Analogue
Electronics
(15B11EC411)
Lecture 5: Common Collector
Configuration
Table of Contents
• CC Amplifier
• BJT Configurations
• CC Amplifier or Emitter Follower
• CC Amplifier - 𝑅𝑖𝑛
• CC Amplifier - 𝐴𝑣𝑜 &𝑅𝑜
• CC Amplifier - 𝐺𝑣
• Analysing CC or Emitter Follower Amplifier
• Summary
• Summary and Applications amplifier
CC Amplifier
CC Amplifier
I/P current is 𝐼𝐵 and I/P voltage is 𝑉𝐵𝐶
O/P current is 𝐼𝐸 and O/P voltage is 𝑉𝐶𝐸
We know, 𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵 then, 𝐼𝐸 = 𝛼𝐼𝐸 + 𝐼𝐶𝐵𝑂 + 𝐼𝐵
1 1
1 − 𝛼 𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶𝐵𝑂 𝐼𝐸 = 𝐼𝐵 + 𝐼
1−𝛼 1 − 𝛼 𝐶𝐵𝑂
1
Now, =𝛾
1−𝛼
Substituting 𝐼𝐸 = 𝛾𝐼𝐵 + 𝛾𝐼𝐶𝐵𝑂 Where, 𝛾 is a current amplification factor in common collector
configuration
CC Amplifier
For I/P characteristics reverse biased characteristics of 𝐼𝐵 plotted
against and I/P voltage 𝑉𝐵𝐶
For O/P characteristics one can replace current is 𝐼𝐸 with 𝐼𝐶 by using
𝐼𝑐 = 𝛼𝐼𝐸 and plot characteristics against O/P voltage is 𝑉𝐶𝐸
The characteristics is same as of CE as 𝛼 ≈ 1 and I/P current is 𝐼𝐵
BJT Configurations
Relation between 𝛼, 𝛽, and 𝛾
𝐼𝐶 𝐼𝐶 𝐼𝐸
𝛼= 𝛽= 𝛾=
𝐼𝐸 𝐼𝐵 𝐼𝐵
Then,
𝛼 𝛼 1
𝛽= 𝛾=
1−𝛼 1−𝛼
𝛽 𝛽 𝛾 =1+𝛽
𝛼=
1+𝛽
𝛾−1 𝛽 =𝛾−1 𝛾
𝛼=
𝛾
CC Amplifier or Emitter Follower
CC amplifier-Application in the design of both small-signal amplifiers and amplifiers that are required to
handle large signals and deliver substantial amounts of signal power to a load.
The emitter follower is fed with a signal source (𝑣𝑠𝑖𝑔 , 𝑅𝑠𝑖𝑔 ) and has a load resistance 𝑅𝐿 connected
between emitter and ground.
We are assuming that RL includes both the actual load and any other resistance that may be present
between emitter and ground. Normally the actual RL would be much lower in value than such other
resistances and thus would dominate.
Since the BJT has a resistance Re connected in its emitter, it is most convenient to use the T model to
represent the BJT.
