BY: Sai Kiran Reddy Dwarampudi TRANSISTOR BY: Sai Kiran Reddy Dwarampudi

Recall p-n junction P N W + - Vappl < 0 Vappl > 0 I I V V Forward bias, + on P, - on N (Shrink W, Vbi) Allow holes to jump over barrier into N region as minority carriers Reverse bias, + on N, - on P (Expand W, Vbi) Remove holes and electrons away from depletion region I V I V

Transistor Trans—relation b/w input and output Istor—is taken from the RESISTOR Output resistance by input resistance

Origin of the names the Emitter 'emits' the electrons which pass through the device the Collector 'collects' them again once they've passed through the Base ...and the Base?...

Currents

Conventional View

types Bc 107---driver stage 2n2222--switching Bc547 Bc548

CONFIGURATIONS COMMON BASE COMMON COLLECTOR COMMON EMITTER

Common Base NPN

Common Collector NPN How does IC vary with VCE for various IB? Note that both dc sources are variable Set VBB to establish a certain IB

Common Emitter NPN

Common Emitter Characteristics We can therefore draw an input characteristic (plotting base current IB against base-emitter voltage VBE) and an output characteristic (plotting collector current Ic against collector-emitter voltage VCE)

IDEAL CE INPUT (Base) Characteristics

IDEAL CE OUTPUT (Collector) Characteristics

ACTUAL CE OUTPUT Characteristics IB =

Various Regions (Modes) of Operation of BJT Active: Most important mode of operation Central to amplifier operation The region where current curves are practically flat Saturation: Barrier potential of the junctions cancel each other out causing a virtual short (behaves as on state Switch) Cutoff: Current reduced to zero Ideal transistor behaves like an open switch * Note: There is also a mode of operation called inverse active mode, but it is rarely used.

DC  and DC   = Common-emitter current gain  = Common-base current gain  = IC  = IC IB IE The relationships between the two parameters are:  =   =   + 1 1 -  Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

The DC Operating Point For a transistor circuit to amplify it must be properly biased with dc voltages. The dc operating point between saturation and cutoff is called the Q-point. The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied. Fig 5-2a & Fig 5-4

NEED FOR STABLIZING Q-POINT Ambience temperature Human Error Cut-in voltage [ -2.5mv/c] STABILITY FACTOR: Measure of stability of operating point

biasing Fixed bias Collector to base bias Fixed bias with emitter resistor Self bias or voltage divider bias

Fixed Bias

Collector to base bias

Emitter Bias

Self bias

Transistors as Switches