Switching Circuits

• If VIN < 0.7V, the transistor is off, VBE = VIN

  If VIN ≥ 0.7V, the transistor is on, VBE = 0.7V
• Linear region When the voltage VIN , increases about 0.7V, a base current starts to flow. The transistor behaves as a current amplifier and the base current causes a larger amplifier current to flow through the collector and load. As VIN increases further, more current flows into the base and this allows a further increase in the collector emitter current.
• The transistor acts as a current amplifier, each transistor has a current gain, called hFE and this is defined by the following current gain formula: hFE = Ic / Ib
• The transfer characteristic of the MOSFET is very similar to that of the npn transistor – the main difference being that the linear region is very small, making it unlikely that the MOSFET will operate in this region.
• The only formula we need to design MOSFET circuits is the formula which relates the drain current, ID to the input voltage, VGS. The symbol gM represents the transconductance of a MOSFET. Transconductance is the characteristic relating the current through the output of a MOSFET to the voltage across the input and is measured in Siemens, S. (Id = gm(Vgs - 3))
• MOSFETs have very high input resistances so very little or no current flows into the input terminal making them ideal for use as electronic switches
• If V+ is greater than V- then Vout will be high. (Comparators)
• MOSFET

  A) Drain

  B) Gate

  C) Source

  
• MOSFETs are ‘voltage operated devices’
• Transistors are 'current operated devices'
• Id = Gm(Vgs - 3)

  • Id: drain current
  • Gm: Transconductance (siemens, S)
  • Vgs: Input voltage