# BJT II ```INTRODUCTION TO ELECTRONICS
EHB 222E
Bipolar Junction Transistors II
(BJT)
Asst. Prof. Onur Ferhanoğlu
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO
ELECTRONICS
1
BJTs – Exercise 1
Determine node voltages &amp; branch currents (assume β = 100)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
2
BJTs – Exercise 2
Determine node voltages &amp; branch currents (assume β = 30)
pnp transistor!
Assume ACTIVE:
• Base current is small -&gt; VB ~ 0V
• VE = VB + 0.7 = 0.7V
• IE = (5-0.7)/1kΩ = 4.3 mA
• IC max ~5V/10 k Ω = 0.5 mA
IE &gt;&gt; IC -&gt; NOT ACTIVE!
Assume SAT
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
3
BJTs – Exercise 2 continued
Determine node voltages &amp; branch currents (assume β = 30)
pnp transistor!
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
4
BJTs – Exercise 3
Determine node voltages &amp; branch currents (assume β = 100)
Simplify base circuit using Thevenin`s theorem
Cancel voltage sources to find Req (Thevenin)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
5
BJTs – Exercise 3 continued..
Determine node voltages &amp; branch currents (assume β = 100)
Loop equation
Assume active
Isolate IE from 1st equation
VC &gt; VB
ACTIVE verified
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
6
BJTs – Exercise 4
Determine node voltages &amp; branch currents (assume β = 100)
1st stage same as previous question
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
7
BJTs – Exercise 4 continued..
Determine node voltages &amp; branch currents (assume β = 100)
1st stage same as previous question
Assume identical solution
• Since part of collector current will
flow to Q2, VC1 will be different
• If IC1 &gt;&gt; IB2
(Same as previous)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
8
BJTs – Exercise 4 continued..
Determine node voltages &amp; branch currents (assume β = 100)
Q2 -&gt; pnp
• Emitter is connected to +
15V -&gt; VEB2 forward biased
Since IC2 ends up at ground
VCE is possibly reverse biased
-&gt; Assume Q2 ACTIVE
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Q2 ACTIVE as assumed
9
BJTs – Exercise 5
Determine node voltages &amp; branch currents (assume β = 100)
Q1 &amp; Q2 cannot be simultaneously conducting
10V / ~ 0 resistance -&gt; infinite current along supplies
Assume Q2 ON Q1 OFF
• Current will flow from ground through Q2
• VB2 is negative as VEB is forward biased
• But, current should not move from negative voltage
to +5 V, through 10kΩ
Assume Q1 ON Q2 OFF
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
10
BJTs – Exercise 5
Determine node voltages &amp; branch currents (assume β = 100)
Q1 ACTIVE or SAT?
VCB &gt; 0 -&gt; reverse biased -&gt; ACTIVE
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
11
BJT in amplifier design
Simple amplifier circuit with BJT`s
Amplification
(highest slope region)
Segment Y-Z:
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
12
Biasing the BJT
Simple amplifier circuit with BJT`s
DC voltage &amp; current at Q
Signal to be amplified is superimposed
on the bias voltage
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
13
Biasing the BJT – graphical illustration
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
14
Small-signal voltage gain
Amplifier output = Amplifier gain x Amplifier input
Amplifier gain:
Av is maximum when VCE = VCE SAT
Not preferred for the sake of signal symmetry
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
15
Exercise 6
IC =
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Exercise 6 continued..
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Exercise 6 continued..
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
18
Exercise 6 continued..
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
19
Graphical Analysis
• For each value of vBE, the circuit will be operating at the point of intersection of
the ic &amp; vCE graph and the straight line (load line)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
20
Small signal operation
• Base-emitter junction is forward biased
• Reverse biased at collector base function is established using the source VCC
• vbe, which is superimposed on VBE, is to be amplied
• First consider DC bias -&gt; vbe = 0
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
21
Small signal operation
• Base-emitter junction is forward biased
• Reverse biased at collector base function is established using the source VCC
• vbe, which is superimposed on VBE, is to be amplied
• When vbe is applied
for small vbe (&lt;10mV)
small-signal approximation
transconductance
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
22
Small signal operation - transconductance
Small signal operation is restricted to
A linear segment of the curve
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
23
Base current &amp; resistance
• Determine the resistance seen by vbe
Small-signal resistance between base and emitter,
looking into the base
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
24
Emitter current &amp; resistance
Small-signal resistance between base and emitter,
looking into the emitter
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
25
Voltage gain
• The transistor senses the input voltage vbe and causes a proportional current gmvbe
• Output voltage becomes:
Substitute for gm
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
26
Small-signal transistor models: Hybrid-π
• BJT is represented as a voltage controlled current source, which includes input
resistance looking into the base (rπ)
KCL at emitter yields:
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
27
Small-signal transistor models: Hybrid-π variation
• BJT is represented as a current controlled current source, which includes input
resistance looking into the base (rπ)
KCL at emitter yields:
Yields to correct
expression for ie
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
28
Small-signal transistor models: T-model
• Alternative models may be more convenient for other situations
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
29
Small-signal transistor models: T-model variation
• Alternative models may be more convenient for other situations
Collector current:
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Exercise 7
Determine voltage gain (vo/vi), assume β = 100
STEP 1) Determine DC operating point
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Exercise 7 continued..
Determine voltage gain (vo/vi), assume β = 100
STEP 2) Determine small-signal parameters
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Exercise 7 continued..
Determine voltage gain (vo/vi), assume β = 100
STEP 3) Eliminate DC sources
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
33
Exercise 7 continued..
Determine voltage gain (vo/vi), assume β = 100
STEP 4) Replace with small-signal equivalent model
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
34
Exercise 7 – solving directly on the circuit graph
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Accounting for the Early-effect
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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