The Transistor as An Amplifier
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Basic Idea
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Step 1: Set the transistor at a certain DC level BIASING
Step 2: Inject a small signal to the input and get a bigger
output COUPLING
Biasing point will determine AC Gain, I/P Impedance, O/P
Impedance, and Maximum Output Swing
Large Signal Operation
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If vI < Vt, the transistor is OFF and we are at point A
If vI increased the transistor will be in saturation region A-B
vO = vDS = VDD –RD.iD
as vI ↑ Æ vO ↓
If vI increased beyond point B, the transistor enters triode region
B-C
For small signal the amplification will be linear
To work as a switch the transistor will work either at points A,
or point C (will be studied later)
(a) Basic structure of the common-source amplifier. (b) Graphical construction to determine the transfer characteristic of the amplifier
in (a), (c) Transfer characteristic showing operation as an amplifier biased at point Q.
Maximum Symmetrical Swing
Two load lines and corresponding bias points. Bias point Q1 does not leave sufficient room
for positive signal swing at the drain (too close to VDD). Bias point Q2 is too close to the
boundary of the triode region and might not allow for sufficient negative signal swing.
Example
Gain =
Δv O
2.2
=−
= −14.7V /V
Δv I
0.15
MOS Amplifier Biasing
1. Biasing by Fixing VGS
The use of fixed bias (constant VGS) can result in a large
variability in the value of ID. Devices 1 and 2 represent
extremes among units of the same type.
2. Biasing by Fixing VG and Using RS
Biasing using a fixed voltage at the gate, VG, and a resistance in the
source lead, RS: (a) basic arrangement; (b) reduced variability in ID; (c)
practical implementation using a single supply;; (e) practical
implementation using two supplies.
MOS Amplifier Biasing (cont.)
3. Biasing by Drain-Gate Feedback
4. Biasing Using Constant Current
(a) Biasing the MOSFET using a constant-current source I. (b)
Implementation of the constant-current source I using a current mirror.