Bipolar Junction Transistors
Dr. S. M. Haider Aejaz
Department of Electrical Engineering
GC University, Lahore
haideraejaz@gcu.edu.pk
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Overview
1 4-8 The BJT Inverter (Transistor Switch)
2 Lecture Assignment
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Introduction
The CE transistor configuration can be used as an inverter as well as
a switch
As an inverter, the circuit converts a low voltage level to high and a
high voltage level to low.
As a switch, the circuit can be used to turn ON or OFF a device.
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BJT Inverter
The figure shows a simple inverter circuit. Note the value of VCC , which is
5 V and the input signal is applied through RB to the base with the two
levels of 0 V and 5 V. What do you expect the behaviour of the transistor
when the input is 0 V or 5 V?
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BJT Inverter
The 0 V at the base is insufficient to forward–bias the BE junction, so the
transistor is in cut–off mode. When the input voltage is 5 V, the transistor
is designed to be in saturation. VCE (sat) is almost 0 (typically about 0.1
V).
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BJT Inverter - Analysis
With 0 V at the input (VLO ), the transistor will definitely be in
cut–off as the BE junction cannot be forward–biased.
When the input is 5 V (VHI ), we have the following,
VHI −VBE
IB = RB
IC = βIB
If this current is high enough, the transistor will be saturated
VCC
Using KVL between the collector and emitter, IC = IC (sat) = RC ,
where VCE (sat) being very small is assumed equal to 0.
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BJT Inverter - Example 4-16
Verify that the circuit in Figure 4–46 behaves like an inverter when the
input switches between 0 V and +5 V. Assume that the transistor is
silicon and that β = 100
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BJT Inverter - Solution
We have to verify that the transistor is saturated when the input voltage is
5 V.
IB = 5−0.7
100k = 43µA
With this value of IB , IC = βIB = 4.3mA
Now, VCE = 5 − (4.3m)(5k) = 5 − 21.5 = −16.5V
This means that the transistor is well in to saturation. So, the actual value
of VCE is 0 V and IC = IC (sat) = 1mA.
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BJT Inverter - Design
VHI −VBE βRC
RB ≤ VCC
VCC RB
RC ≥ β(VHI −VBE )
Since the β of a transistor often varies, we use its minimum possible
value in the above equations, so that saturation is ensured over its
entire range.
Thus, the transistor is often overdriven in saturation.
Overdriving affects the speed response of the transistor while
switching.
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BJT Inverter - Example 4-17
An inverter having RC = 1.5kΩ is to be designed so that it will operate
satisfactorily with silicon transistor whose β–values range from 80 to 200.
What value of RB should be used? Assume that VCC = VHI = +5V
Do it yourself!
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The Transistor as a Switch
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The Transistor as a Switch
It can be seen that the circuit is the same CE configuration.
The input is provided at the base through RB
The output is observed at the Collector
VCC at the input causes the transistor to saturate and there is 0 V
across the transistor (closed switch)
0 V at the input causes the transistor to go in to cut-off mode.
Hence, all the voltage appears across the transistor
Loads operating at different voltages can be connected using a relay
as shown in Fig. 4-48.
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The Transistor as a Switch
Current flowing through the transistor energizes the relay which will turn
the connected device ON. When the transistor will be in cut-off, no
current flows and hence the relay is OFF.
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To do and submit!
Solve Example 4–17.
Why a diode is connected across the relay coil in Fig 4-48?
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