Electronic II (ECE235b)
Bipolar Junction Transistor (BJT)
Anestis Dounavis
The University of Western Ontario
Faculty of Engineering Science
Bipolar Junction Transistor (BJT)
5.7 Single stage BJT Amplifiers
There are basically 3 configurations for implementing single-stage BJT amplifiers:
1. Common emitter,
2. Common base,
3. Common collector.
All 3 configurations use the same structure with the same biasing configurations:
5.7.2 Characterizing BJT amplifiers
Before we examine BJT amplifiers, it is important to know how to characterize the performance of
amplifiers as circuit building blocks.
 A number of amplifier circuits studied are nonunilateral amplifiers. That is, they have
internal feedback that may cause their input resistance to depend on the load resistance.
Similarly, internal feedback may cause the output resistance to depend on the value of the
resistance of the signal source feeding the amplifier.
Four types of Unilateral Amplifiers:
a) Voltage amplifier:
Open Circuit voltage gain
Ideal characteristic
b) Current amplifier:
Ri  
Short Circuit current gain
Ideal characteristic
c) Transconductance amplifier:
Ri  0
vo
vi io  0
R0  0
Ais 
io
ii v o  0
R0  
Gm 
Short Circuit transconductance
Ideal characteristic
d) Transresistance amplifier:
Avo 
Ri  
R0  
Rm 
Open Circuit transresistance
Ideal characteristic
Ri  0
io
vi vo  0
vo
ii io  0
R0  0
For unilateral amplifiers the input resistance Ri, does not depend on the load resistance and the output
resistance Ro does not depend on the resistance of the signal source.
(a)
(b)
(c)
(d)
Table 1.1 The Four Amplifier Types
To accommodate nonunilateral amplifiers, table 5.5 presents a general set of parameters and
equivalent circuits that we will employ in characterizing and comparing transistor amplifiers
A.
B.
C.
Table 5.5
Definitions
Circuit A:
vi
ii

Input resistance: Rin 

Output resistance of proper amplifier: Ro 

Open circuit voltage gain: Avo 
vx
i x vi  0
vo
vi R L  
Circuit B:
io
v i RL  0
Short circuit transconductance: G m 

Circuit C:
vx
i x v sig  0

Output resistance Rout 

Open circuit overall voltage gain: Gvo 
vo
v sig RL  
Other parameters
Ri 

Input resistance with no load:

Voltage gain: Av 

Other parameters defined pg 462-463.
vi
i i RL  
vo
vi
Remarks:
1. Signal source has an open-circuit voltage vsig and an internal resistance Rsig. These parameters
can be an actual signal source, or the Thevenin equivalent of the output circuit of another
amplifier.
 RL can be an actual load or the input resistance of a succeeding amplifier stage in a
cascade amplifier.
2. Parameters Ri, Ro, Avo, Ais and Gm pertain to the amplifier properties; that is they do not depend
on the load values of Rsig and RL.
 Rin, Rout, Av, Ai, Gvo, and Gv may depend on one or both Rsig, R .
3. For nonunilateral amplifiers, Rin may depend on R , and Rout may depend on Rsig.
 For unilateral amplifiers Rin=Ri and Rout=Ro.
4. The resistance of the signal source Rsig and the input resistance Rin determine the current ii that
the amplifier draws from signal source. It also determine the voltage value vi.
5. The gain Av is the overall gain seen at the load RL.
L
L
Example 5.17
Vsig=10mV, Rsig=100k, and load resistance RL=10k. The following measurements were made
Vi(mV)
Vo(mV)
Without RL
9
90
With RL connected
8
70
Find the parameters for circuit A, (i.e. Rin, Avo, Ro).
5.7.3 Common-Emitter (CE) Amplifier
The Common-Emitter configuration is shown in Figure 5.60.
Figure 5.60 (a) A common-emitter amplifier using the structure of Fig. 5.59. (b) Equivalent circuit
obtained by replacing the transistor with its hybrid-p model.