CC Amplifier or Emitter Follower
Our interest lies to find out following parameters
𝑣𝑖
𝑅𝑖𝑛 =
𝑖𝑖
𝑣𝑥
𝑅𝑜 = ቤ
𝑖𝑥 𝑣 =0
𝑖
𝑣𝑜
𝐴𝑣𝑜 = ቤ
𝑣𝑖 𝑅
𝐿 =∞
𝑣𝑜
𝐺𝑣 =
𝑣𝑠𝑖𝑔
CC Amplifier - 𝑅𝑖𝑛
Replacing the circuit with T-model
Effect of 𝑟𝑜 is neglected as it will become parallel to the
load resistance as 𝑅𝐿 is low so 𝑟𝑜 can be neglected
1. Input resistance
𝑖𝑒 = 𝑖𝑏 + 𝑖𝑐
𝑣𝑖 𝑣𝑖
𝑅𝑖𝑛 = =
𝑖𝑖 𝑖𝑏 𝑖𝑒 = 𝑖𝑏 + 𝛽𝑖𝑏
𝑖𝑒 𝑖𝑒 = 𝑖𝑏 (1 + 𝛽)
𝑖𝑏 = and
(1 + 𝛽)
𝑣𝑖
𝑖𝑒 =
(𝑟𝑒 + 𝑅𝐿 ) 𝑖𝑒 (𝑟𝑒 + 𝑅𝐿 )
𝑅𝑖𝑛 = = (𝛽 + 1)(𝑟𝑒 + 𝑅𝐿 )
𝑖𝑒 /(𝛽 + 1)
CC Amplifier - 𝐴𝑣𝑜 &𝑅𝑜
2. Open circuit voltage gain:
But we will first find voltage gain 𝐴𝑣
𝑣𝑜 𝑖𝑒 𝑅𝐿 𝑅𝐿
𝐴𝑣 = = =
𝑣𝑖 𝑖𝑒 (𝑟𝑒 + 𝑅𝐿 ) 𝑟𝑒 + 𝑅𝐿
Now,
𝑣𝑜
𝐴𝑣𝑜 = ቤ then 𝐴𝑣𝑜 ≈ 1
𝑣𝑖 𝑅
𝐿 =∞
𝑣𝑥
3. The output resistance 𝑅𝑜 = ቤ 𝑅𝑜 = 𝑟𝑒
𝑖𝑥 𝑣 =0
𝑖
CC Amplifier - 𝐺𝑣
4. Finding overall voltage gain
𝑣𝑜
𝐺𝑣 = Considering for load resistance 𝑅𝐿
𝑣𝑠𝑖𝑔
𝑅𝑖𝑛 𝑣𝑠𝑖𝑔
𝑣𝑜 = 𝑖𝑒 𝑅𝐿 𝑣𝑖 =
𝑅𝑠𝑖𝑔 + 𝑅𝑖𝑛
and, 𝑣𝑖 𝑅𝑖𝑛 𝑣𝑠𝑖𝑔
𝑖𝑒 = =
(𝑟𝑒 + 𝑅𝐿 ) (𝑅𝑠𝑖𝑔 + 𝑅𝑖𝑛 )(𝑟𝑒 + 𝑅𝐿 )
𝑖𝑒 𝑅𝐿 𝑅𝑖𝑛 𝑅𝐿 (𝛽 + 1)(𝑟𝑒 + 𝑅𝐿 )
𝐺𝑣 = =
𝑖𝑒 (𝑅𝑠𝑖𝑔 + 𝑅𝑖𝑛 )(𝑟𝑒 + 𝑅𝐿 ) (𝑅𝑠𝑖𝑔 + (𝛽 + 1)(𝑟𝑒 + 𝑅𝐿 ))(𝑟𝑒 + 𝑅𝐿 )
𝑅𝐿 (𝛽 + 1)
𝐺𝑣 =
𝑅𝑠𝑖𝑔 + (𝛽 + 1)𝑟𝑒 + (𝛽 + 1)𝑅𝐿
Analysing CC or Emitter Follower Amplifier
For Example: I/P source is of 200 𝑚𝑉 with internal resistance of 100 𝐾Ω and the load resistance is 1𝐾Ω
If we directly put the source with load
200
200 𝑣𝑖 = × 2 ≈ 4𝑚𝑉
𝑖= 100 + 1
100 + 1
Which is severe I/P distortion
Alternate approach is to use an emitter follower amplifier between the source and the load
Emitter follower gives a voltage gain of unity
This condition is valid when signal is sufficient in strength
Analysing CC or Emitter Follower Amplifier
For Example: I/P source is of 200 𝑚𝑉 with internal resistance of 100 𝐾Ω and the load resistance is 1𝐾Ω
For 𝑅𝑖𝑛 = 100𝐾Ω 𝑅0 = 10Ω
200 × 100
𝑣𝑖 = = 100𝑚𝑉
100 + 100
𝑣𝑜𝑜
𝐴𝑣𝑜 =1=
𝑣𝑖
𝑣𝑜𝑜 𝑅𝐿 1000
𝑣𝑜𝑜 = 100 𝑚𝑉 𝑣𝑜 = = 100 × ≈ 99 mV
𝑅𝐿 + 𝑅𝑜 1000 + 10
𝑣𝑜 is 99% of 𝑣𝑖
Analysing CC or Emitter Follower Amplifier
Gain of emitter follower can be used draw an equivalent circuit
𝑣0 𝑅𝐿 (𝛽 + 1) 𝑣0 𝑅𝐿
𝐺𝑣 = = 𝐺𝑣 = =
𝑣𝑠𝑖𝑔 𝑅𝑠𝑖𝑔
𝑣𝑠𝑖𝑔 𝑅𝑠𝑖𝑔 + (𝛽 + 1)𝑟𝑒 + (𝛽 + 1)𝑅𝐿 + 𝑟 + 𝑅𝐿
(𝛽 + 1) 𝑒
Analysing CC or Emitter Follower Amplifier
Thevenin representation of the Obtaining 𝐺𝑣𝑜 from the
output of the emitter follower. equivalent circuit
The emitter follower with
