Semiconductors II Transistors pnp npn Circuit symbol of

Semiconductors - II Transistors ( pnp & npn ) Circuit symbol of transistors Different configurations Input / Output characteristics Parameters & gains Transistor as a switch Transistor as an amplifier Transistor as a oscillator sigmaprc@gmail. com

2 Transistor A transistor is three terminal device made up of semiconducting materials. The two kinds of transistors are (a) pnp transistor (b) npn transistor Three regions of a transistor A transistor has three regions (a) emitter (b) base (c) collector sigmaprc@gmail. com

Emitter 3 (a) Emitter supplies the majority charge carriers ( i. e. holes in case of a pnp transistor and electrons in case of npn transistor ) (b) Emitter is heavily doped (c) Emitter is of moderate size Base (a) Base is the mid region of a transistor (b) Base is very lightly doped (c) Base is very thin Collector (a) Collector collects the majority charge carriers crossing over the base (b) Collector is moderately doped (c) Collector is largest in size sigmaprc@gmail. com

4 Circuit symbol of a transistor (a) Position of arrow indicates the location of emitter (b) Direction of arrow indicates type of transistor ( pnp or npn ). Direction of arrow is based on the conventional direction of flow of current. Movement of holes is treated as the direction of flow of current. sigmaprc@gmail. com

5 Transistor configurations Based on specific requirements, transistors may be used in any one of the following configurations: Common base Common emitter Common collector In any of the above configurations, two terminals on LHS are for input and two on the RHS are for the output. sigmaprc@gmail. com

Input characteristics of a transistor ( CE configuration ) Base current IB is studied as a function of base emitter voltage ( VBE ) VCE is held constant in the output section sigmaprc@gmail. com

Input characteristics of a transistor ( CE configuration ) Initially the base current is zero as there is no potential difference applied across the base emitter junction and barrier potential across it prevents diffusion of charges. With increase in VBE the barrier potential is overcome and there is an increase in IB ( this is similar to the forward bias p-n junction characteristic ) sigmaprc@gmail. com

Output characteristics of a transistor ( CE configuration ) Collector current IC is studied as a function of collector emitter voltage ( VCE ) IB is held constant in the input section sigmaprc@gmail. com

Output characteristics of a transistor ( CE configuration ) IC increases initially with increase in VCE and then reaches almost a constant value. Initial ( linear ) increase of IC which depends on applied VCE represents a saturated state that is independent of IB. sigmaprc@gmail. com

Output characteristics of a transistor ( CE configuration ) When the base current is nearly zero, IC is also zero and independent of VCE. ( zero base current implies no forward VBE) Zero IC independent of applied VCE represents a cutoff state. sigmaprc@gmail. com

Output characteristics of a transistor ( CE configuration ) In between these two regions the transistor attains a state when IC is nearly independent of VCE and depends only on IB. This region where transistor responds to IB is known as active state. From the graph it is observed that a small change in IB ( m. A ) results in a large change in IC ( m. A ). ( Amplified response ! ) sigmaprc@gmail. com

Parameters associated with transistor operation Input resistance ( ri ) Output resistance ( ro ) Current amplification( b ) sigmaprc@gmail. com

Current gains : a , b and g Current amplification( b ) – common emitter configuration b is always greater than 1 Current gain( a ) – common base configuration a is always lesser than 1 Current gain( g ) – common collector configuration sigmaprc@gmail. com

Relation between a, b and g sigmaprc@gmail. com

Transistor as a switch Applying KLL to output Applying KLL to input VBB is taken as the input VCE is taken as the output sigmaprc@gmail. com

Transistor as a switch Cutoff region: input ( Vi ) is low and output ( Vo ) is high. Saturation region: input ( Vi ) is high and output ( Vo ) is low. Switching action : Low input → High output High input → Low output sigmaprc@gmail. com

Transistor as an amplifier a. c. signal to be amplified is connected in the input section of the transistor. VCE is taken as the output Transistor is operated in the active region only. A small change in IB ( m. A ) results in a large change in IC ( m. A ). Output and input are out of phase by p. ( negative slope of VO - VI plot in the active region ) sigmaprc@gmail. com

Transistor as an amplifier Voltage gain Power gain sigmaprc@gmail. com

Transistor as an oscillator Part of output is given as positive feedback, using LC coupling, to the input channel Frequency of oscillations Power supply used in the circuit compensates for damping in LC oscillations. Transistor is operated in cutoff and saturations alternately. sigmaprc@gmail. com

Transistors available in the market sigmaprc@gmail. com

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