Electronics 1

Lecture 4

Bipolar Junction Transistors. Structure, Characteristics, Basic Circuit Configurations, Biasing

Literature1. Tony R. Kuphaldt: Lessons In Electric Circuits, Volume II – AC, 20072. Tony R. Kuphaldt: Lessons In Electric Circuits, Volume III – Semiconductors, 20093. Tietze U, Schenk Ch: Electronic Circuits – Handbook for Design and Applications,

Springer, 2008, ISBN: 35400042974. Horowitz, Hill W: The Art of Electronics, Cambridge University Press, 1989,

ISBN: 0521370957

Bipolar Junction Transistor

Symbol and layer structure of bipolar junction transistors

Biasing for normal active operation:

Structure

Discrete device Integrated circuit

Electron motion directions (opposite to conventional current flow directions)

Structure

Structure and doping profile of a planar NPN transistor

Operation principle of bipolar junction transistors

Electron streaming paths (the conventional current flow positive direction is opposite).

Biasing for normal active operation

E B C

a) Recombinationb) Current through base terminalc) Electrons flooding the depletion regiond) Electrons having reached the depletion region are accelerated towards the collector by

the barrier potential

Operation principle of bipolar junction transistors

Simple npn transistor models: diode-controlled resistance, respectively diode-controlled current source

Currents through bipolar junction transistors

Small signal equations:

Transistor Characteristics

Hybrid quadrupole model parameters

Definition of quadrupole H-parameters (hybrid p)

Short circuit input impedance

Unloaded voltage transfer

Short circuit current transfer

Unloaded output admittance

H parameter (hybrid p) model of bipolar junction transistors.

rBB: base lead resistance (5…50 W)

re: dynamic resistance of base-emitter diode; UT = 26 mV.

B, b: current gain relative to base current (for small signal transistors 50…500, for power transistors 20…50);

m: voltage feedback coefficient (negligible for state-of-the-art devices);

gm: steepness(10…500 mS).

Hybrid quadrupole model of bipolar junction transistors

Parameters of the transistor model

10…100 mS

Short circuit input impedance:

Unloaded voltage transfer:

Unloaded output admittance:

Short circuit current transfer (current gain)

(in kW range)

DC biasing techniques

Voltage divider resistor pair. Negative serial current feedback

Negative parallel voltage feedback

Common emitter amplifier

For alternating signals the emitter is at ground potential.Input signal: between base and ground.Output signal: between collector and ground.Inverting voltage amplifier.

DC characteristic

Common emitter amplifier

” Audio” amplifier without biasing – output signal clipped

DC bias ensures undistorted output signal

Small signal voltage gain:

Common emitter amplifier

Small signal high gain class AAmplifier operation

Q: quiescent point, operation point with zero input signal.AB: load line, the operation point glides on it as a function of the input signal.

DC biasing

Small signal parameters

Base-emitter impedance, Transfer admittance, Collector-emitter impedance,input impedance steepness (gm) output impedance

Common emitter amplifier with bypassed emitter resistance

Voltage gain: Current gain:

For alternating signals the capacitors and the power supply act as shor circuit bypasses.Cin: input connecting capacitor;Cout: output connecting capacitor;Ce: emitter resistance bypass.The negative current feedback of Re works only in DC mode.

Amplifier load: RfTransistor load impedance:

At room temperatureUT = 26 mV

Common emitter amplifier without emitter resistance bypass

The negative current feedback of REworks for alternating signals too.

Common collector amplifier

For alternating signals the collector is at ground potential through the power supply capacitance.Input signal: between base and ground.Output signal: between emitter and ground.Noninverting current amplifier.

DC characteristic

The output voltage is equal to the input voltage minus the base-emitter diode forward voltage.

Common collector amplifier

Current gain:

Common collector amplifier with DC bias

Common collector amplifier

Common base amplifier

Small alternating signal case

The base is at ground potential.Input signal: between emitter and ground.Output signal: between collector and ground.Noninverting voltage amplifier.

DC characteristic

Common base amplifier

Miller-effect

Cascode amplifier

For high frequency applications:- the bandwidth of the common emitter amplifier is limited by the increases collector-base capacitance due to the Miller effect;- the common base amplifier has a low input impedance (few tens of W).

Földelt bázisú Földelt emitteres Kaszkód

The load impedance of the common emitter stage is the input impedance of the common base stage (26 W at 1 mA emitter current), therefore its voltage gain is 1.The common base stage provides the voltage gain.The input impedance of the amplifier is that of the common emitter stage.

Cascode amplifier

Cascode Common emitter

Signals Bode diagrams