GaAs MESFET karıştırıcılarda AF geribeslemenin gürültü sayısı üzerine etkisi
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Abstract
ÖZET Bu çalışmada, FET ve MESFET kullanılarak gerçekle nen kuvvetlendiricilere göre gürültü sayıları yüksek olan karıştırıcıların gürültü sayılarını düşürmek amacıyla uygun ara-frekans geribesleme yöntemleri ve geri besi emenin gürültü sayısı üzerine etkisi araştırılmıştır. îkinci bölümde. FET'li karıştırıcının gürültü sayısını asal tan iki ara-frekans geribesleme devresi verilmiştir. Bunlar kapasitif ve endüktif bölmeli ara-frekans geribesleme devreleridir. Analis sonuçlarına göre kapa sitif bölmeli ara-frekans geribeslemeli FET karıştırıcı nın gürültü sayısının, kaynak iç direnci 100 Ohm alındığında, yaklaşık 3,5 dB düşürülebileceği gösterilmiştir. Üçüncü bölümde, MESFET'li karıştırıcının gürültü sayısının kapasitif bölmeli ara-frekans- geribeslemesi uygulanarak düşürülebileceği gösterilmiştir. Analis sonuçlarına göre, iletim açısının 70 ve kaynak iç direncinin 10 Ohm olması durumunda, gürültü sayısında yaklaşık 5 dB lik bir asalma olacağı bulunmuştur. IV SUMMARY THE EFFECT OF IF FEEDBACK ON MESFET MIXER'S NOISE FIGURE In this work, the main purpose was the reduction of noise figures of JFET and MESFET mixers together with an appreciable amount of conversion gain. To accomplish this aim, the idea proposed by Strut t £93 was used. His idea was the reduction of noise figure by the application of feedback from output signal to input at intermediate-fre quency CIF3 in the triode mixer stages, but he did not made an attempt to clarify his idea. By the same purpose, van der Ziel T 33 applied RF feedback to FET mixer and he experimentally proved that it can be obtainable an appre ciable amount of noise reduction. But, theoretically or experimentally in puplished literature there is no work of applying IF feedback to the FET mixer circuits. To see the results of IF feedback in JFET and MESFET mixer stage, it is applied this kind of feedback to the aforementioned mixer stages by extending the the ory proved by Ziel to our circuits under some assumptions. In chapter 3, it is applied IF feedback to the JFET mixer. It is found two feedback circuit giving same re sults; the first one is the capacitive feedback of the IF output signal current CFigure 13 and the second is the inductive feedback of the IF output signal current. Noise figure analysis for capacitive feedback mix er circuit is performed as follows. It is assumed that IF noise produced by signal source and FET* s input con ductivity g can be neglected for the sake of simplicity. Induced gate noise and the correlation between the drain and the gate noise is assumed to be very small. At the output RF signal is short-circuited. In Figure 2 it is <s/ D Û <k == <s T 9s V 9n T1gt ?H- 9mlvl Q. 'do v R Figure 1. Capacitive divided feedback FET mixer equivalent circuit.IF equivalent circuit is shown. C and C are feedback elements, R IF load, i output noise current without feedback, g v output signal current. 9n Q Q 9mlvl 'do Figure 2. FET mixer IF equivalent circuit. By using Miller's theorem it is easily seen that the feedback presents an input conductance sf= (1+g R>.o>2C2E mi t 2 2 2 <i> to the input conductance. Where g is the conversion transconductance of the device anc? u>. intermediate-frequ ency. According to van der Ziel 'Ell 5, i,the noise for short-circuited output without feedback, fs i* =£.4kT.g. Af do mo (2) where g is the RF transfer conductance and ^=2/3 for a Junction°gate FET. Voltage across the capacitor C4 is found as ; v =- i 1oiCR(-i )Eg -wCCK-ju, «C+C +CRg +g EC>3 v i do n ». 