Analogue ElectronicETEC 3824 Transistors
Learning Outcomes At the end of the lesson, students should be able to : • Sketch and explain the operation of transistor circuits • Describe the function of a transistor in various practical applications
The Junction Transistor • A bipolar junction transistor consists of three regions of doped semiconductors. • A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector). • The device can be characterized as a current amplifier, having many applications for amplification and switching
Transistor Operation • A transistor in a circuit will be in one of three conditions : • Cut off (no collector current), useful for switch operation • In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications • In saturation (collector a few tenths of a volt above emitter), large current useful for "switch on" applications
Load line Characteristic curves
CUTOFF occurs when the base-to-emitter bias prevents current from flowing in the emitter circuit. For example, in the PNP transistor, if the base becomes positive with respect to the emitter, holes are repelled at the emitter-base junction. This prevents current from flowing in the collector circuit.
SATURATION occurs in a PNP transistor when the base becomes so negative, with respect to the emitter, that changes in the signal are not reflected in collector-current flow.
Transistor Switch Example • The switch is open
Transistor Switch Example • The switch is open • There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is across the transistor
Transistor Switch Example • The switch is closed. • The base resistor is chosen small enough so that the base current drives the transistor into saturation. • In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.
AMPLIFICATION is the process of increasing the strength of a signal. An AMPLIFIER is the device that provides amplification without appreciably altering the original signal.
Transistor as Current Amplifier • The larger collector currentIC is proportional to the base current IB • according to the relationship IC = βIB • more precisely it is proportional to the base-emitter voltage VBE . • The smaller base current controls the larger collector current, achieving current amplification
PROPER BIASING OF A TRANSISTOR • Enables the transistor to be used as an amplifier. • To function in this capacity, the emitter-to-base junction of the transistor is forward biased, while the base-to- collector junction is reverse biased.
PROPER BIASING OF A TRANSISTOR • NPN TRANSISTOR OPERATION is basically the action of a relatively small emitter-base bias voltage controlling a relatively large emitter-to-collector current.
NPN Transistor PNP Transistor
PROPER BIASING OF A TRANSISTOR time time
PROPER BIASING OF A TRANSISTOR linear operation – no distortion
IMPROPER BIASING OF A TRANSISTOR Non-linear operation - distortion
BASIC TRANSISTOR AMPLIFIER • producing a large change in collector current for a small change in base current. • This action results in voltage amplification because the load resistor placed in series with the collector reacts to these large changes in collector current which, in turn, results in large variations in the output voltage
one most widely used because it improves circuit stability and at the same time overcomes some of the disadvantages of base-current bias and self-bias
Inverting amplifier Phase relationship between input and output 180º
Non-inverting amplifier Phase relationship between input and output 0º
Reference • http://www.tpub.com/content/neets/14179/css/14179_104.htm • http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html