Qualitative analysis of small-signal modified Sziklai pair amplifier

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 50, April 2012, pp Qualitative analysis of small-signal modified Sziklai pair amplifier Beena Pandey, Susmrita Srivastava, Satyendra Nath Tiwari, Jitendra Singh & Sachchida Nand Shukla Department of Physics & Electronics, Dr R M L Avadh University, Faizabad , UP, India sachida_shukla@yahoo.co.in Received 17 August 2011; revised 16 January 2012; accepted 6 February 2012 A small-signal modified Sziklai pair (complementary Darlington pair) amplifier is proposed with an additional biasing resistance in the circuit. The proposed amplifier produces significantly high voltage gain with narrow bandwidth. Poor response of conventional Darlington pair amplifiers at higher frequencies is found to be absent in the proposed amplifier circuit. Variations in voltage gain as a function of frequency and different biasing resistances, bandwidth and total harmonic distortion of the amplifier have also been studied. Proposed amplifier may be useful for various analog communication applications. Keywords: Small Signal amplifiers, Sziklai amplifiers, Complementary Darlington pair amplifiers 1 Introduction Amplification of small-signals through Darlington pair amplifiers have been observed as an important phenomenon of electronics 1-7. A small-signal Darlington pair amplifier is usually known for higher β values but suffers from the problem of poor response at higher frequencies 3-6. Numerous books and research papers have explored the usefulness of Darlington pair amplifiers 1-7 but least efforts have been made to configure smallsignal Sziklai pair (complementary Darlington pair) amplifiers 8,9. The Sziklai pair 8 named after its inventor George Sziklai of Hungary, is a compound unit of two bipolar transistors of opposite polarities (one npn and other pnp transistor). This compound unit is also sometimes known as compound feedback pair or complementary Darlington pair. Polarity of this compound unit is always determined by the driver transistor. Therefore, a pnp driver with npn output transistor behaves like a pnp transistor and vice versa 8,9. The current gain factor ( ) of the Sziklai pair is slightly less than Darlington pair topology, because this compound unit has a small amount of in-built negative feedback which reduces the gain 9. However, Sziklai pairs hold a better linearity than Darlington pairs when used in linear circuits. Another major advantage of Sziklai pair over the Darlington pair is that the base turn-on voltage is only half of the Darlington's turn-on voltage 8,9. In electronics industry, Sziklai pairs are normally used in the push pull output stage of power amplifiers or to configure quasi-complementary-symmetry power amplifiers 1,8. However, in the present paper Sziklai pair amplifier as small-signal voltage amplifier using an additional biasing resistance in the circuit, has been studied. 2 Experimental Details A commonly used CC/CE Darlington pair amplifier having two npn transistors Q1 and Q2 in compound configuration, has been studied in the present paper. This amplifier circuit is referred here as reference amplifier as shown in Fig. 1. However, in the proposed amplifier, Q1 transistor of the reference amplifier is replaced by a pnp transistor. In addition, an additional biasing resistance R A between collector of transistor Q1 and ground is introduced and emitter of upper pnp transistor Q1 is directly connected with dc supply voltage to obtain the proposed Sziklai Pair amplifier circuit of Fig. 2. Both the amplifier circuits are properly biased using potential divider network with biasing parameters as presented in Table 1. Respective observations are made by feeding the amplifier circuits with 1V ac input signal source from which, a small and distortion less ac signal of 1mV for reference amplifier (Fig. 1) and 10 mv for proposed amplifier (Fig. 2) at 1 khz frequency is drawn as input for amplification purpose. All the observations mentioned in the present paper are furnished through PSpice simulation software 10 (Student version 9.2).

