978-1-4799-5760-6/14/$31.00 ©2014 IEEE Two-Stage Small-Signal Amplifier with Darlington and Sziklai pairs SachchidaNand Shukla, Beena Pandey Department of Physics and Electronics, Dr. Ram Manohar Lohia Avadh University Faizabad - 224001, U.P., India sachida_shukla@yahoo.co.in Abstract— A New circuit model of two-stage small-signal amplifier using Darlington and Sziklai pairs is proposed for the first time. Proposed circuit is obtained by cascading a small- signal Darlington pair amplifier with that of Sziklai pair based small-signal amplifier with minor modifications. Proposed amplifier essentially uses two additional biasing resistances in its design. It crops considerably high voltage and current gain (345.523 and 464.357 respectively) with audible range bandwidth of 43.363 KHz for AC input signals swinging in 10-30mV range at 1KHz or lower frequency. Proposed circuit-architecture successfully removes the poor response problem of conventional Darlington pair amplifiers at higher frequencies and narrow bandwidth problem of recently announced (by authors) circuits of small-signal Sziklai pair amplifier. Variations in voltage gain with frequency and various biasing components like biasing resistances, DC supply voltage and coupling capacitors are discussed in length. Temperature sensitivity of performance parameters, THD and small-signal AC equivalent circuit analysis of the proposed circuit are elaborately studied. Proposed design can be implemented in cascadable gain blocks for receivers, 715Hz-44KHz frequency range power sources and in the circuits where reproduction of signal with simultaneously high voltage and current gain is the prime requirement. Keywords— Two-stage small-signal amplifiers, Sziklai pair Amplifiers, Darlington Pair, Circuit design, Simulation I. Introduction Small-signal amplifiers are key contributors to a number of important electronics and communication circuits [1]-[2]. Sometimes voltage/current gain of single-stage amplifiers are found insufficient for practical applications, and therefore, two or more amplifier stages are used to get the desired level of gain [1]-[3]. In the last few decades, electronics industry has witnessed Darlington pairs as useful audible-range device for single/multi-stage small-signal and power amplifiers [3]-[5]. This paired unit of identical BJTs realizes higher β values but suffers from poor-response-problem at higher frequencies [3]- [5]. In recent years, Darlington pairs are being gradually replaced by Sziklai pairs in single or multi-stage power amplifiers due to having almost similar range of current gain, input resistance, output resistance and voltage gain with extra advantages of better linearity and only half of the base turn-on voltage than Darlington's unit [5]-[9]. However, use of Sziklai pairs in single or multi-stage small-signal amplifiers is still under the developmental phase [3],[6]-[9]. Unlike Darlington pairs, Sziklai pairs hold one NPN and other PNP transistor in CE-CE connection [6]-[7]. Polarity of this unit is determined by the driver transistor. Thus, Sziklai pair with PNP driver and NPN output transistor acts as a PNP transistor and vice versa [6]-[7]. Principally, both Darlington and Sziklai pairs enjoy high input resistance, low output resistance and voltage gain approximately equal to unity [4]- [6]. However, the current gain factor β of Sziklai pair (β Q1 β Q2 +β Q1 ) is slightly less than Darlington pair (β Q1 β Q2 +β Q1 +β Q2 ) topology but at higher β values both are approximated to β Q1 β Q2 [7]. Recently, authors have developed three different circuits of Sziklai pair based small-signal amplifiers [3],[8]-[9]. Present exploration carries an intention to combine desirable features of small-signal Darlington and Sziklai pair amplifiers, and therefore, focuses around the development and analysis of a two-stage small-signal amplifier using Darlington pair in stage-1 and Sziklai pair in stage-2. II. Description of Circuits The present investigation includes a comparative study of two-stage small-signal proposed amplifier (Fig.3) with conventional Darlington pair amplifier (Fig.1) and a modified Sziklai pair amplifier (Fig.2) [5],[8]. Fig.1. Small-signal Darlington pair amplifier (Reference circuit) The amplifiers of Fig.1 [5] and Fig.2 [8] are treated herein as reference circuits. However, proposed two stage amplifier (Fig.3) is obtained by cascading reference amplifier circuits of Fig.1 and Fig.2 with minor modifications [1]-[3]. Devices Q11, Q12 and Q22 in respective circuit designs of Fig.1, Fig.2 and Fig.3 amplifiers are NPN BJTs (Q2N2222 with β=255.9) whereas Q21 is a PNP BJT (Q2N2907A with β=231.7) [10]. PSpice simulation [10] is used to receive respective observations. Every amplifier is fed by 1V AC input signal IEEE-ICSE2014 Proc. 2014, Kuala Lumpur, Malaysia 13