模电课件ch5.2

vO Cutoff Forward-active mode vO Cutoff Forward-active mode
O
Saturation vI
O
Saturation vI
5.2.3 Bipolar Transistor Biasing
VCC
1. Single base resistor biasing • There are the coupling capacitors in some circuits, shown in fig. 2. DC and AC Equivalent Circuits
5.2.1 Common-Emitter Circuit
2. Determining Q-Point Using Graphical Analysis • Output load line is obtained around C-E loop. VCE=VCC－ICRc •This equation produces a straight-line relationship on output characteristic curve, the intersection provides Qpoint. We get VCEQ and ICQ
Input loop
VCC
VTH I B RTH VBE I E RE I E (1 ) I B
VCC RB RC IB
+
CB vs
+
+
RB RC iB
+
iC
+
vCE

VBE

IC
+
RB RC ib
+
ic
+
VCC
VCE

vce

VBE
vS
+
vbe

DC equivalent circuit
AC equivalent circuit
5.2.3 Bipolar Transistor Biasing 1. Single base resistor biasing
• Coupling capacitor: Acting as a short circuit in ac analysis, and as an open circuit in dc analysis • dc voltage sources : The resistance in ac analysis is zero，acting as a short circuit in ac analysis.
VBE VBE(T0 25 C ) (T T0 ) 2.2 10 3 V
Q1 Q IB iB =0 vCE/V iC/mA
T curve shifts left
2. Voltage divider biasing and biasing stability
iC/mA
vs
+
VCC RB1 CB
+
RC B IBQ
+
ICQ VCEQ
+
If voltage of b point keep unchangeable， when T changes, then Q-point will be stable.
Principle:
RB2
VBE

RE
T ICQ IE VE、VB stable VBE IBQ ICQ
# Why an amplifier must be established a appropriate dc biasing?
5.2.1 Common-Emitter Circuit
1. Determining Q-Point Using Approximation Equations • If we use this method, we must know current gain .
IB iB =0 vCE/V
Finally：
ICBO ICEO T VBE IB IC

2. Voltage divider biasing and biasing stability
(2). Voltage divider biasing a. Q-Point Stability Goal: When T changes, ICQ is stable.
VBB 1.3V

5.2.2 Basic Transistor Application
2. Amplifier • If transistor operates in forward-active mode, output waveform is sinusoidal.. • If Q-point is too high or low, output waveform is distorted.
2. Voltage divider biasing and biasing stability (1) Temperature Influences Q-Point
a. Temperature influences ICBO I CBO I CBO(T0 25 C ) e k (T T0 ) T Curves shift up b. Temperature influences input i-v
0.2 0
Active
Saturation 0.7 1.9 vI
5.2.2 Basic Transistor Application
2. Amplifier (P331) • Assume transistor operates in forward-active mode.
VBB vI VBE 0.6 vI iB RB RB
iC VCC=5V RC 5k
I C I B
(VI 0.7) RC
RB
VI
RB 150k IB
VO
=120
vO /V Cutoff
• When VI0.7V, T is cut off. • When VO= 0.2V, yield VI= 1.9V. For VI1.9V, T is saturation. • Voltage Transfer Characteristic show in Fig.
(1) Temperature Influences Q-Point
c. Temperature influences When T increases 1 °C ，
Q1 Q
increases 0.5%1.0%
When T distance between two curves become larger.
（Feedback）
(2). Voltage divider biasing
Condition of keeping VB Stability:
RB1 CB vs
+ +
VCC RC B IBQ
+
ICQ VCEQ
+
I1 >>IB ， VB >>VBE
and
RB2 VB VCC is stable RB1 RB2
RB2
VBE

RE
VCC IC RTH
+
b. Find Q-Point Forming Thevenin equivalent cபைடு நூலகம்rcuit
VTH RB2 VCC RB1 RB2
IB
RC
RTH RB1 // RB 2
VTH

IE
RE
(2). Voltage divider biasing
5.2 DC Analysis of Transistor Circuits What are DC and AC Analyses?
There are two kinds of quantities in the circuit. One is a dc quantity, other is a ac signal, so we use the different notation to represent them. E.g. IB , VBE__DC values ib , vbe__Instantaneous ac values iB , vBE__Total instantaneous values iB=IB+ib , vBE=VBE+vbe Ib , Vbe__phasor values
VCC=5V iC ΔvI
+ +
v O VCC iC RC VCC i B RC 2.6 4( v I )
RB 150k iB
RC 5k
vO
AC component Voltage gain
vO 4(vI ) vO 4 Av vI
=120
iC
VCC Rc Q Slope IB Q 1 Rc
ICQ
VCEQ
VCC
vCE
5.2.2 Basic Transistor Application
1. Voltage Transfer Characteristic (P312)
VI VBE IB RB
VO 5 I C RC 5
VBB VBE IB RB
RB
+
B
+
C
IC RC
+
IB
VBE E

I C I B VCE VCC I C RC