2 PANDEY et al.: SZIKLAI PAIR AMPLIFIER 273 Fig. 1 Conventional darlington pair amplifier (reference amplifier) Fig. 2 Sziklai pair amplifier (proposed amplifier) Table 1 Biasing parameters and configurational details 3 Results and Discussion The amplifiers of Figs 1 and 2 are found to provide fair and distortion-less results up to 10 and 30 mv ac input signals at 1 khz frequency,respectively. Variation of maximum voltage gain as a function of frequency for both the amplifiers is shown in Fig. 3. It is found that the reference amplifier produces maximum voltage gain, 8.51 maximum current gain and 99 khz bandwidth 4,6, while the proposed amplifier produces a maximum voltage gain of , maximum current gain with 4.80 khz bandwidth. It is also observed that at the defined values of biasing parameters (Table 1), the proposed amplifier produces 110.6µA peak output current and volts peak output voltage. Although Sziklai pair unit in the proposed amplifier has CE-CE configuration but the output voltage/ current waveform shows phase reversal due to one pnp and another npn transistor. Similarly, the output waveform corresponding to reference amplifier shows phase reversal due to CC-CE configuration (both transistors are npn) of Darlington pair unit 1. Transistors small-signal amplification parameters and corresponding to the Darlington pair and Sziklai pair topologies in Figs 1 and 2, respectively are also estimated using the following formula as presented in Tables 3 and 4 along with the transistors driver voltages. However, Table 2 consists of the default values of, and transistors turn-on voltages as defined in PSpice models of Q2N2907A and Q2N2222 transistors. Components Reference Amplifier Proposed Amplifier (Configuration-Darlington Pair) (Configuration-Sziklai Pair) Q1 Q2N2222 (npn with =255.9) Q2N2907A (pnp with =231.7) Q2 Q2N2222 (npn with =255.9) Q2N2222 (npn with =255.9) R S R 1 47k 33k R 2 5k 100k R C 10k 10k R E 2k 2k R A Not Available 500 R L 10k 10k C 1, C 2 1µF 1µF C E 10µF 0.1µF Biasing Supply +15V dc +18V dc Input ac Signal range for distortion-less output 1-10mV (1kHz) 10-30mV (1kHz) Table 2 - parameters and turn-on voltages (V T ) for transistors of the compound units as defined in PSpice device models Compound unit configuration Transistor Q1 of the compound unit Transistor Q2 of the compound unit Compound Unit 1 1 V T 2 2 V T dc dc Darlington pair V V Sziklai pair V V

3 274 INDIAN J PURE & APPL PHYS, VOL 50, APRIL 2012 = Q1 Q2 + Q1 = Q1 Q2 + Q1 + Q2 = /(1+ ) for Sziklai pair for Darlington pair for both topologies The estimated and values in Table 4 corresponding to the proposed Sziklai pair amplifier are found to be adequately in accordance with the prescribed range for small-signal amplifiers 1,11. It is further found that in the absence of added resistance, but keeping other biasing parameters intact, the proposed amplifier produces a voltage gain of 0.41, current gain 0.021, bandwidth khz with an output current of the order of nano-ampere. Perhaps high driving voltages for Q1 and Q2 (Table 3) and the inclusion of added resistance R A in the proposed amplifier circuit are responsible for the significant enhancement in the voltage gain 3,4. The proposed amplifier is also found to effectively remove the problem of poor response of a conventional Darlington pair amplifier (Fig. 1) at higher frequencies. Total Harmonic Distortion percentage (THD%) has also been calculated for the reference and proposed amplifiers using following formula 1,2. A D = th n %n harmonic distortion=% n 100% A1 Fig. 3 Variation of maximum voltage gain as a function of frequency The THD for reference amplifier was estimated for 10 significant harmonic terms and found to be 0.734% while that for proposed amplifier was estimated for 8 significant harmonic terms and found to be 1.72%. Variation of maximum voltage, current gains and bandwidth with temperature is also measured and listed in Table 5. For reference amplifier, the Table 3 Transistor currents and driving voltages ( V D ) based on simulation results Compound unit configuration Transistor Q1 of the compound unit Transistor Q2 of the compound unit I CQ1 I BQ1 I EQ1 V D I CQ2 I BQ2 I EQ2 V D Darlington pair 1.31 A 20.6nA 1.29 A 1.44V 188 A 1.33 A A 0.97V Sziklai pair 30.8mA A 36.81mA 17.2V A 6.97mA 5.69mA 13.8V Table 4 and parameters based on simulation results Compound unit configuration Transistor Q1 of the compound unit Transistor Q2 of the compound unit Compound Unit 1ac 1ac 2ac 2ac ac ac Darlington pair Sziklai pair Table 5 Variation of maximum voltage and current gains with temperature Temperature, ( C) Darlington pair amplifier (Fig. 1) Sziklai pair amplifier (Fig. 2) Voltage gain Current gain Bandwidth, khz Voltage gain Current gain Bandwidth, khz

