Analog systems and applications — lecture-XV.

Bipolar junction transistors, lecture XV and XVI.

This article belongs to a series of lectures on analog electronics, the paper goes by the name “Analog Systems and Applications” for the physics honors degree class. All lectures of this series will be found here. This is the 15th and 16th lecture of the series. The lectures were delivered on 14th March and 19th March 2018.

Recapitulation

So far we have been discussing the conductors, semiconductors, holes, intrinsic semiconductors (lecture-1); extrinsic (p and n type) semiconductors and classification of semiconductors (lecture-2); conductivity and mobility of semiconductors (lecture-3); pn junction diodes, their biasing and depletion layer (lecture-4); energy levels and band theory as well as reverse breakdown (lecture-5); field, potential and width calculation in pn step junction (lecture-6); current voltage characteristics and resistance and diode equation (lecture-7); half wave rectifiers and their parameters (lecture-8 and 9); center tapped full wave rectifier (lecture-10); bridge type full wave rectifier (lecture-11); current and voltage sources and Norton’s and Thevenin’s theorem (complimentary lecture); Zener diode and voltage regulation (lecture-12); the light emitting diodes (LED, lecture-13); and finally the photodiodes (lecture-14). This finished our unit-1 of the CBCS (credit based course structure) syllabus of Analog Systems and Applications course.

The terrain

Now we will begin our sojourn into the other units where most important aspects have been covered (more extensively so in unit-3, as in unit-1). Time permitting the left out lectures will be prepared by me (first off line and then the web version).

A note of defense: preparing each lecture note for a classroom delivery takes on an average 4 hours of study, 1-2 hours of writing the notes, 1-2 hours of classroom delivery. In addition to prepare the web-version (which includes, typesetting, making formulas where I use LaTex and HTML code and diagrams where I mostly use inkscape) takes anywhere between 10-20 hours. Thats like 18 to 28 hours of constant toil per lecture. That explains why out of more than 500 hand-ready lecture notes I have (therefore the number of lectures I have actually delivered in 2016-2018) I have been able to make web-version of only some 50-some in number. That between 2017-2020.

Lets begin the lectures we have at hand (lectures 15 and 16 unit 2). Today we will discuss another line of application of the pn semiconductor diodes: the so called bipolar junction transistors or BJT in short.

Lecture XV

Bipolar junction transistors.

There are two ways in which pn junction diodes can be joined together sharing a common P or N terminal. This produces 3 layer, 2 junction, 3 terminal devices. Such devices are known as Bipolar Junction Transistors or BJT in short. They can act either as an “insulator” or a “conductor” when a small signal voltage is applied. Thus the BJT is capable of two functions: (i) switching (a digital electronics capability) and (ii) amplification (an analogue electronics capability).

The BJTs operate under 3 different regions;

(i) Active region: here the transistor operates as an amplifier, IC = β IB.

(ii) Saturation region: here the transistor operates as “ON” of a switch: IC = I(saturation).

(iii) Cut-off region: here the transistor operates as “ON” of a switch: IC = 0.

Note: the transistor is coined from two words, Transistor = Transfer + Varistor.

No its not from Trans + Sister. SOmebody must have joked.

There are two basic types of BJT, the PNP and the NPN depending on which layer is shared in the connection of the diodes (obviously N in PNP and P in NPN is the terminal thats shared).

The 3 terminals of the BJT are emitter (E) base (B) and collector (C). The PNP and NPN transistors operate on the same principle, what differs between them is their “biasing” and “polarity of power supply”.

Lets now see some cool diagrams which evince how our devices look in their element or circuit symbols.

The circuit symbols and element symbols of the PNP transistor.

The circuit and element symbol of PNP transistor.
The circuit and element symbol of PNP transistor.

The circuit and element symbol of PNP transistor.

The circuit and element symbol of NPN transistor.
The circuit and element symbol of NPN transistor.

There are 3 ways a BJT can be connected in an electronic circuit having one terminal (out of three) common between the “input” and “output”. They are;

(i) Common Base configuration: has voltage gain but no current gain.

(ii) Common Emitter configuration: has both voltage and current gain.

(ii) Common Collector configuration: has no voltage gain but has current gain.

Lets study them in detail.

Common Base (CB) configuration.

In this configuration, BASE is common to both input and output signal. Input is applied between BASE and EMITTER. Output is taken from BASE and EMITTER.

