current gain and decrease stability. The value of R5 thus obtained was 9.4 MQ""' 10 M.Q.

RAINA et el. : DESIGN AND FABRJCATION OF LOW NOISE AMPLIFIER 173

R5 R8 IOMO 470k +

C6 20.UF

Fig. !-General bias scheme

+24V

The value of (R4+R3) is selected to provide a given base voltage at the Q-point. With a base voltage of 3.9 V and a base current of 0.16 JJ-A, the value of R4+R3=20 MQ, so that R3=R4=10 M.Q. The DC current is held constant against temperature variation by both the shunt degeneration action of R3+R4 and the series emitter degeneration action of R8.

In case of a bypass capacitor C6 across the emitter resistance R8, this arrangement permits R8 to be removed from the circuit as far as the signal is concerned, but leaves R8 in the circuit (in regards to direct current). The capacitive reactance of the capacitor is represented by XE. As a rule XE=R8110. However, for stiff value XE = R8/20=23500 Q; C6=1+(6.28x30x23500)::::: 20 JlF.

The value of C3 forms a high-pass filter with R5.

The high frequency limit can be ignored. When the reactance drops to approximately one half of the resistance, the output will drop to approximately 90% of the input (or approximately 1 dB loss). The low

frequency limit of say 30 Hz requires a capacitance value of 1/3.2 FX3 ; where X3 is the capacitive reactance of C3. As a rule, X3 should be 111 Oth of R5.

This will provide an approximate 1 dB drop at the low frequency. Thus, C3=1 +(3.2x30x1000000)=0.1 JJ-F; C5=1+(3 .2x30x1.47000) ::::: 8 JJ-F.

Based on the bias consideration as described above, the circuit diagram of the low noise amplifier is shown in Fig. 2. The transistors used are SG 107, CIL231, and BC108; whereas diodes used are: CA34, available from the local market. The SG 107 was chosen as an equivalent to 2N2848 whose speci­fications were found to be suitable and useful for the low rioise amplifier design .

The emitter-to-base voltage is the test voltage rather than the operating voltage7

. Some current is always flowing in the base-emitter junction, being 0.3 -0.6 V. Therefore, while making bias connections, this factor has to be borne in mind. Also, while considering base-emitter bias, the input signal that would be available has to be taken into consideration. Care has also to be taken to see that the transistor is not operated at or near the maximum current rating. The fabrication was done under simulated operating conditions by applying small input signal through a signal generator and setting the bias values in actual condition and checking the output across the collector on an oscilloscope. Each stage was fabricated separately and all stages are coupled together to find the total output corresponding to a certain known input8

-10

.

As shown in Fig. 2, the signal path is first routed through a low pass filter with R1=47 k.Q; R2=4.7 kn; C1=C2=0.1 JJ-F. Amplification follows through a high­gain two-stage low noise amplifier. The first stage has high input impedance, while the second stage feeds into an emitter-follower buffer stage. A potentiometer is inserted in the output circuit of the emitter follower. This control enables the gain to be set to an optimum level. This is basically a common collector (or we call an emitter follower) to match high impedance to a low impedance of the two stages of the amplifier. The potentiometer in the circuit varies the impedance, and where the impedance is perfectly matched, it gives high amplification at the collector output of TR4.

A further stage of amplification is used at TR5 to bring the signal level to the region of 15 V peak-to­peak. Finally, an _emitter follower is used to couple the signal to the phase sensitive detector at a low impedance level.

174 INDIAN J RADIO & SPACE PHYS, JUNE 2003

r C2 R!5

SPECIFICATION

GAIN 3000 - ( VOUT I VIN)

NOISE LESS THAN 10 mV PEAK TO PEAK OUTPUT lt.IPEDANCE IOO.ll. INPUT IMPEDANCE • 1.5 mll SUPPLY • 2~V D.C.

Rl8

+2~V

-<>

f-----11f---....---<>~~~k

NOISE LEVEL

LOW NOISE AMPLIFIER

SENSITIVE DEn:CTOR

toi-EXTERNAL IOO.UA t.IETER

Fig. 2---Circuit diagram of the low noise amplifier

The low frequency response is improved by splitting the collector resi stance such that the main part of it is bypassed by low frequency compensating capacitor having a value of 8 Jl F. The effect of this capacitor is to increase the gain at low frequencies, thus compensating for the loss in gain due to other bias and coupling capacitors. The emitter resistances have been bypassed by large capacitors to avoid a.c. regeneration. This shunt degeneration linearizes, but reduces the current gain and the input and output impedances and extends the frequency response.

Based on the circuit of Fig. 2, a printed circuit board was developed using software called SMARTWORK (version 2.1 by Wintek Corporation). The software requires a PC with at least 640 kB memory and a CGA card for its operation.

Sample copy of the printer output is shown in Fig. 3. These were then handed over to a PCB dealer who manufactured the actual PCB on glass-epoxy according to the layout. Typical DC voltages measured with a multimeter are shown in Table 1. The waveforms were also measured with an oscilloscope. An input of 5 m V was fed directly into the amplifier and various voltages measured are shown in Table 2.

The low noise amplifier generates very little noise of its own. The input noise level is 10 mY peak-to­peak when there is no input signal. This noise level can be considered as inherent to the amplifier.

