CHAPTER 18 BJT-TRANSISTORS

Function of Transistors

Main Applications of Transistors

BIPOLAR JUNCTION TRANSISTORS (BJTs) BJT is constructed with three doped semiconductor regions separated by two pn junction There are three regions : Emitter Base Collector There are two type of BJT: npn pnp

Basic construction of BJT

การป้อนไฟให้กับ Transistor แบบ BJT AC bias ป้อนสัญญาณไฟกระแสสลับ DC bias ป้อนสัญญาณไฟกระแสตรง กระตุ้นให้ BJT อยู่ในช่วงทำงานได้ แม้สัญญาณ input AC จะอยู่ในช่วง ติดลบ ขา BE: Forward bias ขา BC: Reverse bias Q-point อยู่ในช่วงใช้งาน Active Region

Transistor DC Biasing The BE junction is forward-biased The BC junction is reverse-biased (Vbase ( B ) > Vemitter ( E) ) (Vbase ( B ) < Vemitter ( E) ) (Vcollector ( C ) > Vbase ( B ) ) (Vcollector ( C ) < Vbase ( B ) )

Transistor Operation

DC current gain Alpha and Bata The Collector current is equal to times the emitter current The collector current is equal to the base current multiplied by has a value between 0.950 and 0.995 has a value between 20 and 200

Transistor Voltages (DC bias) 2 Voltage Sources

Transistor Voltages (DC bias) Voltage divider Sources Use only a single dc source to provide forward-reverse bias to the transistor Resistor R1 and R2 form a voltage divider that provides the base bias voltage

Transistor Voltages (DC bias) Voltage divider Sources

Input Resistance at the Base Voltage divider Sources

Base Voltage V

THE BIPOLAR JUNCTION TRANSISTOR AS AN AMPLIFIER

THE BIPOLAR JUNCTION TRANSISTOR AS AN AMPLIFIER When both junction are forward-biased, the transistor is in the saturation region of its operation When VCE exceeds 0.7 V, the base-collector junction becomes reverse-biased and the transistor goes into the active or linear region When IB=0 the transistor is in the cutoff region

Load Line Operation Load Line: VCE vs IC

Quiescent or Q-Point

Quiescent or Q-Point Load Line AC Sources DC Sources IB = 400 uA

Signal Operation on the Load Line Q-point ต่ำเกินไป จ่าย IB น้อยเกินไป Q-point สูงเกินไป จ่ายไปให้ IB มากเกินไป Q-point เหมาะสม ค่ากำลังขยาย beta มากเกินไป

BJT Transistor AC Amplifier Class A วงจรทำงานตลอดเวลา full cycle of AC input signal (360o) Class B วงจรทำงาน half cycle หยุด half cycle (180o) Class C วงจรทำงานน้อยกว่า half cycle (< 180o)

BJT Transistor AC Amplifier: Class A ขยายสัญญาณได้ 3 แบบ Common Base Amplifier (CB): Voltage Amp. Common Emitter Amplifier (CE): Power Amp. Common Collector Amplifier (CC): Current Amp.

การวิเคราะห์การขยายสัญญาณ AC (i/p) ใช้ Equivalent Circuit มาเป็นตัวแทน BJT (AC model) Simple model Hybrid Π model h parameter model

Simple Model (small signal analysis) BJT ( npn type) Simple model C E C E B B

Simple Model ถ้าโจทย์ไม่ให้ = Dynamic AC resistance ที่ขา E-B Parameter in BJT equivalent circuit ถ้าโจทย์ไม่ให้ = Dynamic AC resistance ที่ขา E-B = Dynamic AC resistance ที่ขา C-B = ค่าสูงมากระดับ MΩ ถ้าโจทย์ไม่ให้

Amplifier Gain Calculation Process Calculating DC bias DC current and voltage in DC bias circuit Checking load line and Q-point Calculating AC amplifier gain re’ from IE(DC) Voltage gain (AV) = Current gain (Ai) = Power gain (AP) = AV Ai

Common Base Amplifier ( CB Amp ) BJT B vin vOUT E C B Simple model for AC small signal analysis

CB Amp. Circuit Vcc

Common Base Amplifier ( CB Amp )

Common Emitter Amplifier ( CE Amp ) B C BJT E vin vOUT B C E Simple model for AC small signal analysis

CE Amp. Circuit without CE Vcc R1 R2

Common Emitter Amplifier ( CE Amp: no CE )

Common Emitter Amplifier ( CE Amp: no CE )

CE Amp. Circuit with CE Vcc

Common Emitter Amplifier ( CE Amp: with CE from E-GND ) Higher voltage gain

Common Emitter Amplifier ( CE Amp: with CE from E-GND )

Common Collector Amplifier ( CC Amp ) Common Emitter Amplifier B E BJT C vin vOUT B E C Simple model for AC small signal analysis

CC Amp. Circuit Vcc

Common Collecter Amplifier ( CC Amp)

Common Emitter Amplifier (CC Amp)

THE BJT AS A SWITCH Conditions in cutoff Conditions in saturation

BJT PARAMETERS AND RATINGS IF the temperature goes up, goes up, and vice versa.

BJT PARAMETERS AND RATINGS Maximum Transistor Ratings PD(max) = maximum power dissipation