Transcript ECE 333 Linear Electronics

ECE 333 Linear Electronics
Chapter 7 Transistor Amplifiers
How a MOSFET or BJT can be used to make an
amplifier  linear amplification  model the linear
operation Three basic ways  Practical circuits by
discrete components
1
Introduction
• The basic principles of using MOSFET and BJT
in amplifier design are the same.
• Active region
- MOSFET: saturation or pinch-off region)
- BJT: active mode
2
7.1 Basic Principles
• 7.1.1 The basis for amplifier operation
– The basic application of a transistor in amplifier
design is that when the device is operated in the
active region, a voltage-controlled current source
is realized.
3
4
• 7.1.2 Obtaining a voltage amplifier (NMOS and
npn amplifiers)
5
6
• 7.1.3 The voltage-transfer characteristics
– VTC is non-linear:
For BJT:
7
• 7.1.4 Obtaining Linear Amplification by Biasing
the Transistor
– A dc voltage VGS is selected to obtain operation at
a point Q on the segment AB of the VTC
– Q: bias point or dc operation point, or quiescent
point
– The signal to be amplified is vgs(t)
8
Figure 7.3 Biasing the MOSFET amplifier at a point Q located on the segment AB of the VTC.
9
Fig. 7.4 The MOSFET
amplifier with a small
time-varying signal vgs(t)
10
• 7.1.5 The Small-Signal Voltage Gain
(* because at B point, VGS is
largest for saturation)
11
Example 7.1
Solution: VGS=0.6V, VOV=0.2V
12
(b)
The max negative swing at the drain is 0.2V. The
positive side is fine with 0.2V (0.6V is still less
than VDD)
Max
More precise analysis
13
• For BJT:
14
Example 7.2
(Read it by yourself)
15
• 7.1.6 Determining the VTC by Graphical Analysis
16
17
7.2 Small-Signal Operation and Models
• 7.2.1 The MOSFET Case
18
• The signal current in the drain terminal
Small-signal condition:
19
• If the small-signal condition is satisfied:
Voltage gain
20
Fig. 7.12
21
• Separating the DC analysis and the signal
analysis
• Small-signal equivalent circuit models
22
• With MOSFET channel modulation
23
• The Transconductance gm
24
• Example 7.3: small-signal voltage gain?
25
IG = 0 
26
27
28
• Modeling the Body effect
29
• 7.2.2 The BJT Case
30
• Collector current and Transcoductance
If:
31
If:
32
33
• The base current and the input resistance at
the base
• The emitter current and the input resistance
at the emitter
34
• Example 7.5: determine vo/vi . Known β=100
35
1. At the quiescent operating point
Since VC > VB  CBJ is reverse biased, the device
is operating in the active mode
36
2. Determine the small-signal model
3. Determine signal vbe and vo
37
Small signal at output
Voltage gain
* The voltage gain is small because RBB is
much larger than rπ
38
7. 3 Basic Configurations
39
• 7.3.2 Characterizing Amplifiers
Output resistance
Overall voltage gain
40
• 7.3.3 The common-source (CS) and commonemitter amplifiers
common-source
41
• Common-emitter amplifier
42
• 7.3.4 CS and CE amplifier with a Rs or Re
With load resistance:
43
• 7.3.5 The common-Gate (CG) and the
Common-Base (CB) Amplifiers
44
• The source and emitter followers (commondrain or common-collector amplifiers)
(* because infinite Rin)
45
7.4 Biasing
1. To establish in the drain (collector) a dc
current that is predictable, and insensitive to
variations in temperature and to large
variations in parameter values between
devices of the same type;
2. To locate the dc operating point in the active
region and allow required output signal
swing without the transistor leaving the
active region.
46
• The MOSFET case
- E.g., biasing by fixing VG and connecting a Rs
47
• Example 7.11
Solution: design the resistance by distributing
VDD into 3 equal part on RD, transistor VDS and RS
(each part = 5 V)
48
When Vt = 1.5 V
49
• 7.5 Discrete-Circuit Amplifiers
(self-reading)
50