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Linear Circuit Models for Bipolar Junction Transistors

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					              Linear Circuit Models for Bipolar Junction
                                  Transistors

Introduction

Bipolar Junction Transistors (BJTs), Field Effect Transistors (FETs) and vacuum
tubes all have similar circuit models. The heart of each model is a dependent
source. In fact, the importance of these electronic devices, at least from a circuit
theoretic point of view, is that they are practical dependent sources that mimic
the behavior (more-or-less) of ideal dependent sources in much the same way
that practical resistors, capacitors and inductors mimic the behavior of their ideal
counterparts. The dependent-source-like behavior permits small input signals to
these devices to control much larger signals and thereby accomplish two key
processes of electronics: amplification and switching.


The Linear Circuit Model

We concentrate on modeling BJTs because they are slightly more difficult to
model than FETs or vacuum tubes. Consider the (nonlinear) i-v characteristics of
the collector-emitter port of a BJT that resides in an otherwise linear circuit
(whose i-v characteristic is represented by the linear load line in the following
figure):
                                                                                             2




For amplification applications, we wish to operate the transistor somewhere in
the region where the curves are more-or-less equally spaced and parallel, the so-
called active region. The active region stretches between the saturation region
near the vertical axis where the curves bend and coalesce and the cut-off region
near the horizontal axis where the base current is zero and the collector current
is small. The load line, which is the i-v characteristic of the linear circuit that the
transistor sees at its collector-emitter port, normally lies mostly in the active
region. Because we assume all of the circuit except for the transistor is linear, the
load line is a straight line. (In fact, the slope of the load line is just 1 Rth , where
Rth is the Thevenin resistance of the linear circuit seen by the collector-emitter
port. The Thevenin voltage of this circuit is the intersection of the load line with
the Vce axis.)

With no signal applied, Vce (the collector-to-emitter voltage) and I c (the collector
current) take on constant values Vceq and I cq , respectively. The point Vceq , I cq  is
called the quiescent point (because of the absence of the signal) or simply the
operating point. The intersection of the load line with the collector-emitter i-v
characteristic for a given base current, I b , determines the operating point, the
unique point that lies on the i-v characteristics of both the transistor and the rest
                                                                                              3



of the circuit. Often, but not always, we will choose the operating point to lie near
the center of the active region. (In the figure above, we have shown a possible
operating point Vceq , I cq    6V ,0.6 mA that requires a base current I b  20  A .)


In setting the operating point, we must be careful that the power dissipated by the
transistor at the operating point, Vceq I cq , does not exceed the power dissipation
rating specified by the manufacturer on the data sheet.

We model the i  v characteristics in the region around the operating point with a
set of equally spaced parallel lines that approximates the actual i-v
characteristics in the active region:




This model is clearly horrible in some regions (in the saturation region,
especially), but is not too bad in the active region. The major merit of this
obviously oversimplified model, as we will see later, is that it allows us to
construct a circuit model of the transistor by using only linear circuit elements (as
opposed to nonlinear circuit elements) and allows us to apply the powerful and
relatively simple methods of linear network analysis to circuits that contain
transistors. The model turns out to be more useful than we might reasonably
expect because we so frequently try to design analog electronic circuits to
behave in a linear fashion. The linear circuit model therefore lets us carry out an
approximate design and then calculate the ideal behavior of such circuits. We
                                                                                             4



use special designer tricks (such as negative feedback) to minimize the effects of
the nonlinearities in actual transistor i-v characteristics on the performance of the
circuit.

For convenience, we can term the above model our geometrical model since it
approximates the transistor i-v characteristics by equally spaced parallel lines. In
this model, the quantity I c is the increase in collector current, I c , for an
increase, I b , in the base current, I b . This geometrical model of the transistor
can be transformed into an algebraic model by writing the equations
corresponding to the straight lines.