To establish a signal ground (or AC ground) at the emitter, a large capacitor CE, usually in the
F or tens of F is connected between the emitter and ground.
This capacitor provides a low impedance to ground at all signal frequency of interest and is
called a bypass capacitor.
At low frequencies the capacitor is less effective as an AC short.
Capacitors Cc1 and Cc2 are included not to disturb the DC bias conditions.
Next, we must determine the terminal characteristics of the CE amplifier, its input resistance, voltage
gain, and output resistance.
 Note this amplifier is unilateral (Rin=Ri and Rout=Ro).
vi
 RB || r  r
ii
Input resistance:
Rin 
Output resistance:
Rout  RC || ro  RC
Output voltage:
v 0   g m v (r0 || RC || RL )
Open circuit voltage gain:
Voltage gain:
Av 
Avo 
if R B  r
if ro  RC
vo
  g m ( r0 || RC )   g m RC
vi RL  
vo
  g m ( r0 || RC || RL )   g m RC || RL
vi
Relationship between vsig and vi:
v i  v  v sig
if ro  RC
if ro  RC
Rin
RB || r
 v sig
Rin  Rsig
RB || r  Rsig
vo
R B || r
 ( r0 || RC || RL )

g m ( r0 || RC || RL )  
vsig
R B || r  Rsig
r  Rsig
The common emitter configuration provides large voltage and current gain, but Rin is low and Rout is
relatively high.
Overall voltage gain:
Gv 
5.7.4 Common-Emitter Amplifier with an Emitter Resistance
Figure 5.61 includes a resistance RE in the signal path between the emitter and ground. The resistance
RE can be used as a design parameter to change the characteristics of amplifier. RE is refered to as the
emitter degeneration resistance.
Figure 5.61 (a) A common-emitter amplifier with an emitter resistance Re. (b) Equivalent circuit
obtained by replacing the transistor with its T model.


Due to the resistance RE, the T-model is used since this significantly reduced the complexity of
the analysis.
In addition, the resistance ro is omitted to simplify the calculation
Input resistance:
Rin 

Rib 
v i ( re  Re )i e ( re  Re )(1   ) ib


 ( re  Re )(1   )
ib
ib
ib
vi
 RB || Rib  RB || (re  Re )(1   )
ii
Note the multiplication of (1+) is known as the resistance-reflection rule.
Output resistance:
Rout  RC
Output voltage:
v 0  ic ( RC || R L )  ie ( RC || R L )
Open circuit voltage gain:
v
 i eRC
  RC

RC
g m RC
g m RC
Avo  o





vi RL   ( re  Re )i e ( re  Re ) re (1  Re / re ) (1  Re / re ) (1  g m Re )

The resistance RE decreases the voltage gain.
Voltage gain:
Av 
v o  i e ( RC || RL )   ( RC || R L ) ( RC || RL )



vi
( re  Re )i e
( re  Re )
( re  Re )
since =1
Rin
RB || ( re  Re )(1   )
 vsig
Rin  R sig
RB || ( re  Re )(1   )  Rsig
Relationship between vsig and vi:
v i  vsig
Overall voltage gain:
Gv 
Relationship between v and vi:
v
re

v i re  Re
vo
Rin
 ( RC || RL )

vsig
Rin  Rsig ( re  Re )
The resistance RE of the CE amplifier results in the following characteristics:
1. The resistance RE increases the input resistance Rib by:
Rib (with Re )
( r  Re )(1   )
R
 e
 1  e  1  g m Re
Rib ( without Re )
re (1   )
re
2. The voltage gain from base to collector, Av is reduced by the factor (re  Re ) /   (1  g m Re ) .
3. For the same nonlinear distortion, the input signal vi can be increased by the factor
(re  Re ) / re  (1  g m Re ) .
4. High-frequency response is significantly improved (Chapter 6).
Example: Common-emitter with emitter resistance including ro.
For the common-emitter amplifier with an emitter resistance shown in Figure 5.61a) determine Rin
including ro in the analysis.
Problem 5.130
For the common-emitter amplifier shown let Vcc=9V, R1=27k, R2=15kRE=1.2 k and RC=2.2k.
The transistor has =100 and VA=100V.
1. Calculate the DC bias current IE.
2. If Rsig=10k, RL=2 k, find Rin, Rout, the voltage gain vo/vsig, and current gain io/ii.
Remove the capacitor at the emitter and recalculate Rin, the voltage gain vo/vsig, and current gain io/ii.
Figure P5.130
5.7.5 The Common-Base (CB) Amplifier
The Common-Base configuration is shown in Figure 5.62.
Figure 5.62 (a) A common-base amplifier using the structure of Fig. 5.59. (b) Equivalent circuit
obtained by replacing the transistor with its T model.


