Bipolar Junction Transistors (BJTs)
4.12 Single-Stage BJT Amplifiers
Common-Emitter Amplifier with
a Resistor in the Emitter
Step 1: Choosing the equivalent circuit;
Step 2: Simplifying the circuit by
ignoring r0;
Step 3: Determining the input
resistance Ri
vb = ie ( re + Re )
since
ib = (1 − α )ie =
Ri ≡
ie
β +1
vb
= ( β + 1)( re + Re )
ib
Notice that the input resistance
looking into the base is (b+1) times
the total resistance in the emitter,
which is known as the resistancereflection rule.
Electronic Circuits,
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
Lesson 20&21
Bipolar Junction Transistors (BJTs)
2.12 Single-Stage BJT Amplifiers
Common-Emitter Amplifier with a Resistor in the Emitter (cont.)
Step 4: Determining the Voltage Gain;
v o = −α i e R C
v b = ie ( re + Re )
We knew that
α ≈1
vo
− α RC } − RC
=
≈
vb
re + R e
re + R e
Therefore
The voltage gain between base and
collector is equal to the ratio of the total
resistance in the collector to the total
resistance in the emitter. This statement
is a general one and applies to any BJT
amplifier circuit.
To obtain the overall voltage gain, we
multiply
vb
Ri
=
vs
Ri + R s
Since
R i = ( β + 1)( re + R e )
Electronic Circuits,
( β + 1)( re + Re )
vo vo vb
RC
=
⋅ =−
⋅
v s vb v s
re + Re R s + ( β + 1)( re + Re )
R
( β + 1)
=− C ⋅
1 R s + ( β + 1)( re + Re )
β RC
≈−
R s + ( β + 1)( re + Re )
Av =
The gain is insensitive to b, particularly when Rs is small
Step 5: Determining
Ai = io / ib = − β ,
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
R o = RC
Lesson 20&21
Bipolar Junction Transistors (BJTs)
2.12 Single-Stage BJT Amplifiers
Common-Emitter Amplifier with a Resistor in the Emitter (cont.)
Electronic Circuits,
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
Lesson 20&21
Bipolar Junction Transistors (BJTs)
2.12 Single-Stage BJT Amplifiers
Common-Base Amplifier
Step 1: Choosing the equivalent circuit;
Step 2: Simplifying the circuit by ignoring r0;
Step 3: Determining the input resistance Ri
VT
IE
Step 4: Determining voltage gain:
Ri = re =
v o = −α ie RC
since
thus
vs
ie = −
R s + re
v
α RC
Av = o =
v s R s + re
Low input resistance, not suitable for voltage
amplifier;
High output impedance and about unit
current gain, suitable for current buffer;
Step 5: Current gain and output resistance
− α ie
i
Ai = o =
=α
− ie
ii
Rarely used alone because no voltage
gain, nor current gain;
R o = RC
Electronic Circuits,
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
Lesson 20&21
Bipolar Junction Transistors (BJTs)
2.12 Single-Stage BJT Amplifiers
Common-Collector Amplifier
Very commonly used for both small signal and large signal
and digital circuits. The main purpose of the CC circuit is to
connect a source having a large resistance Rs to a load with
a relatively low resistance.
Step 1: Choosing the equivalent circuit;
Step 2: Redraw circuit diagram that
lead to faster circuit analysis;
Step 3: It can be found that the circuit is a
simplified version (RC=0) of the CE amplifier with
a resistor in the emitter (Re=r0//RL).
Step 4: Determining circuit parameters:
Ri = ( β + 1)[ re + ( r0 // R L )]
For the case
re << R L << r0
Ri = ( β + 1) R L
The amplifier has a relatively large input resistance. The
equivalent load to the source is magnified by (b+1) times.
Electronic Circuits,
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
Lesson 20&21
Bipolar Junction Transistors (BJTs)
2.12 Single-Stage BJT Amplifiers
Common-Collector Amplifier (cont.)
Following the rule of voltage divider, we have
vb
( β + 1)[ re + ( r0 // R L )]
=
v s R s + ( β + 1)[ re + ( r0 // R L )]
1
The overall voltage gain
re → 0
}
( r0 // R L )
v o vb vo
=
⋅ ≈ 1⋅
≈ 1
Av =
v s v s vb
re + ( r0 // R L )
By inspection that r0 in parallel with re, which is in series
with Rs reflected into the emitter by dividing it by (b+1),
we can find the output resistance
⎡
Rs ⎤
Rs
≈
+
→0
Ro = r0 // ⎢ re +
r
e
⎥
β + 1⎦
β +1
⎣
We can show that
i
ro
Ai ≡ o = ( β + 1)
ib
ro + R L
R L << r0
}
≈ β +1
The circuit is ideally suitable for applications as a voltage
buffer amplifier
Electronic Circuits,
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
Lesson 20&21
Bipolar Junction Transistors (BJTs)
Grounded
Emitter
rπ = β
Ri
RC // ro (H)
Ro
Av
Ai
VT (M)
IC
−
β ( RC // ro )
R s + rπ
−β
(H)
ro
(H)
RC + ro
Single-Stage BJT Amplifiers
Common
Emitter
Common
Base
( β + 1)( re + Re ) (M)
R o = RC
(H)
− β RC
(H)
R s + ( β + 1)( re + Re )
−β
(H)
Most versatile and useful configuration. It provides both voltage gain
and current gain. The drawback is
the high output resistance and
poor high-frequency response.
Electronic Circuits,
re =
VT (L)
IE
Ro = RC (H)
α RC
R s + re
α
Common
Collector
( β + 1)[ re + ( r0 // R L )] (H)
⎡
Rs ⎤
r0 // ⎢ re +
⎥ → 0 (L)
+
β
1
⎣
⎦
re → 0
(L)
(L)
Useful to be
current buffer
Dept. of Elec. Eng., The Chinese University of Hong Kong, Prof. K.-L. Wu
}
( r0 // R L )
≈ 1 (L)
re + ( r0 // R L )
( β + 1)
ro
ro + R L
(H)
Ideal for voltage
buffer. Useful to be
the last stage of a
multistage amplifier
Lesson 20&21