Transistor as Switch
Objective
Theory
Transistor is a three terminal, bipolar, current controlled device. It works in three different
regions: Active, Cutoff and Saturation according to applied biasing condition.
Transistor as switch: ON
Transistor will become ON (saturation) when a sufficient voltage V is given to input. During this
condition the Collector Emitter voltage Vce will be approximately equal to zero, ie the transistor
acts as a short circuit. For a silicon transistor it is equal to 0.3v. Thus collector current Ic =
Vcc/Rc will flow.
Transistor as switch: OFF
Transistor will be in OFF ( cutoff ) when the input Vin equal to zero. During this state transistor
acts as an open circuit and thus the entire voltage Vcc will be available at collector.
Schematic
In this circuit, a square wave input is applied. When input is high, the transistor is turned on and
works in saturation region. So maximum current IC flows through transistor as well as LED. Hence
LED emits the light. When input is low (low means not enough to turn on the transistor), the
transistor remains in cutoff. So current IC is zero thus LED does not emit the light. As the input is
square wave, the LED will turn on and off alternately. If output is observed on CRO from the
collector, then it will be also a square wave but out of phase by 180° with input. Thus transistor
is working as a switch which can be made on or off by an external input.
Procedure
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Connect the circuit as shown in figure.
Apply 1 Hz square wave signal to the base and ground from function generator.
Apply 5V VCC to collector and ground.
Observe the indication of LED and see the output waveforms on CRO.
Observations
Draw input and output waveforms.
Conclusion
The transistor switching operation is performed based on the voltage applied at the base terminal
of a transistor. When a sufficient voltage is applied between the base and emitter, collector to
emitter voltage is approximately equal to 0. Therefore, the transistor acts as a short circuit. The
collector current flows through the transistor.
Similarly, when no voltage or zero voltage is applied at the input, transistor operates in cutoff
region and acts as an open circuit. In this type of switching connection, load (LED) is connected
to the switching output with a reference point. Thus, when the transistor is switched ON, current
will flow from source to ground through the load(LED).
Graph 3.2: Output for Inverting Voltage Amplifier
Phase Shift=_____________
Questions
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What is the effect as Rf is increased?
How does the voltage gain vary as the op amp is changed?
Does the inverting voltage amplifier exhibit a predictable and stable voltage gain?
What can be said about the input impedance of this type of amplifier?
Based on the simulation, what is the relationship between gain, feedback and distortion
for this circuit?