Obtaining 𝑅𝑜𝑢𝑡 from the Rin and Rout determined
equivalent circuit with simply by looking into the
𝑣𝑠𝑖𝑔 set to zero. input and output terminals,
respectively.
Analysing CC or Emitter Follower Amplifier
Thevenin Representation of the Emitter-Follower output-
A more general representation of the emitter-follower output is shown in Figure(a).
Here 𝐺𝑣𝑜 is the overall open-circuit voltage gain that can be obtained by setting RL = ∞) in the circuit of
Figure(b), as illustrated in Figure(b).
The result is 𝐺𝑣𝑜 = 1. The output resistance 𝑅𝑜𝑢𝑡 is different from 𝑟𝑜 .
To determine 𝑅𝑜𝑢𝑡 we set 𝑣𝑠𝑖𝑔 to zero (rather than setting 𝑣𝑖 to zero).
Again we can use the equivalent circuit in Figure(b) to do this, as illustrated in Figure(c).
Finally, we show in Fig. (d) the emitter-follower circuit together with its 𝑅𝑖𝑛 and 𝑅𝑜𝑢𝑡. Observe that 𝑅𝑖𝑛 is
determined by reflecting 𝑟𝑒 and 𝑅𝐿and to the base side (by multiplying their values by (𝛽 + 1)). To determine
𝑅𝑜𝑢𝑡 grab hold of the emitter and walk (or just look!) backward while 𝑣𝑠𝑖𝑔 = 0.
We note that unlike the CE and CB amplifiers we studied earlier, the emitter follower is not unilateral. This is
manifested by the fact that 𝑅𝑖𝑛 depends on 𝑅𝐿 and 𝑅𝑜𝑢𝑡 depends on 𝑅𝑠𝑖𝑔
Summary
Summary and Applications amplifier
The CE configuration is the one best suited for realizing the bulk of the gain required in an amplifier.
Depending on the magnitude of the gain required, either a single stage or a cascade of two or three stages
can be used.
Including a resistor in the emitter lead of the CE stage provides a number of performance improvements at
the expense of gain reduction.
The low input resistance of the CB amplifier makes it useful only in specific applications, it has a much better
high-frequency response than the CE amplifier. This superiority will make it useful as a high-frequency
amplifier, especially when combined with the CE circuit.
The emitter follower finds application as a voltage buffer for connecting a high resistance source to a low-
resistance load and as the output stage in a multistage amplifier, where its purpose is to equip the amplifier
with a low output-resistance.
References
1. Sedra, Smith, Microelectronics Circuits, Sixth Edition, Oxford.
