Bipolar Junction Transistors ( BJT ). EBB424E Dr. Sabar D. Hutagalung School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia. Transistors. Two main categories of transistors: bipolar junction transistors (BJTs) and field effect transistors (FETs).
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Dr. Sabar D. Hutagalung
School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia
A point-contact transistor was the first type of solid state electronic transistor ever constructed.
It was made by researchers John Bardeen & Walter Houser Brattain at Bell Laboratories in December 1947.
The point-contact transistor was commercialized and sold by Western Electric and others but was rather quickly superseded by the junction transistor.
Transistor Size (3/8”L X 5/32”W X 7/32”H)
No Date Codes. No Packaging.
The “Planar Structure” developed by Fairchild in the late 50s shaped the basic structure of the BJT, even up to the present day.
NPN Bipolar Transistor
Apply a Collector-Base voltage
Apply an Emitter-Base voltage
Some electron fall into a hole
Some electron fall into a hole
And the Q in the base is
So, current is
and b is
a is called the common-base current gain
Collector-emitter is a family of
curves which are a function of
Base-emitter junction looks like a forward biased diode
Current in active region depends (slightly) on vCE
VA is a parameter for the BJT (50 to 100) and called the Early voltage
Due to a decrease in effective base width W as reverse bias increases
Account for Early effect with additional term in collector current equation
Nonzero slope means the output resistance is NOT infinite, but…
IC is collector current at the boundary of active regionEarly Effect
It is called the common-emitter configuration because (ignoring the power supply battery) both the signal source and the load share the emitter lead as a common connection point.
It is called the common-collector configuration because both the signal source and the load share the collector lead as a common connection point. Also called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance.
This configuration is more complex than the other two, and is less common due to its strange operating characteristics.
Used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.
Likewise, we can write KVL around the collector circuit.
1. Draw the load lines on the transistor characteristics
2. For the input characteristics determine the Q point for the input circuit from the intersection of the load line and the characteristic curve (Note that some transistor do not need an input characteristic curve.)
3. From the output characteristics, find the intersection of the load line and characteristic curve determined from the Q point found in step 2, determine the Q point for the output circuit.
The Load Line intersects the Base-emitter characteristics at VBEQ = 0.6 V and IBQ = 20 µA
Now that we have the Q-point for the base circuit, let’s proceed to the collector circuit.
The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at VCEQ = 5.9 V and ICQ = 2.5mA, then β = 2.5m/20 µ = 125
Note the asymmetry around the Q-point of the Max and Min Values for the base current and voltage which is due to the non-linearity of the base-emitter characteristics
From this graph, we find:
At Maximum Input Voltage:
VBE= 0.63 V, iB= 24 µA
At Minimum Input Voltage:
VBE= 0.59 V, iB= 15 µA
Recall: At Q-point:
VBE= 0.6 V, iB= 20 µA
∆iΒmax = 24-20 = 4 µA;
∆iBmin = 20-15 = 5 µA
Using these max and min values for the base current on the collect circuit load line, we find:
At Max Input Voltage: VCE= 5 V, iC= 2.7mA
At Min Input Voltage: VCE= 7 V, iC= 1.9mA
Recall: At Q-point: VCE= 5.9 V, iB= 2.5ma
From the values calculated from the base and collector circuits we can calculate the amplifier gains:
Current flow in a pnp transistor biased to operate in the active mode.
(a) A schematic illustration of pnp BJT with 3 differently doped regions. (b) The pnp bipolar operated under normal and active conditions. (c) The CB configuration with input and output circuits identified. (d) The illustration of various current component under normal and active conditions.
Current flow in an pnp transistor biased to operate in the active mode.
Two large-signal models for the pnp transistor operating in the active mode.