Electronics Technology Fundamentals Chapter 24 Switching Circuits

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Introductory Concepts – P1 Switching Circuits – circuits designed to respond to (or generate) nonlinear waveforms The BJT as a Switch

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Introductory Concepts – P2 The BJT as a Switch (Continued)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Introductory Concepts – P3 The JFET as a Switch

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Introductory Concepts – P4 The MOSFET as a Switch

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Introductory Concepts – P5 The CMOS Switch Insert Figure 24.8

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Introductory Concepts – P6 A Switching Application: The LED Driver – couples a low-current output to a relatively high-current device

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P1 Review of Rectangular Waveform Characteristics

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P2 BJT Switching Time

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P3 BJT Switching Time (Continued) Propagation Delay – the overall delay between the input and output transitions, as measured at the 50% points on the two waveforms Four sources of BJT propagation delay Delay Time (t d ) – the time required for the BJT to come out of cutoff Rise Time (t r ) – the time required for the transition from cutoff to saturation Storage Time (t s ) – the time require for the BJT to come out of saturation Fall Time (t f ) – the time required to make the transition from saturation to cutoff

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P4 BJT Switching Time (Continued) Upper cutoff frequency can be found using: To pass a square wave, f C2 must be at least 100 times the square wave frequency Where f max = the upper frequency limit for a discrete switching circuit Where t r = the rise time of the device

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P5 BJT Switching Time (Continued)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P6 Improving BJT Switching Time I B should be very high initially (to reduce delay time) and then settle down to some level just below that required for saturation (to reduce storage time) The reverse bias current should be very high initially (to reduce storage time) and then decrease to the minimum value required to keep the transistor in cutoff (to reduce delay time)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P7 Improving BJT Switching Time (Continued) – desired I B and reverse-bias characteristics achieved by speed-up capacitor (C S ) Insert Figure 24.13

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P8 Improving BJT Switching Time (Continued) Insert Figure 24.14

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P9 Improving BJT Switching Time (Continued) – the value of the speed-up capacitor must be chosen so that the RC time constant is very short compared to the minimum pulse width of the input

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P10 JFET Switching Time JFET spec sheets usually list values of t d, t r, t s and t f Some JFET spec sheet list only turn-on time and turn-off time Turn-on Time (t on ) – sum of delay and rise time Turn-off Time (t off ) – sum of storage time and fall time

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P11 Switching Devices Switching Transistors – groups of BJTs and JFETs designed to have extremely low values of t d, t r, t s and t f For Example: 2N2369 t d = 5 ns t r = 18 ns t s = 13 ns t f = 15 ns

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P12 Buffer Circuits – a switching circuit that produces an output that is in phase with its input

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Basic Switching Circuits: Practical Considerations – P13

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P1 A voltage-level detector that is similar to a comparator, but with some key differences

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P2 Schmitt Trigger Overview Upper Trigger Point (UTP) – the voltage where a positive-going transition makes the output change from - V out to +V out Lower Trigger Point (LTP) – the voltage where a negative-going transition makes the output change from +V out to -V out Insert Figure 24.17

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P3 Schmitt Trigger Overview (Continued) – the range of voltages between the UTP and LTP is referred to as hysteresis

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P4 Schmitt Trigger Overview (Continued)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P5 Noninverting Schmitt Trigger

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P6 Noninverting Schmitt Trigger (Continued)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P7 Noninverting Schmitt Trigger (Continued) – modification so that the magnitudes of the UTP and LTP are not equal Insert Figure 24.21

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P8 Inverting Schmitt Trigger

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Schmitt Triggers – P9 Inverting Schmitt Trigger – modification so that the magnitudes of the UTP and LTP are not equal

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P1 Multivibrator – a switching circuit designed to have zero, one or two stable output states.

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P2 Multivibrator (Continued) Astable Multivibrator (free-running multivibrator) – a circuit that switches back and forth between two dc levels as long as it has a dc supply voltage Monostable Multivibrator – a circuit that has one stable output state Bistable Multivibrator – a circuit that has two output states

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P3 The 555 Timer – a switching circuit contained in an 8-pin IC

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P4 The 555 Timer (Continued) – comparator operation

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P5 The 555 Timer (Continued) – flip-flop operation Insert Figure 24.29

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P6 The Monostable Multivibrator – one-shot operation

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P7 The Monostable Multivibrator (Continued)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P8 The Monostable Multivibrator (Continued) – Determining a Valid Trigger Signal (no control voltage used)

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P9 The Monostable Multivibrator (Continued) – Determining a Valid Trigger Signal (control voltage used) where V con = the voltage at the control voltage input

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P10 The Astable Multivibrator – free-running operation

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P11 The Astable Multivibrator (Continued) – timer produces a steady train of pulses

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P12 The Astable Multivibrator (Continued) –

Electronics Technology Fundamentals, 3 rd ed. Paynter and Boydell © 2009 Pearson Higher Education, Upper Saddle River, NJ All Rights Reserved Multivibrators: The 555 Timer – P13 Voltage-Controlled Oscillator (VCO) – a free- running multivibrator whose output frequency is determined by a dc input control voltage (V con ) Insert Figure 24.35