Chapter 5 Bipolar Junction Transistors (BJTs) This lecture covers

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Chapter 5 Bipolar Junction Transistors (BJTs) This lecture covers Powered By Docstoc
					Course Notes for EE 0257 Analysis and Design of Electronic Circuits



 Chapter 5: Bipolar Junction Transistors (BJTs)                       The transfer characteristics
                                                                      The basic configuration of a BJT amplifier is shown below, which is
                                                                      referred as common-emitter (CE) circuit or grounded-emitter
This lecture covers Section 5.3-5.4                                   configuration.
   1.   The transfer characteristics
   2.   Graphic analysis
   3.   Operation as a switch
   4.   DC analysis




Notes:




                                                                      The transfer function shown above can be broken down into three
                                                                      regions and explained here. In general, the relationship between the
                                                                      output vO (vCE) and the input vI (vBE) can be first and universally
                                                                      related by the I-V equation for the collector resistance RC:
                                                                                                vO = vCE = VCC − RC iC
                                                                      • For 0<vI<0.5V, the EBJ is shut-off, no current flows into the base,
                                                                          so the entire BJT is shut-down and acting as an open-circuit. The
                                                                          output will be close to VCC. That is from point X to Y.
                                                                      • When 0.5V<vI, the BJT enters the active mode (Y-Z region), the iC
                                                                          and vI become an exponential function:

Chapter 5: Bipolar Junction Transistors (BJTs)                                                                                           1
Course Notes for EE 0257 Analysis and Design of Electronic Circuits
                                                   vI
                                                                              c). Find the positive increment of vBE (above VBE) that drivers the BJT
                                      iC = I S e        VT

                                                                    vI
                                                                              to the edge of the saturation where vCE=0.3V
                                      vO = VCC − RC I S e                VT
                                                                              d). Find the negative increment in vBE that drives the BJT to within 1%
•   This will lead to a sharp exponential drop to the output until vO         of cut-off.
    drops to 0.4V below the base (vI). The CBJ is turned on (forward
    biased) and the BJT enters saturation.
•   In the saturation region, as we studied in the previous class, the
    saturation resistance RCEsat is very small, so the circuit behaves like
    a close-circuit. Since VCEsat changes little between 0.1-0.2V, the
    saturation currents almost remain constant.

The Amplifier Gain
Apparently, to operate the BJT as a amplifier, it must be biased in the
active region (YZ). It will be ideal to be biased right at the center of
the active region so we get the maximum output signal swing. The bias
point is labeled as Q or quiescent point. In this region, the transfer
function remain relatively linear:
                         V BEI
                IC = I S e       VT

                                         vI
                vO = VCC − RC I S e           VT

                                                             V BE
                    dv                       1                I R     V
                AV = O                   =−    I S e VT RC = − C C = − RC
                    dvI      v I =V BE
                                            VT                 VT      VT
The above equation is correct as long as the output amplified signal
voltage does not pull the BJT into the cut-off or saturation. Otherwise,
the positive or negative part of the output signal will be clipped off.

Example: Consider a common-emitter circuit using a BJT having
IS=10-15 A, a collector resistance RC=6.8kΩ and a power supply
VCC=10V.
a). Determine the bias voltage VBE needed to operate the BJT at
VCE=3.2V, what is the value of IC?

b). Find the voltage gain Av at this bias point. If an input sine-wave
signal of 5-mV is superimposed on VBE, find the amplitude of the
output sine-wave signal.
Chapter 5: Bipolar Junction Transistors (BJTs)                                                                                                     2
Course Notes for EE 0257 Analysis and Design of Electronic Circuits


The Graphical Analysis
Similar to the MOSFET, a graphical analysis is illustrative to see how
a BJT works as amplifier and the impact of the bias point to the signal
dynamic range. A typical BJT configuration is shown below.




•   First, the linear load I-V load line of the resistance RB will intersect
    the exponential I-V curve of the EBJ diode. This will determine the
    input bias point.
•   A superimposing of input AC signal vi will produce a vBE change
    as shown below.
•   Once the bias point VBE is determine, we shift the output iC vs. vCE,
    again, the collector resistance determine the output load line.
•   The intersect of the output load line with the I-V curve of the BJT
    at the bias point VBE will yield the output bias point.
•   The output current and voltage changes are then determined using
    the maximum and minimum values of vBE along the output load
    line shown below.
•   The location of the bias point Q will determine the allowable
    output signal swing.




