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Chapter 19

Chapter 19. Solid-State Switching Circuits. The BJT as a Switch. The BJT can be used as a switch by driving it back and forth between saturation (closed) and cutoff (open). Open and Closed BJT Switch. When operated in saturation, the BJT acts as a closed switch.

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Chapter 19

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  1. Chapter 19 Solid-State Switching Circuits

  2. The BJT as a Switch • The BJT can be used as a switch by driving it back and forth between saturation (closed) and cutoff (open).

  3. Open and Closed BJT Switch • When operated in saturation, the BJT acts as a closed switch. • When operated in cutoff, the BJT acts as an open switch.

  4. Saturation • Transistor saturation can be guaranteed by designing the circuit so that: • +Vpk = VCC • IB is greater than IC(sat) / hFE

  5. The JFET as a Switch • The JFET switch • Has higher input impedance than a BJT switch. • Produces a positive output pulse when it receives a negative input pulse.

  6. JFET Switch Output When the circuit input is 0 V:

  7. The MOSFET Switch • The MOSFET switch has high input impedance (like the JFET switch) with the input/output polarity relationship of the BJT switch.

  8. The CMOS Switch • The CMOS switch does not require resistors in the output circuit. • The “off” MOSFET acts as the drain resistor for the “on” MOSFET.

  9. A BJT Switch as an LED Driver • Driver – A circuit used to couple a low-current output to a relatively high-current device.

  10. Practical Waveform Measurements • Pulse width (PW) – The time spent in the active (high) dc voltage state. • Space width (SW) – The time spent in the passive (low) dc voltage state. • Period (T) – The sum of PW and SW. Also referred to as cycle time.

  11. Duty Cycle • Duty cycle – The ratio of pulse width (PW) to period (T), measured as a percentage.

  12. Waveform Time Measurements • Delay time (td) • Rise time (tr) • Storage time (ts) • Fall time (tf)

  13. Wave Shape vs. Operating Frequency Cutoff frequency: Practical frequency limit:

  14. Reducing BJT Switching Time • BJT delay time is reduced by: • Applying a high initial value of base current. • Using the minimum value of reverse bias required to hold the component in cutoff. • BJT storage time is reduced by: • Limiting base current to a value lower than that required to completely saturate the BJT. • Applying a high initial reverse bias to the component. • Rise time and fall time are functions of BJT construction, and cannot effectively be reduced.

  15. Speed-Up Capacitors • Speed-up capacitor – A component used to reduce delay time and storage time.

  16. Buffers • Buffer – A switching circuit that does not produce a voltage phase shift.

  17. Schmitt Triggers • Schmitt trigger – A voltage-level detector. • The output of a Schmitt trigger changes state when • When a positive-going input passes the upper trigger point (UTP) voltage. • When a negative-going input passes the lower trigger point (LTP) voltage.

  18. Trigger Point Voltages • Trigger point voltages may be equal or unequal in magnitude, and are opposite in polarity.

  19. Hysteresis • Hysteresis – A term that is often used to describe the range of voltages between the UTP and LTP of a Schmitt trigger.

  20. Noninverting Schmitt Trigger(Symmetrical trigger points)

  21. Noninverting Schmitt Trigger(Asymmetrical trigger points)

  22. Inverting Schmitt Trigger(Symmetrical trigger points)

  23. Inverting Schmitt Trigger(Asymmetrical trigger points)

  24. Multivibrators • Multivibrator – A circuit designed to have zero, one, or two stable output states. • There are three types of multivibrators. • Astable (or Free-Running Multivibrator) • Monostable (or One-Shot) • Bistable (or Flip-Flop)

  25. Astable Multivibrators • Astable multivibrator – A switching circuit that has no stable output state. • The astable multivibrator is a rectangular-wave oscillator. • Also referred to as a free-running multivibrator.

  26. Monostable Multivibrators • Monostable multivibrator – A switching circuit with one stable output state. • Also referred to as a one-shot. • The one-shot produces a single output pulse when it receives a valid input trigger signal.

  27. Bistable Multivibrators • Bistable multivibrator – A switching circuit with two stable output states. • Also referred to as a flip-flop. • The output changes state when it receives a valid input trigger signal, and remains in that state until another valid trigger signal is received.

  28. The 555 Timer • 555 timer – An 8-pin IC designed for use in a variety of switching applications.

  29. Comparator Input/Output Relationships

  30. Flip-Flop Input/Output Relationships

  31. 555 Timer One-Shot

  32. 555 Timer One-Shot Waveforms

  33. Intermittent One-Shot Operation • A valid input trigger fulfills one of these relationships: • Invalid input signals may result in intermittent operation.

  34. Decoupling Capacitors • Decoupling capacitor – A capacitor connected between the supply voltage and ground pins of an IC to prevent intermittent high-frequency operation.

  35. 555 Timer Astable Multivibrator

  36. Astable Multivibrator Waveforms

  37. Astable Multivibrator Relationships

  38. Voltage-Controlled Oscillators (VCOs) • Voltage-controlled oscillator (VCO) – A free-running oscillator whose output frequency is controlled by a dc input voltage.

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