Electronics 1 Lecture 4 Bipolar Junction Transistors. Structure, Characteristics, Basic Circuit Configurations, Biasing Literature 1. Tony R. Kuphaldt: Lessons In Electric Circuits, Volume II AC, 2007 2. Tony R. Kuphaldt: Lessons In Electric Circuits, Volume III Semiconductors, 2009 3. Tietze U, Schenk Ch: Electronic Circuits Handbook for Design and Applications, Springer, 2008, ISBN: 3540004297 4. 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 Electron motion directions (opposite to conventional current flow directions) Discrete device Integrated circuit
Structure Structure and doping profile of a planar NPN transistor
Operation principle of bipolar junction transistors E B C Biasing for normal active operation Electron streaming paths (the conventional current flow positive direction is opposite).
Operation principle of bipolar junction transistors a) Recombination b) Current through base terminal c) Electrons flooding the depletion region d) Electrons having reached the depletion region are accelerated towards the collector by the barrier potential 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
Hybrid quadrupole model of bipolar junction transistors H parameter (hybrid p) model of bipolar junction transistors. r BB : base lead resistance (5 50 W) r e : dynamic resistance of base-emitter diode; U T = 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); g m : steepness(10 500 ms).
Parameters of the transistor model Short circuit input impedance: Unloaded voltage transfer: (in kw range) Short circuit current transfer (current gain) Unloaded output admittance: 10 100 ms
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 A Amplifier 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 (g m ) output impedance
Common emitter amplifier with bypassed emitter resistance For alternating signals the capacitors and the power supply act as shor circuit bypasses. C in : input connecting capacitor; C out : output connecting capacitor; C e : emitter resistance bypass. The negative current feedback of R e works only in DC mode. Voltage gain: Current gain: Amplifier load: R f Transistor load impedance: At room temperature U T = 26 mv
Common emitter amplifier without emitter resistance bypass The negative current feedback of R E works 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. The output voltage is equal to the input voltage minus the base-emitter diode forward voltage. DC characteristic
Common collector amplifier Common collector amplifier with DC bias Current gain:
Common collector amplifier
Common base amplifier The base is at ground potential. Input signal: between emitter and ground. Output signal: between collector and ground. Noninverting voltage amplifier. DC characteristic Small alternating signal case
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