Switching circuits

1.4 Output circuits

1.6 Diodes

Switching circuits

  1. Uses

    1. As an interface between analogue & digital sub-systems
    2. As a transducer driver, which amplifies a low current to drive a high-current circuit

  2. Transistors

    1. NPN transistor

      1. Circuit diagram: NPN transistor symbol
      2. Leads: corresponding to the diagram with the flat side pointing to the right
      3. A small base current be used to control a much larger load current
      4. The transistor is off if VIN < 0.7 V and on if VIN ‰ 0.7 V

    2. N channel MOSFET

      1. Circuit diagram: MOSFET symbol
      2. Leads: corresponding to the diagram with the flat side pointing to the right
      3. MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor
      4. A MOSFET is on at any non-zero voltage and draws very little current, so is ideal for driving heavy loads from the output of a logic system, since they cannot drive high currents

  3. Thyristor

    1. Circuit diagram: Thyristor symbol
    2. Leads:
    3. A small gate voltage can be used to latch a large load current whilst forward biased
    4. A thyristor can be reset by reversing the current through it

  4. Voltage comparator

    1. Comparators have a greater sensitivity than transistor switches
    2. The output is low if the non-inverting input is lower than the inverting output and high if the non-inverting input is higher than the inverting input
    3. Comparators can be used with input sensors so that when the input voltage rises above a certain threshold, the comparator outputs a signal to the rest of the circuit
    4. Comparators are unable to drive a very high current so a transistor is often used as a transducer driver