4 PANDEY et al.: SZIKLAI PAIR AMPLIFIER 275 bandwidth remains unchanged but both varieties of gains increase with rising temperature. The similar situation persists for proposed amplifier up to 80 C temperature and beyond this critical temperature (80 C), the bandwidth begins to widen and voltagecurrent gains significantly decrease. This observation verifies the usual behaviour of transistor parameter h FE with temperature 12 but beyond critical temperature the proposed amplifier deviates from this usual behaviour perhaps due to addition of R A in the circuit. The variation of maximum voltage gain as a function of added resistance R A is shown in Fig. 4. The maximum of the voltage gain corresponding to added resistance R A for proposed amplifier is observed at R A =0.5 k. The overall property is that the maximum voltage gain linearly decreases up to R A =50 k and thereafter, it tends towards saturation. Thus, the proposed amplifier is found to produce considerable response for R A 1 k. Variation of maximum voltage gain with dc supply voltage is shown in Fig. 5. It is observed that the maximum voltage gain for proposed amplifier has a nonlinear rising tendency for increasing values of dc Fig. 4 Variation of maximum voltage gain with added resistance R A supply voltage up to 20 V and beyond this critical limit it decreases with a slow pace. However, the maximum voltage gain for reference amplifier possesses linear rising tendency with dc supply voltage. Both the amplifiers under discussion fairly respond up to 40V of Vcc. Therefore, the optimum performance of the proposed amplifier is received for V voltage range of the dc supply voltage. Variation of maximum voltage gain as a function of R E for both the amplifiers is shown in Fig. 6. The maximum voltage gain for reference amplifier has a decreasing tendency (almost exponentially) at increasing values of R E. However, the voltage gain for the proposed amplifier increases with R E and respective response curve is found to be inverted replica of the curve corresponding to reference amplifier. Both the amplifiers fairly respond up to 25 k value of R E and above this value, the output voltage and current waveforms show distortion. Variations of maximum voltage gain with collector resistance R C and load resistance R L are also estimated (not shown in form of figures). It is found that maximum voltage gain has a nonlinear rising tendency for increasing values of collector resistance R C for both the amplifiers up to 10 k and beyond this critical limit, the voltage gain gradually acquires a saturation tendency. The reference and proposed amplifier circuits performs well below 40 k of R C. It is also observed that overall voltage gain of the proposed amplifier is always higher than the reference amplifier gain at any value of R C. Similarly for R L, it is observed that voltage gain value rises up linearly in low resistance range up to 50 k value of R L but for higher R L values it gradually acquires a sustained level. This rising and saturation of the voltage gain with R L is well in accordance of the usual behaviour of small signal Fig. 5 Variation of maximum voltage gain with supply voltage Vcc Fig. 6 Variation of maximum voltage gain with emitter resistance R E

5 276 INDIAN J PURE & APPL PHYS, VOL 50, APRIL 2012 amplifiers 2,3,4,6,7,11. It is further observed that the basic nature of the variation of maximum voltage gain with R L or R C for both amplifier circuits (Figures 1 and 2) is similar but the overall voltage gain of the proposed amplifier is always found to be higher than the reference amplifier gain at every value of R L or R C. 4 Conclusions Sziklai pair topology is normally used to design quasi-complementary-symmetry push-pull Class-B power amplifiers but in the present paper it is explored to design a small-signal amplifier. The proposed amplifier effectively removes the problem of poor response of conventional small-signal Darlington pair amplifiers at higher frequencies. It shows a considerable response for additional biasing resistance R A 1 k and significantly produces high voltage gain with narrow bandwidth and a current gain greater than unity. The optimum performance of the proposed Sziklai pair amplifier is received for V of dc supply voltage and the maximum voltage gain increases almost exponentially with emitter resistance R E by keeping R E 25 k for distortionless response. References 1 Boylestad R L & Nashelsky L, Electronic Devices & Circuit Theory, Pearson Education Asia, 9 th edn, (2008) 299, 304, David A Bell, Electronic Devices & Circuit Theory, Prentice Hall of India, 3 rd edn, (2002) Sayed ElAhl AMH, Fahmi M M E & Mohammad S N, Solid State Electronics, 46 (2002) Tiwari, S N & Shukla S N, Bull of Pure & Appl Sci, 28D, No1 (2009) Chris T A & Robert G M, IEEE Journal of Solid State Circuits, 24 (1989) Tiwari S N, Dwivedi A K & Shukla SN, J Ultra Scientist of Phys Sci, 20 (2008) Tiwari S N, Pandey, B, Dwivedi, AK & Shukla S N, J Ultra Scientist of Phys Sci, 21 (2009) Sziklai G C, Push-pull complementary type transistor amplifier, US Patent 2,762,870, September 11, Horowitz P & Winfield H, The Art of Electronics, Cambridge University Press ISBN , Rashid M H, Introduction to PSpice Using OrCAD for Circuits & Electronics, Pearson Education, 3 rd Ed (2004) Motayed A, Browne T E, Onuorah A I & Mohammad S N, Solid State Electronics, 45 (2001) Barua A G & Tiru B, Indian J Pure & Appl Phys, 44 (2006) 482.