Electronic InstrumentationBasic Circuits with BJT Transistors

* In this presentation definitions and examplesfrom Wikipedia, HowStaffWorks and some other sources

were used

Lecturer: Dr. Samuel Kosolapov

Items to be defined/refreshed/discussed

• 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/

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Pin allocations may be different

http://www.talkingelectronics.com/pay/BEC/Page21.html

BJT equivalent circuitshttp://electronics-course.com/bipolar-junction-transistor-bjt

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BJT as an (invertor) switchhttp://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/

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Need for BJT Switch:Arduino max per pin current ~ 40 mAThis current may be too small to drive solenoids, motors, etc.

BJT can provide current gain (Ic >> Ib)

BJT as an (invertor) switchhttp://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/

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Work Line: Vcc = Vce + Ic*Rc

Vin = 0 (LOW) : Vout = Vcc (HIGH)Vin = ~VCC (HIGH) : Vout ~0 (LOW)

BJT as an (invertor) switch. Equations for Fast Evaluationshttp://blog.oscarliang.net/bjt-bipolar-junction-transistor-beginner-tutorial/

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BJT Switch Large Signal (DC) Equivalent Circuit

Irb = Ib ; Ic = b*Ib; Ie = Ib + Ic ; Vb = Ib*Rb + VbeVc = 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

Practical BJT switching circuithttp://www.electronics-tutorials.ws/transistor/tran_4.html

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(Varduino_out – Vswitch_in ) / Rb = Ib <= 40 E-3A;We do not want to burn Arduino PinWorst 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 LOADThe calculation trick is that transistor is

STILL in the active region: Ic = b * IbSuppose, we want Ic = 100 mA and b = 100 Ib -= 1

mA

(Varduino_out – Vswitch_in ) / Rb = IbRb maximal value:

Rb < (5-0.7) / (Ic/b)

Rb < (5 – 0.7) / 1E-3 ~ 43 kW

Practical BJT switching circuitshttp://www.electronics-tutorials.ws/transistor/tran_4.html

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Basic BJT Amplifier Configurationshttp://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html

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Common Input and Output pin Common Emitter; Common Base; Common Collector

http://www.talkingelectronics.com/pay/BEC/Page21.html

Small Signal BJT Amplifier Operationhttp://www.slideshare.net/_IrfanAnsari/l7-bjt-amplifier-26289398

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Small Signal BJT Amplifier Operation: Graphical Analysishttp://www.slideshare.net/_IrfanAnsari/l7-bjt-amplifier-26289398

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Generalized Common Emitter: Detailed Analysishttp://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html

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Actual circuit

Small-Signal Equivalent Circuitready for Voltage Node Analysis

In the Medium Frequencies(Decoupling capacitors are C1 and C2 “large”.

Ce value will be set LATERParasite capacitors are “negligibly small”)

BJT Model with Y-parameters used

Generalized Common Emitter: Voltage Node Equations

http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html

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Or

Generalized Common Emitter: Voltage Gain

http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html

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Case Re = 0 (Or Ce is LARGE) (Classic Common Emitter)

Case Re != 0 (Or Ce is absent) (Generalized Common Emitter)

Important concept: desensitivity:If b is LARGE Av ~ Rc/Re : No dependence on b

Input Resistance: LOW (~ 1 kW)Many sensors cannot be

connected directly to CE

Generalized Common Emitter: Input Resistance

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Case Re = 0 (Or Ce is LARGE) (Classic Common Emitter)

Case Re != 0 (Or Ce is absent) (Generalized Common Emitter)

Input Resistance: LOW (~ 1 kW)Many sensors cannot be

connected directly to CE

Input Resistance: MEDIUM (~ b*Re ~ 50 kW) More sensors can be connected directly to CE

(But Av is small)

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

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Or

Common Collector: Voltage Gain. Input and Output Resistance

http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html

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If b LARGE Av ~ 1 Buffer

Input Resistance is LARGE (~ 500 kW )

Output Resistance is SMALL (~ 10 W )

Common Collector: Example. Using High Input and Low Output Resistance

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Input Resistance is LARGE (~ 500 kW )

~ No loading effect for Photo resistor sensor

Output Resistance is SMALL (~ 10 W )

Enough current for LED

Common Base Amplifier

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Useful for High Frequencies (Because of absence of Miller Effect)

BJT Amplifiers Summary (see Sedra and Smith book)

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BJT Amplifiers Practical Examples with Arduino. Sensors. Microphone Amplifier

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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)

BJT Amplifiers Practical Examples with ArduinoActuators: Speaker

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Small Speaker can be connected to ArduinoBut : Arduino can deliver up to 40 mA current per pin protected resistor ~ 100 W neededSimple 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

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

BJT Amplifiers Practical Examples with ArduinoActuators: Speaker. Connection with Transistor

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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)

BJT Amplifiers Practical Examples with ArduinoActuator. Powerful External Relay Driver

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BJT Amplifiers Practical Examples with ArduinoActuator. Motor Driver

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BJT Amplifiers Practical Exampleswith 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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