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BJT Switching Circuits

-. BJT 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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BJT Switching Circuits

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  1. - BJT 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. BJT – disposición física

  6. Característica en conexión directa

  7. Completo: directa + reversa

  8. Modelo Ebers-Moll

  9. Regiones de operación

  10. En corte (simplificado)

  11. En saturación (simplificado)

  12. En saturación (simplificado)

  13. Velocidad de conmutación de un Diodo NOTA: diodos PN no shottky

  14. Velocidad de conmutación de un BJT

  15. Conmutación del BJT

  16. Conmutación del BJT Td->sale del corte 0.1ICS Tr->acumulación de carga en la base Ts->eliminación de sobresaturación Tf->reducción de carga en la base

  17. Transistor Schottky En saturación

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

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

  20. 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.

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

  22. 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.

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

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

  25. 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.

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

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

  28. Noninverting Schmitt Trigger(Symmetrical trigger points)

  29. Noninverting Schmitt Trigger(Asymmetrical trigger points)

  30. Inverting Schmitt Trigger(Symmetrical trigger points)

  31. Inverting Schmitt Trigger(Asymmetrical trigger points)

  32. Schmidt trigger con transistores

  33. 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)

  34. 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.

  35. 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.

  36. 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.

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

  38. Comparator Input/Output Relationships

  39. Flip-Flop Input/Output Relationships

  40. 555 Timer One-Shot

  41. 555 Timer One-Shot Waveforms

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

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

  44. 555 Timer Astable Multivibrator

  45. Astable Multivibrator Waveforms

  46. Astable Multivibrator Relationships

  47. 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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