To begin, let's start with the line corresponding to I b  0 . Recall the slope-
intercept form of straight lines:

           Ic   slopeVce   intercept 

The slope clearly has dimensions of Siemens, and hence is a conductance. We
call it G . The intercept, for I b  0 , is I c* . The algebraic form of the straight line
that corresponds to I b  0 , therefore, is:

           I c  GVce  I c*


In the straight lines for which I b  0 , only the intercept changes – the slope is
unchanged. More specifically, as I b increases by I b , we must add I c to the
intercept. The number of I c 's that we must add is I b / I b . Thus, the appropriate
intercept for a line corresponding to a given I b is

            Ib
                I c  I c*
           I b

so that we can write

                               I c
           I c  G Vce             Ib  I c
                                           *

                               I b
                                                                                       5



or

        Ic = G Vce +  I b + I c *

where

               I c
         
               I b

is the customary  (or h fe ) of the BJT. We term this equation for the collector
current an algebraic model of the transistor (in the active region) in that it gives
I c as a linear function of I b and Vce in terms of transistor parameters G ,  and
I c* . Since G is small (the lines are nearly horizontal and hence have nearly zero
slope), I c is a relatively weak function of Vce and varies mainly with I b .

We now seek an equivalent circuit that exhibits the same i-v characteristics at the
collector-emitter terminals as the algebraic equation. The GVce term just
corresponds to a conductance, G :




We can include the  I b term as a dependent current source:




The I c* term can be included as an independent current source:
                                                                                        6




For simplicity in notation, we can include the I c* source in the dependent source:




This circuit model exhibits approximately the same i-v characteristics at the
collector-emitter port as does the original transistor if its operation is limited to the
active region of its i-v characteristics.

What about the base-emitter port? The characteristic curve looks like this:
                                                                                        7



(This i-v characteristic is typical of solid state PN junctions, exactly what appears
across the emitter-base terminals of the BJT.) The geometrical model for this i-v
characteristic in the active region is a straight line:




We again use the slope intercept formula but, following tradition, we write Vbe as
a function of I b instead of vice versa:

       Vbe   slope Ib  intercept 

The slope clearly has the dimensions of Ohms and hence is a resistance, r . (In
the above graph, the slope of the line is 1 r since it shows I b vs Vbe instead of
vice versa.) The intercept is Vbe . Thus, we have
                                *




       Vbe  r I b  Vbe
                       *




This equation is our algebraic model of the i-v characteristics of the BJT at the
base-emitter port. The circuit that has this i-v characteristic is:
                                                                                    8




For transistors made of silicon, typically Vbe  0.7 V .
                                             *




The overall linear equivalent circuit for the BJT is:




This linear circuit is equivalent to the original BJT in the sense that if it is
substituted for the BJT, the currents and voltages in the rest of the circuit in
which the transistor resides are unchanged by the substitution to the extent that
the nonlinearities in the transistor can be neglected. (We'll see later that negative
feedback can reduce the effects of the nonlinearities.)


Application of the Linear Equivalent Circuit to the Analysis of a BJT
Common Emitter Amplifier

For specificity, consider an NPN BJT common emitter amplifier:
                                                                                          9




In this circuit, vth  t  and Rth might represent the Thevenin equivalent circuit of a
preceding amplifier stage, a signal generator or a sensor of some sort. We
replace the transistor by its linear equivalent circuit:




At this point, we might ask why not just use this equivalent circuit for the
transistor and forget the transistor, especially since the equivalent circuit is linear
(a highly desirable feature in most amplifiers) and the transistor is not. The
answer is that we cannot buy the dependent source at Radio Shack. In fact,
when we buy a transistor, all we really want is the dependent source, but TI or
Motorola or whoever insists on giving us r , Vbe , I c* and G , which we really do
                                               *


not want, but have to take anyway. Nevertheless, the linear equivalent circuit is
                                                                                      10



extremely convenient for approximate analysis of the circuit, as we are about to
see.

To facilitate the analysis of the circuit, we divide the independent sources into
two groups, signal and bias, and apply the superposition theorem to permit
calculation of their effects separately. The signal sources are all independent
sources that generate what we consider to be signals. In our case, vth  t  is the
only signal source. The bias sources are all other independent sources. In our
case, the bias sources are Vbe , I c* and Vcc . In applying the superposition theorem,
                             *


all dependent sources must, of course, be left on all the time.