Input current (IE) is large as it is the sum of both base current (IB) and collector current (IC). Thus collector output is less than emitter output. Current gain is 1 or less. So common base configuration attenuates the input signal.

Lets represent this configuration by the following circuit diagram.

Circuit diagram for the common base configuration of a PNP transistor. Note that the electron flows opposite to the arrows shown depicting current (which is called conventional current).

Circuit diagram for the common base configuration of a PNP transistor.
Circuit diagram for the common base configuration of a PNP transistor.

Its a “non-inverting” voltage amplifier circuit as the input signal Vin and output signal voltage Vout are “in phase”. This configuration has an unusual “voltage gain” characteristics and common base configuration are not found very commonly.

The input characteristics here is that of a forward biased diode while output characteristics represents an illuminated photodiode. (Read about photodiodes here) It has a high ratio of output to input resistance (RL =”load or output” and Rin = “input”). This leads to a resistance gain.

Voltage gain is given by: \boxed{A_V = \frac{V_{out}}{V_{in}} =\frac{I_C \times R_L}{I_E \times R_{in}}=\alpha \Big(\frac{R_L}{R_{in}}\Big)} where α = IC/IE is current gain and RL/Rin is the resistance gain.

Common base arrangement is used in single stage amplifier circuits such as “microphone pre-amplifier” and radio frequency (RF) amplifiers due to its very good high frequency response.

Lecture XVI

Common Emitter (CE) configuration.

  • Its also known as grounded-emitter configuration
  • Input signal is applied between base and emitter.
  • Output is taken from collector and emitter.
  • Its the most commonly used configuration for transistor based amplifier application.
  • It produces the highest power gain of all 3 BJT configuration. Thats because input impedance is low (forward biased PN) and output impedance is high (reverse biased PN). Thus current gain is higher.

Here is a circuit diagram for the common emitter type configuration.

Circuit diagram for the common emitter configuration of a NPN transistor. Note that the electron flows opposite to the arrows shown depicting current (which is called conventional current).

Circuit diagram for the common emitter configuration of a NPN transistor.
Circuit diagram for the common emitter configuration of a NPN transistor.

Current flowing out of the transistor is equal to the current flowing into the transistor, thus; IE = IC + IB.

  • Load resistance (RL) is connected in series with the collector. The current gain is large; IC/IB. Its denoted by the symbol β.
  • α = IC/IE thats always less than 1.
  • Small change in base current IB results in much larger change in collector current (IC). Thus small changes in IB controls the current in the E-C (emitter-collector) circuit. Typically 20 < β < 200 for most general purpose transistors. If β = 100 it means when 1 electron flows from B-terminal 100 electrons flow between E-C terminals.
  • α = IC/IE, β = IC/IB, since IE = IC + IB, α = [IC/IB]/[(IC + IB)/IB] or α = β/+1) and β = α/(1-α). Also IC = αIE= βIB.
  • This configuration has greater input impedance and current gain than the common base configuration. Also power gain is highest among the 3 configurations, but voltage gain is much lower. Common emitter configuration is an inverting amplifier circuit. Vin and Vout are out of phase (1800).

Common Collector (CC) configuration.

  • This is known as grounded collector configuration. Collector is common through the supply. Input is given directly to base. Output is taken from emitter load. Therefore it is known as emitter follower (and also voltage follower) circuit.
  • It is useful for impedance matching applications as it has very high input impedance. (100s of ) but low output impedance.

Here is a circuit diagram for the common collector type configuration.

Circuit diagram for the common collector configuration of a NPN transistor. Note that the electron flows opposite to the arrows shown depicting current (which is called conventional current).

Circuit diagram for the common collector configuration of a NPN transistor.
Circuit diagram for the common collector configuration of a NPN transistor.
  • It has a current gain of approximately β.
  • RL is in series with emitter so current through it is IE. IE = IC + IB. γ = IE/IB = (IC + IB)/IB = β + 1.
  • Its a non-inverting circuit. Vin and Vout are in phase.
  • Voltage gain is always less than unity.
  • It has a large current gain (similar to common emitter configuration).

Summary

CharacteristicsCBCECC
Input impedancelowmediumhigh
Output impedancevery high highlow
Phase (in degree)0180 0
Voltage gainhigh medium low
Current gain low medium high
Power gainlowvery high medium

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