ECE

• Transistor DC Bias Circuit

  • VBB forward - biases the base-emitter junction
  • VCC reverse - biases the base-collector junction

  
• Two Basic Types of Transistors


  1. Bipolar Junction Transistor (BJT)

  2. Field-Effect Transistor (FET)
• The invention of the transistor was the beginning of a technological revolution that is still continuing. All of the complex electronic devices and systems today are an outgrowth of early developments in semiconductor transistors.
• Bipolar Junction transistor (BJT) structure

  The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.

  
• Bipolar Junction transistor (BJT) structure

  • The pn junction joining the base region and the emitter region is called the base-emitter junction.

  • The pn junction joining the base region and the collector region is called the base-collector junction.

  
• Bipolar Junction transistor (BJT) structure

  • The BJT is constructed with three doped semiconductor regions separated by two pn junctions, as shown.

  • The three regions are called emitter, base, and collector.

  
• The BJT is used in two broad areas:


  • as a linear amplifier to boost or amplify an electrical signal and

  • as an electronic switch.

  
• BJT Operation

  
• BJT Operation


  • Free electrons easily diffuse through the forward-based BE junction into the lightly doped and very thin p-type base region, as indicated by the wide arrow.

  • The base has a low density of holes, which are the majority carriers, as represented by the white circles.

  
• BJT Operation


  • A small percentage of the total number of free electrons injected into the base region recombine with holes and move as valence electrons through the base region and into the emitter region as hole current, indicated by the red arrows.

  • When the electrons that have recombined with holes as valence

  electrons leave the crystalline structure of the base, they become free electrons in the metallic base lead and produce the external base current.
• BJT Operation


  • Most of the free electrons that have entered the base do not recombine with holes because the base is very thin.

  • As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage.

  
• BJT Operation


  • The free electrons move through the collector region, into the external circuit, and then return into the emitter region

  along with the base current, as indicated.

  • The emitter current is slightly greater than the collector current because of the small base current that splits off from

  the total current injected into the base region from the emitter.
• Transistor Currents


  IE=IC+IB

  
• DC Beta (βDC) and DC Alpha (αDC)

  • The dc current gain of a transistor is the ratio of the dc collector current (IC) to the dc base current (IB) and is designated dc beta (βDC) βDC = IC/IB

  • Typical values of βDC range from less than 20 to 200 or higher.

  • βDC is usually designated as an equivalent hybrid (h) parameter, hFE, on transistor datasheets.
• DC Beta (βDC) and DC Alpha (αDC)


  • The ratio of the dc collector current (IC) to the dc emitter current (IE) is the dc alpha (αDC). The alpha is a less-used parameter than beta in transistor circuits.

  αDC = IC/IE

  • Typical values of αDC range 0.95 to 0.99 or greater, but αDC is always less than 1.
• Transistor DC Model of an npn transistor

  • The input circuit is a forward-biased diode through which there is base current.

  • The output circuit is a dependent current source (diamond-shaped element) with a value that is dependent on the base current, IB, and equal to βDCIB

  
• BJT Circuit Analysis

  • IB: dc base current

  • IE: dc emitter current

  • IC: dc collector current

  • VBE: dc voltage at base with respect to emitter

  • VCB: dc voltage at collector with respect to base

  • VCE: dc voltage at collector with respect to emitter

  
• BJT Circuit Analysis


  • The base-bias voltage source, VBB, forward-biases the base-emitter junction, and the collector-bias voltage source, VCC, reverse-biases the base-collector junction.

  • When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltage drop of VBE ≅ 0.7
• Collector characteristic curves


  A set of curves that show how the collector current, IC, varies with the collector-to-emitter voltage, VCE, for specified values of base current, IB

  
• Collector characteristic curves

  1. Assume VBB is set to produce a certain value of IB and VCC is zero

  2. Both the BE junction and the BC junction are forward-biased

  3. Base current is primarily towards the ground due to less impedance therefore IC is approximately zero

  
• Saturation


  The state of a BJT in which the collector current has reached a maximum and is independent of the base current

  
• Collector characteristic curves


  1. As VCC is increased, VCE increases as the collector current increases

  2. When VCE reaches a sufficiently high voltage, the reverse-biased base-collector junction goes into breakdown

  
• Cutoff


  The nonconducting state of a transistor
• Cutoff


  1. When IB = 0, the transistor is in the cutoff region

  2. Collector leakage current (ICEO) is extremely small and is usually neglected

  3. VCE ≅ VCC

  4. Base-emitter and base-collector junctions are reverse-biased

  
• Cutoff to Saturation

  1. As IB increases due to increasing VBB, IC also increases and VCE decreases due to the increased voltage drop across RC

  2. When the transistor reaches saturation, IC can increase no further regardless of further increase in IB

  3. Base-emitter and base-collector junctions are forward-biased

  
• DC Load Line


  A straight line that represents the voltage and current in the linear portion of the circuit that is connected to a device

  
• DC Load Line


  1. The bottom of the load line is at ideal cutoff where IC = 0 and VCE = VCC

  2. The top of the load line is at saturation where IC = IC(sat) and VCE = VCE(sat)
• Determine whether or not the transistor in figure shown is in saturation


  • Assume VCE(sat) = 0.2 V

  
• DC and AC Quantities

  • Italic capital letters are used for both dc and ac currents (I) and voltages (V)
  • DC quantities always carry an uppercase roman (nonitalic) subscript
  • AC and all time-varying quantities always carry a lowercase italic subscript
  • Transistors have internal ac resistances that are designated by lowercase r' with an appropriate subscript
• Voltage Amplification


  1. The ac base voltage is Vb = Ier'e

  2. The ac collector voltage, Vc, equals the ac voltage drop across RC

  3. Vc ≅ IeRC

  4. Voltage gain is defined as the ratio of the output voltage to the input voltage, the ratio of Vc to Vb is the ac voltage gain, Av, of the transistor

  5. Av ≅ Rc/r'e

  
• The BJT as a Switch


  • In cutoff, there is, ideally, an open between collector and emitter
  • In saturation, there is, ideally, a short between collector and emitter
• Conditions in Cutoff

  Neglecting leakage current, all of the currents are zero, and VCE is equal to VCC
• Conditions in Saturation


  1. The formula for collector saturation current is IC(sat) = βDCIB

  2. The minimum value of base current needed to produce saturation is IB(min) = IC(sat)/βDC

  3. Normally, IB should be significantly greater than IB(min) to ensure that the transistor is saturated

  
• For the transistor circuit in figure shown:

  • What is VCE when VIN = 0 V?
  • What minimum value of IB is required to saturate this transistor if βDC is 200? Neglect VCE(sat)
  • Calculate the maximum value of RB that will put the transistor in saturation assuming βDC = 200 when VIN = 5 V