2 x ar tw ork lno . pcb

V1 . 2 r2 holes : 101 Solder side approximate size : 3 .35 by 1.80 inches

Fig. 3------Printed circuit board of L N A

However, the gain is such that the incoming signal corresponding to the square modulation from the I.F. detector buried in the noise is picked up and amplified to a level where it can operate the phase sensitive detector. In fact, the amplifier is tuned to the modulation frequency having a gain of 60 dB. This gives a pure modulated signal devoid of any noise before being fed to the phase sensitive detector, giving best noise performance. The role of this amplifier is critical, particularly, with respect to the overall sensitivity of the radiometer system, which has been achieved to 1 K.

RAINA et el.: DESIGN AND FABRICATION OF LOW NOISE AMPLIFIER 175

Table !- Typical DC voltages measured under no signal 3 Performance of low noise amplifier A low noise amplifier is a very important

component in the radiometer configuration , particularly when gain fluctuations are considered in the post detection amplification. The inherent noise of the system is 10 mY under no signal conditions. But when a signal of say 5 mY is applied, the gain is of the order of 15 V and amplification is distinct and is seen to be above the 10 mY level of the internal noise. The noise figure of the amplifier is 2 dB. In a highly sensitive system this amplification becomes substantial. This is so, because the synchronous detector will have an inherent drift and to overcome this threshold, the signal voltage must be amplified to a level several times greater than the inherent instabilities of the synchronous detector. This is a special type of amplifier specifically developed for the microwave/mm-wave radiometric work, where one has to detect the signals usually of the order of 10- 17 W and thus has been rightly justified as a low noise amplifier. The performance of the low noise amplifier can be best understood by considering the block diagram of the radiometric system as shown in Fig. 4. The signal received by the antenna is passed through the modulator which switches the signal between the antenna and the reference load. This modulated signal is then fed to the mixer, where the

conditions

Values at

Typical DC voltages TR 1 TR2 TR3 TR4 TR5

v

Emitter to chassis + 3.2 +4 +II +8 +16

Base to chassis + 3.9 +4.7 + 11 .5 + 8.5 + 16.5

Collector to

chassis + 10.4 +12 0 + 16 + 24

Table 2- Voltages measured under signal conditions

Specification

TR 1 Collector (V)

TR2 Collector (V)

TR4 Collector (V)

TR5 Emitter (V)

Measured voltage (peak-to-peak)

v

1.5

3

15

15

Fig. 4--Biock diagram of the radiometric system

176 INDIAN J RADIO & SPACE PHYS, JUNE 2003

incoming signal combines with the local oscillator power to produce a sum and difference frequency, and only the difference frequency'' of 60 MHz is accepted by the I.F. amplifier having a bandwidth of 20 MHz and overall gain of 105 dB . The two noise levels produced by the temperature comparator after amplification at I.F. are detected by a simple amplitude demodulator to restore the modulated wave. This low level 45 Hz square-wave obtained at the output of the 60 MHz I.F. amplifier is completely buried in noise. The purpose of the low noise amplifier is to improve the signal-to-noise ratio and bring the signal level to a sufficiently large amplitude to operate a phase sensitive detector in a satisfactory manner. Thus a low noise amplifier has been designed and tuned to the modulation frequency which is having a gain of 60 dB. This gives a pure modulated signal devoid of any noise and is fed to the phase sensitive detector having an integration time of 6 s. It is, here, in the low noise amplifier that all the spurious noi ses generated in the preceding circuits are eliminated and is thus a crucial component in the radiometric system.

The performance of the fabricated low noise amplifier is outlined below:

Input impedance Input signal Output impedance Output signal Gain Noise Supply

= 1.5 MO =5mY = 100 Q approximately = 15 Y peak-to-peak maximum = 3000 approx. CVou/V;n) =Less than 10 mY peak-to-peak =+24 y

4 Discussion

A low noise amplifier has been fabricated on a PC board and tested by giving a small input signal and the corresponding amplification recorded on an oscilloscope. It is found that with an input signal of 5 mY, the final output obtained is 15 V (peak-to­peak) . Thi s amplifier is used to amplify the detected modulated signal from the I.F. amplifier before giving to the phase sensitive detector. Thi s forms a part of the microwave receiver used for the retrieval of the data by microwave/mm-wave radiometers.

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(1996) 188. 4 Lenk John, D, Handbook of simplified solid stare circuit

design (Prentice Hall Inc ., Englewood Cl iffs, N J), 1971. 5 Raina Sushil, Study of K-band radiometer system and

de velopment of low noise amplifier and a phase sensitive detector for the microwave receiver in M Tech Rep No.454/40192, Radio Science Divi s ion, National Physical Laboratory, New Delhi 110012, pp 1-5 1. 1992.

6 Raina M K & Rai na Sushi I, Rev Sci lnstrum (USA), 7 1 (2000) 1849.

7 Shea R F, Transistor Applications (J ohn Wiley & Sons, Inc NY), 1964.

8 Me lvino Paul , Transistor Circuit Approximations (T ata McGraw Hill , Bombay), 1968.

9 Gray P, Physical Electronics and Circuit Models of Transistors Vo/.2 (John Wiley & Sons, NY), 1965.

10 Millman J & Halkias C, Electronic devices and circuits (McGraw Hill Company Inc., NY), 1967.

II Raina M K & Raina Sushi I, Indian J Radio & Space Phys, 29 (2000) 363.