Transistors

1
Transistors

• Fundamentals
• What transistors do
• How to analyse transistor circuits
• Small and large signals
• Common-Emitter Amplifier
• Review of analysis and design

2
The Bipolar Junction Transistor

• BJT is a current amplifier
• The collector current is controlled by a much
  smaller base current
• The sum of the collector and base currents flow
  into or out of the emitter
• Base-emitter junction looks a lot like a PN
  junction diode

3
Operating Regions - Cut Off

• If the base current is zero, the collector
  current is also zero
• It doesnt matter how big the collector-emitter
  voltage, VCE, is
• i.e. collector-emitter junction looks like an
  open circuit
• In this state, the transistor is in the cut-off
  region

4
Operating Regions - Active

• Base current flows and controls the larger
  collector current
• Collector current is proportional to the base
  current
• Transistor is in the active region
• Operation can be summarised by two equations

5
Operating Regions - Saturation

• Collector current rises in proportion to the base
  current
• As collector current rises, resistor voltage
  rises and collector-emitter voltage falls
• When VCE 0, it cant go any lower and the
  collector current cannot get any higher
• The transistor is saturated
• Collector-emitter junction looks like a short
  circuit

6
Amplification

• BJT amplifiers work by controlling the collector
  current by the base-emitter voltage
• This is only possible in the active region
• Cut-off and saturation regions correspond to the
  transistor turning fully off or on like a
  switch
• In the active region, the transistor is only
  partly on and the current can be controlled

7
Small Signals

• We want circuits with a linear response but real
  transistors arent linear

Current
Voltage
8
Small Signal Collector Current
9
Mutual Conductance

• IC and VBE are exponentially related
• iC and vBE, on the other hand, are approximately
  linearly related
• The constant of proportionality, gm, is known as
  the mutual conductance
• It isnt a real conductance, but it is the ratio
  between a current and a voltage

10
Estimating gm

• The small signal behaviour is estimated by a
  tangent to the exponential IC-VBE curve
• gm is, therefore, simply the gradient of the curve

11
Amplification
Assume that the transistor is biased in the
active region somehow
RC
IC
VC
VBE
12
Simple Common-Emitter Amplifier

• IB provides a d.c. base current to bias the
  transistor in the active region
• CIN couples the input voltage, removing the d.c.
  base bias voltage
• CIN is a short circuit to a.c. signals
• but an open circuit to the d.c. bias current
• vBE is, therefore, equal to vIN

13
Analysis
14
Biasing

• Gain is proportional to gm which is, in turn,
  proportional to IC
• In this circuit,
• Unfortunately, b has a very wide tolerance
• The gain is, therefore, not predictable

15
Reliable Biasing

• Collector current is set accurately regardless of
  b
• CE ensures that the whole of the a.c. input
  voltage is still dropped across VBE
• RB provides the d.c. base bias current
• Usually, the current source is approximated by a
  resistor

16
Practical Amplifier

• To analyse the circuit
• Determine quiescent conditions
• Calculate mutual conductance
• Calculate small signal performance
• Voltage Gain
• Input Impedance
• Output Impedance
• Cut-off frequency

17
The Story so Far

• Small signal analysis is used to simplify
  calculations by linearising the non-linear
  response of the transistor
• Using mutual conductance, gain calculations are
  now only a couple of lines of equations
• Careful choice of the biasing network leads to
  reliable performance
• Next time practical amplifier calculations,
  input output impedances and capacitor
  calculations