1. Common Emitter Amplifier
The common emitter amplifier is excited at the base of the BJT with the output taken at the collector. Other than the circuit below, there many other ways to bias this amplifier
Figure 7.56 (a) A common-emitter amplifier using the classical biasing arrangement of Fig. 7.52(a).

2. Common Emitter Amplifier
We are primarily interest in the small-signal characteristics. The following is the its small-signal equivalent circuit (notice we include ro in the small-signal model:
Input
Resistance
Output
Resistance
Figure 7.56 (b) Equivalent circuit and analysis of a common-emitter amplifier using the classical biasing arrangement of Fig. 7.52(a)..

3. Common Emitter Amplifier
The overall small-signal voltage gain:
At the input
Therefore, the overall small-signal gain:

4. Common Emitter Amplifier
Substitute the Av expression into above equation, we have
When r<<RB1 and RB2, Rin r , then
Recall that r=/gm, then

5. Example 7.12 (Page 463)
Design the basic circuit below for VCC=12 V and use rule of thumb to establish a current IE=1 mA. Assume =100

6. Common Collector Amplifier
The common collector amplifier is excited at the base of the BJT with the output taken at the emitter with a gain close to unit. Thus it is also called Emitter Follower.
Figure 7.59 (a) An emitter-follower circuit.

7. Common Collector Amplifier
The Early effect ro is included in the small-signal model because it has appreciable effect. ix
Figure (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model. Note that ro is included because it is easy to do so. Normally, its effect on performance is small.

8. Common Collector Amplifier
From the small-signal circuit, we can see that
𝑣 𝑖 = 𝑖 𝑒 ( 𝑟 𝑒 + 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿 )
𝑅 𝑖𝑏 = 𝑣 𝑖 𝑖 𝑏 =(+1)( 𝑟 𝑒 + 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿 )
Noting ib=ie/(+1) , we have
The input resistance is
Can be very large!
Figure (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model. Note that ro is included because it is easy to do so. Normally, its effect on performance is small.

9. Common Collector Amplifier
𝑣 𝑜 = 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿 𝑟 𝑒 + 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿 𝑣 𝑖
The output voltage is
𝐴 𝑉 = 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿 𝑟 𝑒 + 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿
From which we can directly determine that
The overall small-voltage gain:
𝐺 𝑉 = 𝑅 𝑖𝑛 𝑅 𝑖𝑛 + 𝑅 𝑠𝑖𝑔 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿 𝑟 𝑒 + 𝑅 𝐸 | 𝑟 𝑜 | 𝑅 𝐿
Clearly, the overall small voltage gain is less than one but close to one.
Figure (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model. Note that ro is included because it is easy to do so. Normally, its effect on performance is small.

10. Common Collector Amplifier
Apply KVL from the output trough the input of the small-signal circuit
Using KCL at the emitter terminal
𝑖 𝑥 = 𝑣 𝑜 𝑅 𝐸 || 𝑟 𝑜 − 𝑖 𝑒
Substitute ie into vo, we can find the output resistance
𝑅 𝑜𝑢𝑡 = 𝑟 𝑜 ||𝑅 𝐸 || 𝑟 𝑒 + 𝑅 𝐵 || 𝑅 𝑠𝑖𝑔 +1 ix
𝑅 𝑜𝑢𝑡 ≈ 𝑟 𝑒 + 𝑅 𝐵 || 𝑅 𝑠𝑖𝑔 +1 𝑖𝑓 𝑅 𝐸 ||𝑟 𝑜 𝑖𝑠 𝑏𝑖𝑔
So the output resistance is very small!
Figure (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model. Note that ro is included because it is easy to do so. Normally, its effect on performance is small.