Electronic Instrumentation
Basic Circuits with BJT Transistors
* In this presentation definitions and examples
from Wikipedia, HowStaffWorks and some other sources
were used
Lecturer: Dr. Samuel Kosolapov
Items to be
defined/refreshed/discussed
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BJT as a switch
Basic BJT Amplifiers Configuration
Common Emitter
Common Collector (Emitter Follower, Buffer)
Common Base
Summary of configurations properties
Examples with Arduino
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Basic BJT (pnp and npn – most of modern transistors)
http://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/
Pin allocations may be different
http://www.talkingelectronics.com/pay/BEC/Page21.html
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BJT equivalent circuits
http://electronics-course.com/bipolar-junction-transistor-bjt
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BJT as an (invertor) switch
http://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/
Need for BJT Switch:
Arduino max per pin current ~ 40 mA
This current may be too small to drive solenoids, motors, etc.
BJT can provide current gain (Ic >> Ib)
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BJT as an (invertor) switch
Work Line: Vcc = Vce + Ic*Rc
http://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/
Vin =
0 (LOW) : Vout = Vcc (HIGH)
Vin = ~VCC (HIGH) : Vout ~0 (LOW)
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BJT as an (invertor) switch. Equations for Fast Evaluations
http://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/
Large Signal (DC) Equivalent Circuit
BJT Switch
Irb = Ib ; Ic = b*Ib; Ie = Ib + Ic ;
Vb = Ib*Rb + Vbe
Vc = Ic*Rc + Vce
Vc = b*Ib*Rc + Vce
If Vce < 0.2 V : Transistor is FULLY ON  Vce ~ 0  Vc = Ic*Rc
(Then equation Ic = b*Ib is not valid). Trick: use it as if it is still valid to make fast evaluations
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Practical BJT switching circuit
http://www.electronics-tutorials.ws/transistor/tran_4.html
(Varduino_out – Vswitch_in ) / Rb = Ib <= 40 E-3A;
We do not want to burn Arduino Pin
Worst case:
Varduino_out = 5V; Vswitch_in = 0.7 V
 Rb minimal value:
Rb > (5 – 0.7) / 40E-3 ~ 100 W
We want to provide LARGE current to the LOAD
The calculation trick is that transistor is
STILL in the active region: Ic = b * Ib
Suppose, we want Ic = 100 mA and b = 100  Ib -= 1
mA

(Varduino_out – Vswitch_in ) / Rb = Ib
Rb maximal value:
Rb < (5-0.7) / (Ic/b)
 Rb < (5 – 0.7) / 1E-3 ~ 43 kW
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Practical BJT switching circuits
http://www.electronics-tutorials.ws/transistor/tran_4.html
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Basic BJT Amplifier Configurations
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
Common Input and Output pin Common Emitter; Common Base; Common Collector
http://www.talkingelectronics.com/pay/BEC/Page21.html
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Small Signal BJT Amplifier Operation
http://www.slideshare.net/_IrfanAnsari/l7-bjt-amplifier-26289398
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Small Signal BJT Amplifier Operation: Graphical Analysis
http://www.slideshare.net/_IrfanAnsari/l7-bjt-amplifier-26289398
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Generalized Common Emitter: Detailed Analysis
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
Actual circuit
Small-Signal Equivalent Circuit
ready for Voltage Node Analysis
In the Medium Frequencies
(Decoupling capacitors are C1 and C2 “large”.
Ce value will be set LATER
Parasite capacitors are “negligibly small”)
BJT Model with
Y-parameters used
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Generalized Common Emitter:
Voltage Node Equations
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
Or
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Generalized Common Emitter:
Voltage Gain
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
Case Re = 0 (Or Ce is LARGE)
(Classic Common Emitter)
Input Resistance: LOW (~ 1 kW)
 Many sensors cannot be
connected directly to CE
Case Re != 0 (Or Ce is absent)
(Generalized Common Emitter)
Important concept: desensitivity:
If b is LARGE  Av ~ Rc/Re : No dependence on b
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Generalized Common Emitter:
Input Resistance
Case Re = 0 (Or Ce is LARGE)
(Classic Common Emitter)
Input Resistance: LOW (~ 1 kW)
 Many sensors cannot be
connected directly to CE
Case Re != 0 (Or Ce is absent)
(Generalized Common Emitter)
Input Resistance: MEDIUM (~ b*Re ~ 50 kW)
 More sensors can be connected directly to CE
(But Av is small)
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Common Collector: Basic Circuits
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
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Common Collector (Emitter Follower): Operation
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
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Common Collector:
Detailed Analysis
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
Or
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Common Collector:
Voltage Gain. Input and Output Resistance
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
If b LARGE  Av ~ 1  Buffer
Input Resistance is LARGE (~ 500 kW )
Output Resistance is SMALL (~ 10 W )
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Common Collector:
Example. Using High Input and Low Output Resistance
Input Resistance is LARGE (~ 500 kW )
 ~ No loading effect for Photo resistor sensor
Output Resistance is SMALL (~ 10 W )
 Enough current for LED
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Common Base Amplifier
Useful for High Frequencies (Because of absence of Miller Effect)
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BJT Amplifiers Summary (see Sedra and Smith book)
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BJT Amplifiers Practical Examples with Arduino.
Sensors. Microphone Amplifier
Electret microphone already has a preamplifier inside.
But if we need more sensitive microphone, additional BJT amplifiers can be used
(discuss designs advantages and disadvantages)
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BJT Amplifiers Practical Examples with Arduino
Actuators: Speaker
With Arduino UNO
Tone Library is typically used to produce
DIGITAL sounds (ON-OFF pulses)
 Linearity is not required
Buzzer volume is small.
If we need strong audio signal, speaker must be used
Small Speaker can be connected to Arduino
But : Arduino can deliver up to 40 mA current per pin
 protected resistor ~ 100 W needed
Simple connection, BUT:
Rsp ~ 10 W; Vsp ~ 0.1 *5V (voltage divider); Psp ~ Vsp*Vsp / Rsp ~ 30 mW
 Power on the speaker will be LOW
 Additional Amplifier is needed
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BJT Amplifiers Practical Examples with Arduino
Actuators: Speaker. Connection with Transistor
Explain diode usage (Hint: Speaker is actually a coil)
Discuss Buffer alternative (R = 100 W)
Both circuits are for amateurs only. Power Amplifier must be used with serious speaker
(details later)
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BJT Amplifiers Practical Examples with Arduino
Actuator. Powerful External Relay Driver
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BJT Amplifiers Practical Examples with Arduino
Actuator. Motor Driver
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BJT Amplifiers Practical Examples
with Arduino Actuators
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Control Questions
• What have I learned ?
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Literature to read
1. TBD
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