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Lecture #3 BJT Biasing Circuits Instructor Banna Benha University - Faculty of Engineering at Shoubra © Ahmad El ECE-312 Electronic Circuits (A) Lecture #3 BJT Biasing Circuits Instructor: Dr. Ahmad El-Banna October 2014 Banna Agenda - Operating Point © Ahmad El Transistor DC Bias Configurations Design Operations Various BJT Circuits 312 2014 Lec#3 , Oct , Troubleshooting Techniques & Bias Stabilization - ECE Practical Applications 2 Banna Introduction - • Any increase in ac voltage, current, or power is the result of a transfer of energy from the applied dc supplies. © Ahmad El • The analysis or design of any electronic amplifier therefore has two components: a dc and an ac portion. • Basic Relationships/formulas for a transistor: 312 2014 Lec#3 , Oct , - • Biasing means applying of dc voltages to establish a fixed level of ECE current and voltage. >>> Q-Point 3 Banna Operating Point - • For transistor amplifiers the resulting dc current and voltage establish an operating point on the characteristics that define the © Ahmad El region that will be employed for amplification of the applied signal. • Because the operating point is a fixed point on the characteristics, it is also called the quiescent point (abbreviated Q-point). Transistor Regions Operation: 1. Linear-region operation: Base–emitter junction forward-biased Base–collector junction reverse-biased 312 2014 Lec#3 , Oct , 2. Cutoff-region operation: - Base–emitter junction reverse-biased Base–collector junction reverse-biased ECE 3.Saturation-region operation: Base–emitter junction forward-biased 4 Base–collector junction forward-biased Banna - © Ahmad El • Fixed-Bias Configuration • Emitter-Bias Configuration • Voltage-Divider Bias Configuration • Collector Feedback Configuration • Emitter-Follower Configuration 312 2014 Lec#3 , Oct , • Common-Base Configuration - • Miscellaneous Bias Configurations ECE TRANSISTOR DC BIAS CONFIGURATIONS 5 Banna Fixed-Bias Configuration - • • Fixed-bias circuit. DC equivalent ct. © Ahmad El • Base–emitter loop. • Collector–emitter loop. 312 2014 Lec#3 , Oct , - ECE 6 Banna Fixed-Bias Configuration Example - © Ahmad El 312 2014 Lec#3 , Oct , - ECE 7 Banna Fixed-Bias Configuration ... - • Transistor Saturation © Ahmad El • Saturation regions: (a) Actual (b) approximate. • Determining ICsat for the fixed-bias configuration. 312 2014 Lec#3 , Oct , • Determining ICsat - ECE 8 Banna Fixed-Bias Configuration ... - • Load Line Analysis © Ahmad El 312 2014 Lec#3 , Oct , - ECE 9 Banna Emitter-Bias Configuration - • BJT bias circuit with emitter resistor. • DC equivalent ct © Ahmad El • Base-Emitter Loop 312 2014 Lec#3 , Oct , - ECE 10 Banna Emitter-Bias Configuration - Collector-Emitter Loop © Ahmad El , Oct 2014 Oct , Lec#3 312 , - ECE 11 Banna Emitter-Bias Configuration - • Improved bias stability (check example 4.5) The addition of the emitter resistor to the dc bias of the BJT provides © Ahmad El improved stability, that is, the dc bias currents and voltages remain closer to where they were set by the circuit when outside conditions, such as temperature and transistor beta, change. 2014 Oct , Lec#3 • Saturation Level 312 , • Load Line Analysis - ECE 12 Banna Voltage-Divider Configuration - • Voltage-divider • Exact Analysis bias configuration. © Ahmad El , Oct 2014 Oct , • DC components of the Lec#3 voltage-divider configuration. 312 , - ECE 13 Banna Voltage-Divider Configuration - • Approximate Analysis • Transistor Saturation © Ahmad El • Load-Line Analysis , Oct 2014 Oct , Lec#3 312 , - ECE 14 Voltage - Divider Divider Configuration Configuration Example 15 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Banna Collector Feedback Configuration - • DC bias circuit with • Base–Emitter Loop voltage feedback. © Ahmad El , Oct 2014 Oct , • Collector–Emitter Loop Lec#3 312 , - ECE 16 Banna Collector Feedback Configuration - • Saturation Conditions Using the approximation I’C = IC © Ahmad El • Load-Line Analysis 2014 Oct , Continuing with the approximation Lec#3 I’C = IC results in the same load line defined for the voltage-divider and emitter-biased configurations. 312 , - The level of IBQ is defined by the ECE chosen bias configuration. 