Since Rsig appears in series with the emitter terminal the T model is used.
To simply the analysis the resistor ro is not included.
vi
 re
ii
Input resistance:
Rin 
Output resistance:
Rout  RC
Output voltage:
v 0  i e ( RC || RL )  
Open circuit voltage gain:
Voltage gain:
Av 
Avo 
vi
( RC || R L )
re
vo

 RC  g m RC
vi RL   re
vo 
 ( R || R L )  g m ( RC || RL )
vi re C
Relationship between vsig and vi:
v i  v  v sig
Overall voltage gain:
Gv 
Short-circuit current gain: Ais 
Rin
re
 v sig
Rin  Rsig
re  Rsig
vo
re
 ( RC || RL )

g m ( RC || RL ) 
vsig
re  Rsig
re  Rsig
io
i
 i 
ii v o  0 ii
In summary
 CB amplifier exhibits a very low input resistance (re).
 A short circuit gain that is nearly unity ()
 Like the CE amplifier the output resistance is Rc.
 The open circuit voltage gain is positive and is equal in magnitude to the CE amplifier
g m ( RC || R L )
Since CB amplifier has low input resistance, the CB alone is not an attractive as a voltage amplifier. A
significant application for the CB circuit is as a unity-gain current amplifier or current buffer.
Problem 5.141: Find Rin and vo/vsig. Assume =100
Figure P5.141
5.7.6 The Common-Collector (CC) Amplifier or Emitter Follower
The Common-Collector configuration is shown in Figure 5.63.
Figure 5.63 (a) An emitter-follower circuit based on the structure of Fig. 5.59. (b) Small-signal
equivalent circuit of the emitter follower with the transistor replaced by its T model augmented with ro.
(c) The circuit in (b) redrawn to emphasize that ro is in parallel with RL. This simplifies the analysis
considerably.



For the common collector or emitter follower, the collector is to be at signal ground, thus Rc has
been eliminated.
For this configuration, it is relatively simple to take ro into consideration, thus ro is included into
the analysis.
This circuit is not unilateral; that is, the input resistance depends on the RL, and the output
resistance depends on Rsig. (Thus care must be exercised in the characterizing the CC amplifier).
vi
 RB || (   1)[ re  ( ro || RL )]
ii
Input resistance:
Rin 
Output resistance:
R sig || RB 

Rout  r0 ||  re 

 1 

Output voltage:
v0 
Open circuit voltage gain:
Voltage gain:
Av 
ro || R L
v
re  ro || RL 
Avo 
vo
ro

vi RL   re  ro
vo
ro || R L

vi re  ro || RL
Relationship between vsig and vi:
v i  v  v sig
Overall voltage gain:
Gv 
Short-circuit current gain: Ais 
Rin
RB || (   1)[ re  ( ro || RL )]
 v sig
Rin  Rsig
RB || (   1)[ re  ( ro || RL )]  Rsig
vo
RB
(   1)( ro || R L )

vsig R sig  R B ( R sig || RB )  (   1)[ re  ( ro || RL )]
see Figure 5.64
io
  1
ii v o  0
In summary
 CC amplifier exhibits a very high input resistance.
 A low output resistance.
 A voltage gain that is less than one. The voltage emitter follows closely the voltage at the input,
which gives the circuit the name emitter follower.
 CC amplifiers are often used as the last stage or output stage in a multi-stage amplifier due to
high input resistance and low output resistance.
 CC amplifier has high current gain.
Figure 5.64 (a) An equivalent circuit of the emitter follower obtained from the circuit in Fig. 5.63(c)
by reflecting all resistances in the emitter to the base side. (b) The circuit in (a) after application of
Thévenin theorem to the input circuit composed of vsig, Rsig, and RB.
Problem 5.144:
For the emitter follower in Figure 5.144, the DC component of vsig is zero, find the DC emitter current.
Assume =100. Neglecting ro, find Rin, voltage gain vo/vsig, current gain io/ii and output resistance Rout.
Figure P5.144
5.7.7 Summary and Comparison




CE configuration is best suited for realizing the bulk of the gain required in the amplifier.
Depending on the gain required several CE stages can be used.
Including Re in the CE amplifier provides a number of performance improvements at the
expense of gain reduction.
CB has low input resistance and is useful for high-frequency applications (Chapter 6).
CE or emitter follower finds application as a voltage buffer, due to its high input resistance and
low output resistance.