The superposition theorem tells us that a part of each voltage or current results
from the signal sources and another part from the bias sources. We reflect this
fact by changing notation to represent the signal part of a voltage or current by
lower case letters and the bias part by upper case letters:

       I  i t   I

       V  v t   V

To apply superposition, we first set the bias sources to zero to obtain the signal
equivalent circuit (sometimes called the a-c equivalent circuit). Note that, for
example, setting Vcc  0 connects Rb1 and Rb 2 in parallel and connects Rc to
ground:
                                                                                       11




where Rb  Rb1 || Rb 2 . From this circuit, we note that the signal equivalent circuit
of the transistor is:




This is the only part of the linear equivalent circuit for the transistor that comes
into play when all the bias independent sources are turned off. We can now look
at how this model relates to two common signal models for BJTs:




Note that
                                                                                          12



        r  hie  rbb '  rb ' e

        G  hoe  g ce

         ib  g m vb ' e

        vb ' e  rb ' e ib

Because  ib  g m vb ' e and vb ' e  rb ' e ib , then  ib  g m rb ' e ib so that

          g m rb ' e

It may seem silly to split hie into two pieces  rbb '  rb 'e  , but when considering the
high frequency performance of BJTs, we'll see that there are physically
meaningful capacitors (shown in dotted lines in the figure) connected between b'
(not b) and c and between b' and e. Thus, the hybrid  equivalent circuit is
convenient because it is more easily extended to the high frequency case. At low
frequencies, our model, the h parameter model and the hybrid  model are
essentially equivalent. We usually use the hybrid  form of the signal equivalent
circuit because it can be used at both high and low frequencies.

We will return to the signal equivalent circuit later. Right now, however, we set
the signal source(s) to zero to find the bias equivalent circuit:
                                                                                         13




From this circuit, we note that the bias equivalent circuit of the transistor is:




This is the only part of the linear equivalent circuit for the transistor that comes
into play when all the signal independent sources are turned off.

This bias equivalent circuit (sometimes called the d-c equivalent circuit) will allow
us to analyze how stable this circuit is from the bias point of view. That is, we can
analyze how much the operating point changes as the transistor parameters vary
because of temperature or of differences in parameter values among transistors
of the same type.

A Rough Estimate of the Parameter r = hie = rbb' + rb'e


Notice that the parameter r  hie  rbb '  rb ' e appears in both the bias and signal
equivalent circuits. We can get a rough estimate of its value by recognizing that
the base-emitter terminals of the BJT connect across a solid state PN junction
                                                                                       14



and by using the i-v characteristic of an ideal solid state PN junction to
approximate the base-emitter characteristic of the actual transistor:

                      eV     
        Ib  I o exp  be  - 1
                      kT     

where

        I o is a small constant (with units of current) that varies from device to
        device

        e  1.6  10-19 C is the charge on an electron

        k  1.38  10-23 J / K is Boltzmann's constant and

        T is the temperature of the device in K

             kT
The quantity    has the units of voltage and at T  300 K (roughly room
              e
temperature),

        kT
            0.026 V
         e

If we take the slope of the line in the linear model, 1 r , to be the tangent to the
I b vs Vbe curve, then we have:

        1         Ib       e       eV 
                       Io    exp  be 
        r         Vbe      kT      kT 

From the ideal PN junction i-v characteristic we find:

                 eV 
        I o exp  be      Io    Ib 
                 kT 

Thus,
                                                                                     15


       1         e
                       Io          Ib 
       r         kT

But at typical operating points, Vbe  k T e so that I b  I o . Thus,

       1   e
             Ib
       r   kT

Now GVce  I cq and I c  I cq so that from our earlier result
                      *




       I c  G Vce   I b  I c
                               *




evaluated at a typical operating point we see that

       I cq   I bq


or

                 1
       I bq          I cq
                 

That is, the GVce term and the I c* term are usually dominated by  I b , the
dependent source term. In that case, we find:

                kT     
       r 
                 e     I cq


At T  300 K (about room temperature), we find:

                             
       r  0.026
                             I cq


This result, though approximate, gives at least a rough idea of the value for
r  hie  rbb '  rb ' e when no more accurate value is available. The result does
accurately reflect the facts that r decreases with high quiescent currents and
increases with high values of  (when little base current is drawn for a given
value of the collector current).

				
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