17 Banna Emitter-Follower Configuration - • Common-collecter (emitter-follower) configuration. © Ahmad El , Oct 2014 Oct , • dc equivalent ct i/p ct Lec#3 312 , - ECE o/p ct 18 Banna Common-Base Configuration - • Common-base configuration © Ahmad El • i/p ct , Oct 2014 Oct , Lec#3 312 , - • Determining VCB & VCE ECE 19 MISCELLANEOUS BIAS CONFIGURATIONS CONFIGURATIONS BIAS MISCELLANEOUS 20 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Summary Table Summary 21 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Summary Table.. Summary 22 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna DESIGN OPERATION DESIGN 23 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Banna Design Operations - • Discussions thus far have focused on the analysis of existing networks. All the elements are in place, and it is simply a matter of solving for the © Ahmad El current and voltage levels of the configuration. • The design process is one where a current and/or voltage may be specified and the elements required to establish the designated levels must be determined. • The design sequence is obviously sensitive to the components that are 2014 Oct , already specified and the elements to be determined. If the transistor and supplies are specified, the design process will simply determine Lec#3 the required resistors for a particular design. • Once the theoretical values of the resistors are determined, the 312 , - nearest standard commercial values are normally chosen and any variations due to not using the exact resistance values are accepted as ECE part of the design. 24 Design Operations Operations Design Example 25 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Banna Design Operations Example.. - • Design of a Current-Gain-Stabilized (Beta-Independent) Circuit © Ahmad El , Oct 2014 Oct , Lec#3 312 , - ECE 26 Banna - © Ahmad El • MULTIPLE BJT NETWORKS • CURRENT MIRRORS 2014 Oct , • CURRENT SOURCE CIRCUITS • Bipolar Transistor Constant-Current Source Lec#3 • Transistor/Zener Constant-Current Source 312 , • PNP TRANSISTORS - • TRANSISTOR SWITCHING NETWORKS ECE VARIOUS BJT CIRCUITS 27 Banna MULTIPLE BJT NETWORKS - • R–C coupling © Ahmad El , Oct 2014 Oct , • Darlington configuration Lec#3 312 , - ECE 28 MULTIPLE BJT BJT MULTIPLE NETWORKS.. 29 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna • MULTIPLE BJT BJT MULTIPLE Feedback Pair Feedback NETWORKS… 30 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna • MULTIPLE BJT NETWORKS BJT MULTIPLE Direct Coupled Direct …. 31 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna CURRENT MIRRORS MIRRORS CURRENT 32 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Banna CURRENT SOURCE CIRCUITS - © Ahmad El • • Transistor/Zener Bipolar Transistor Constant-Current Source Constant-Current Source 2014 Oct , Lec#3 312 , - ECE 33 pnp TRANSISTOR SWITCHING Banna TRANSISTORS NETWORKS - © Ahmad El , Oct 2014 Oct , Lec#3 312 , - ECE 34 TRANSISTOR SWITCHING SWITCHING TRANSISTOR NETWORKS.. 35 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna TROUBLESHOOTING TROUBLESHOOTING TECHNIQUES 36 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Banna TROUBLESHOOTING TECHNIQUES - • For an “on” transistor, the voltage VBE should be in the neighborhood of 0.7 V. • For the typical transistor amplifier in the active region, VCE is usually about 25% to 75% of VCC . © Ahmad El , Oct 2014 Oct , Lec#3 312 , - ECE 37 Banna BIAS STABILIZATION - • The stability of a system is a measure of the sensitivity of a network to variations in its parameters. © Ahmad El • In any amplifier employing a transistor the collector current IC is sensitive to each of the following parameters: , Oct 2014 Oct , Lec#3 312 , - ECE 38 Banna BIAS STABILIZATION .. S(Ico) - • emitter-bias configuration © Ahmad El there is a reaction to an increase in IC that will tend to oppose the change in bias conditions. • feedback configuration 2014 Oct , • fixed-bias configuration Lec#3 a stabilizing effect as described for the emitter-bias configuration. 312 , - • voltage-divider bias ECE the level of IC would continue to rise with temperature, with IB maintaining a fairly constant The most stable of the value—a very unstable situation. configurations 39 Banna BIAS STABILIZATION .. S(VBE)& S(β) - © Ahmad El For fixed-bias , Oct 2014 Oct , Lec#3 312 , - ECE 40 Banna - © Ahmad El • BJT Diode Usage and Protective Capabilities • Relay Driver • Light Control 2014 Oct , • Maintaining a Fixed Load Current Lec#3 • Alarm System with a CCS 312 , • - Voltage Level Indicator • Logic Gates ECE PRACTICAL APPLICATION 41 Banna Practical Application - • BJT Diode Usage and Protective Capabilities © Ahmad El , Oct 2014 Oct , • Lec#3 Relay Driver 312 , - ECE 42 Banna Practical Application.. - • Light Control © Ahmad El , Oct 2014 Oct , • Lec#3 Maintaining a Fixed Load Current 312 , - ECE 43 Banna Practical Application… - • Alarm System with a CCS © Ahmad El , Oct 2014 Oct , • Voltage Level Indicator Lec#3 312 , - ECE 44 • Practical Practical Logic Gates Logic Gates Application…. 45 ECE-312 , Lec#3 , Oct 2014 © Ahmad El-Banna Banna - • For more details, refer to: • Chapter 4 at R. Boylestad, Electronic Devices and Circuit Theory, © Ahmad El 11th edition, Prentice Hall. • The lecture is available online at: • https://speakerdeck.com/ahmad_elbanna • For inquires, send to: 2014 Oct , • [email protected] • [email protected] Lec#3 312 , - ECE 46 .
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