# CB/CG Amplifiers, Emitter/Source followers

```E3 238 Analog VLSI Circuits
Lecture 10: CB/CG Amplifiers, Emitter/Source Followers
Gaurab Banerjee
Department of Electrical Communication Engineering,
Indian Institute of Science, Bangalore
[email protected]
Common Base Amplifier
Note: vbe = -vin
Apply KCL at vout:
gm &gt;&gt; go
Non-inverting gain stage, same magnitude as CE stage
Common Gate Amplifier
Note: vgs = -vin
KCL at vout:
Non inverting gain, same magnitude as CS stage
Note:
Both CE/CS and CB/CG amplifiers have their gain equations in
the form:
•
•
(Effective trans-conductance)/(Effective output conductance)
•
Conductance adds in parallel networks -&gt; easier to work with
Small Signal Terminal Impedances
Common Base Amplifier:
Common impedance analysis
technique -&gt; apply test voltage,
determine test current
Small Signal Terminal Impedances
From (2):
From (1):
very small
Approx. = gm
A few substitutions...
Derive this!
Small Signal Terminal Impedances
Common Gate Amplifier:
rin =&gt; obtained by applying a test voltage at the input
KCL at input:
Small Signal Terminal Impedances
Common Collector Amplifier (Emitter Follower)
Common Collector Amplifier (Emitter Follower)
Gain &lt; 1
Terminal Impedance Values
Good voltage buffer -&gt; Ri large, Ro small
Input Resistance:
Emitter resistance “multiplied
up” and added to base -&gt; High
input resistance!
Terminal Impedance Values
Output Resistance:
Note: We ignore RL here,
assuming RL &gt;&gt; Ro
Low impedance,
dominates
Resistance in base is divided by (β0+1) and reflected to the emitter
=&gt; low output impedance
Common Drain Amplifier (Source Follower)
Body and Source not at
the same potential
Gain &lt; 1
Order of magnitude =&gt;
gm &gt;&gt; gmbs &gt;&gt; gds
=&gt; av ≈ 1
Common Drain Amplifier (Source Follower)
Small, due to high
o/p impedance -&gt;
high gain
Large, due to low
o/p impedance -&gt;
low gain
So, by inspection, the MOS CD amplifier has a low output impedance
given by:
Derive!
```