1 v 1 t* fnl £q -ii>ZCC E32+<u> 2<C+C +CP.Q +g RC>2 ~n t i ı 1 `n rn± C3> VIIt- is easily seer» that vis real when ; 9n C = Ü. <4> 4 6>2CR v Using eq. C43 and CI 5 in eq. C3Ü> gives R gf Vi (l+g R>. (q +g > ( ido> ` (5> ml ~ r> 1 At the instant t, the IF noise at the output can be rep resented as i, =a. cosiij. (t-r> (6) de i After mixing, the output noise current due to voltage v is i -g gf£ q v =, `° pw t - ra.cos[w.(t-T>+WT] <7> -mo i <l+g RXg.+g_> ı p rol n f where uyp is local oscillator frequency. This noise must be added to a. cosa. Ct-iO. Here after, by following the mixer theory proved by van der Ziel [203, the IF noise with feedback is found as - - g i* =i*. (1-3F - ÎÎÜ+F2> C8> d do eg c where F is given by gmo9rR c <l+g RXg +g`>Applying noise figure definition to the circuit in Fig. 1, noise figure of IF feedback mixer is found as F=l+- mo,.2 <g +g >. 2 3a sn mı b 2gmiR 2-2 2 1 - Cl+g RXg +g > (1+g K)2Cg +g >2 rnx n i tni it x <io> Considered as a function of g, this has a minimum value if f a g R(l+g R> _ ~ r> mi aml 9f~ 2 `2 g R-g RCl+g R) mo mi mi <11 > Substuting into CI CD yields g ^S 9 =1 + + { g +g > (1 mın g 2 a n 2 * SmigB mi b mo (12> Without feedback Cg =CD, the noise figure i: g fg F<«. *^ mo -.2 =1 + + ( g +g ) g 2 '`s =n ^ml a C135 which is larger than F. if the condition 3 min 1- < 1 C14D is satisfied. Since g > g in a mixer, this condition can always be realized!`0 BuÇ, contradict or ally this con dition corresponds to the increase in conversion gain. VIIIIt can easily be shown that inductive divided feed back Cby replacing capacitors by inductors} gives the same results for noise figures with and without feedback. In chapter 3, The IF feedbacking in the MESFET mix er is considered. Simplified noise equivalent circuit of the MESFET transistor is shown in Figure 3 [163, [17], [183. In this equivalent circuit, parasitic elements and associated noise sources was neglected. i channel noise and square mean is given by nd represents i* = 4kT Af.g.P nd o m C15^ where g is the transconductance of the transistor. Gate noise induced capacitively by the voltage fluctua tions produced along the channel is represented as a noise source across gate and source electrodes, and its square mean is given by r.g = 4kT Af. uZ. C2/g CI 6} 3d Figure 3. MESFET's simplified noise equivalent circuit. In this equetions, P and Û are paremeters depending on the biasing conditions and MESFET's geometry. Since K.<<lx`C6>.C.} at IF, it is neclected. It is assumed that Lit IF noise produced by signal sources very small and at the the output RF signal is short circuited. Equivalent cir cuit of the IF feedback MESFET mixer is shown in Figure A. IXV R Fiği ur e 4,. MESFET mixer equivalent circuit. Feedback elements are C and C and a. Under the assump- i e ~n t i ons aforesaid, IF equivalent circuit of the mixer is shown in Fiqure 5. /N1 9n -r Q Q <2 Tdo T9*.!*! +/N R Figure S. MESFET mixer IF equivalent circuit. C is the par el lei equivalent of the C and C. capaci tors at IF. Noise analysis is performeâ as in ''Chapter 2. Here some important results are given. It is found v is real if C = 2 o2C R I 1 C17Z> Minimum noise figure of the MESFET mixer is given byV R Fiği ur e 4,. MESFET mixer equivalent circuit. Feedback elements are C and C and a. Under the assump- i e ~n t i ons aforesaid, IF equivalent circuit of the mixer is shown in Fiqure 5. /N1 9n -r Q Q <2 Tdo T9*.!*! +/N R Figure S. MESFET mixer IF equivalent circuit. C is the par el lei equivalent of the C and C. capaci tors at IF. Noise analysis is performeâ as in ''Chapter 2. Here some important results are given. It is found v is real if C = 2 o2C R I 1 C17Z> Minimum noise figure of the MESFET mixer is given byg2 R R ml L s g = <ai> c [C -.2 l+-^-İ=- g R /2 u2C2(R +R. > C +C m d L I a ı a l i 2 J 2 Obtained theoretical results are as follows. l.Mi: er with IF feedback: F. =3.4.7 dB, G =3.35 dB. 3. Mixer Tiran e without feedback: F =3.9 dB, G =8.3 dB. Tmm c XII
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