Chapter 5: Bipolar Junction Transistors (BJTs)                                 3
Course Notes for EE 0257 Analysis and Design of Electronic Circuits
                                                                               drive the BJT completely into saturation to ensure safe operations.
Operation as a Switch                                                          In this case the saturation voltage VCEsat≈0.2V rather than 0.3 V.
To operate a BJT as an On-Off switch, we need to swing it between              And we have to use the β in the saturation region βforced.
the cut-off mode and the saturation mode. Given a circuit shown                                               V − VCEsat
below:                                                                                                I Csat = CC
                                                                                                                  RC
                                                                                                             I Csat
                                                                                                     IB =
                                                                                                            β forced

                                                                           •   beyond Similar to the MOSFET, a graphical analysis is illustrative
                                                                               to see how a BJT


                                                                           Example: The transistor shown below has β from 50 to 150. Find the
                                                                           value of RB resulting in saturation with an overdrive factor of 10.
•   When the input vI<0.5V, the EBJ turns off, therefore no current
    flow through the collector, the vC=VCC.
•   When vI goes beyond 0.5 V, the BJT is turned on. If the input is a
    step TTL signal, the input will lift the vBE up quickly to reach
    around 0.7V.
•   At this point, a small change of vBE will cause a big increase of iC
    and big voltage drop across RC.
•   When the vC is below vBE for about 0.4V. So vC=0.3V, the BJT
    enters the saturation region.
•   At the edge of the saturation (Edge-of-saturation or EOS),
    vC=0.3V, we can use this number to calculate both collector and
    the base currents.
                                      V − 0.3
                         I C ( EOS ) = CC
                                         RC
                                        I C ( EOS )
                        I B ( EOS ) =
                                            β
                       V I ( EOS ) = I B ( EOS ) R B + VBE
•   At the edge of the saturation, we can still use β in active mode to
    perform this analysis. However, in a practical situation, we need to
Chapter 5: Bipolar Junction Transistors (BJTs)                                                                                                  4
Course Notes for EE 0257 Analysis and Design of Electronic Circuits


Example 2: perform analysis of the following BJT circuits, the        Example 3: perform analysis of the following BJT circuits:
minimum β=30 for this BJT:




Chapter 5: Bipolar Junction Transistors (BJTs)                                                                                     5
Course Notes for EE 0257 Analysis and Design of Electronic Circuits

                                                                      Biasing in BJT amplifier circuits
Example 4: perform analysis of the following BJT circuit (β=100).     Like MOS amplifier, the biasing of BJT amplifier must lead to
                                                                      predictable and stable. The circuits must be in-sensitive of temperature
                                                                      drifting. It also need to tolerate large variation of βs in circuits consist
                                                                      of different type of BJTs.

                                                                      Let’s first exam two configurations that is sensitive to temperature and
                                                                      β changes:




                                                                      We see some drawback on these configurations:
                                                                      • The sharp exponential relationship iC=vBE will lead to a large
                                                                        drifting of IC and VCE. So this bias is not temperature stable.
                                                                      • For the second configuration, I B = (VCC − 0.7 ) / RB ,     IC = β I B ,
                                                                        however, if a circuit unit consists of a lot of BJT, the variation of
                                                                        β make this configuration unpredictable.

                                                                      One power-supply bias scheme:




Chapter 5: Bipolar Junction Transistors (BJTs)                                                                                                  6
Course Notes for EE 0257 Analysis and Design of Electronic Circuits
First, let’s see how the introduction of RE increase the temperature       Collector-to-base feedback bias scheme:
stability:
                            R2               RR                                                                         VCC = I E RC + I B RB + VBE
                   V BB =         VCC RB = 1 2
                          R1 + R2          R1 + R2                                                                                     IE
                                                                                                                        = I E RC +         RB + V BE
                           VBB − VBE                                                                                                  β +1
                   IE =
                        RE − RB /( β + 1)
To make the circuit insensitive to temperature and β, we just need to                                                               VBB − VBE
                                                                                                                         IE =
                                                  R                                                                              RE − RB /( β + 1)
                  V BB >> VBE (~ 0.7V ) RE >> B
                                                 β +1
Which is quite achievable, some more design consideration:
• If VBB is too high, then the voltage drop across RC and VCB is
   limited, we will not have enough voltage spans for signal. The          The resistance RB determines the signal span becomes:
   general rule of thumb is VBB (~VE)=1/3VCC, VCB=1/3VCC, and                                                            R
                                                                                                      VCB = I B RB = I E B
   ICRC=1/3VCC.                                                                                                         β +1
• To satisfy the 2nd requirement, we need RB to be small, which also       The feedback resistance RB also provides negative feedback, which
   leads to large drain of current across the voltage divider, and small   also stabilize the circuit. We will explain this in class.
   input resistance. The trade-off leads to the current flow through the
   voltage divider R1 and R2 to be 0.1-1 IE.                               Constant-current bias scheme:
• RE will also provide feedback to stabilize the bias current.
                                                                           The best way to bias a circuit is of course provide constant and never-
If two power supplies are available, the biasing circuit is shown below.   change current IE, this can be done by a constant-current source.




                                                                                                                                VCC − (−VEE ) − VBE
                                                                                                                      I REF =
                                                                                                                                        R




Chapter 5: Bipolar Junction Transistors (BJTs)                                                                                                        7