Bipolar Junction Transistor Characterstics-Biassing and
Experiment - #9
Kehali B. Haileselassie and Kou Vue
ELC ENG 330 – Electronics I
To obtain the input and output Characteristics of a single Bipolar Junction Transistor
(BJT) and to demonstrate the capability of the transistorin order to produce amplification
when it is biased in the active region.
Determine the BJT small-signal parameters: input resistance Rπ and output resistance R0 .
Observe the effect of bias on BJT small signal parameters.
A bipolar junction transistor has three terminals: emitter (E), base (B) and collector (C). In BJT
the current flowing from the emitter(E) to the collector (C) which is controlled by changing
voltage drop between base (B) and emitter (E). Usually, in most circuits the signal base current
(IB) is quite small as compared to collector current(IC). Hence, BJT-based circuits can be used
to amplify the signal since small input power can produces large output power.
The common emitter (CE), common base (CB) and common collector (CC) BJT
Configurations in circuits depending on which BJT terminal is grounded becauseit is used as a
reference point for the input and output signals.
Internally BJT is three layers of semiconductors of different conductivity types. For instance, in
n-p-n gamma-based BJT the emitter is n-type gamma, base is p-type gammaand collector is
again n-type gamma. thereare two pn-junctions inside BJT. It can be bias Base-Emitter or BaseCollector junctions either in forward or reverse direction by applying voltages between terminals.
In n-p-n BJT the positive VBE = VB-VE means forward bias to base-emitter junction which
lowers the energy barrier for electron injunction from emitter to base and as a result electrons are
freely diffuse across the base that they are rapidly removed at the base-emitter junction.
The base-collector junction could be reversely biased as well when VBC = VB-VC is
negative. In other words, the positive Collector accepts electrons coming from Emitter through
Bipolar Junction Transistor operates in three regions (in the active region, in the saturation
region or in the cut-off region). These three regions are defined as follows:
Cutoff: both the emitter and collector junction are reverse biased
Saturation: both the emitter and collector junctions are forward biased
Active: emitter junction is forward biased, collector junction is reverse biased
Part 1: Establishing the DC Operating Point (“Q-Point”).
1. Wiring up the complete circuit and measuring exact values for the resistors.
2. Turn on the 12-volt DC power supply, and record all voltages in the circuit.
3. Measurethe values for voltages and currents in the circuit compare to predicted values.
Part 2: Evaluating AC Small Signal Operation.
1. With the 12 volt DC supply turned off, add the “decoupling capacitors” to the circuit, along
with the load resistor and the input signal generator.
2. Set the signal generator to provide a 100mVpp, 1 kHz signal to the circuit and using the
oscilloscope we verified the amplitude after changing the “Out Term” setting on the signal
generator to “high–Z”.)
3. Temporarily disconnect the signal generator, and then turn on the 12-volt DC supply in order
to verify that the base, collector and emitter DC voltages have not changed from Part 1.
4. Using the oscilloscope, record the amplitude of the AC signal at the output of the signal
generator, the base of the transistor, the collector and across the load resistor.
The values of all voltage in the circuit from our measurement are stated in the table below:
Where VR_1 is the voltage across R=10k
VR_2 is the voltage across R=2.2k
VR_C is the voltage across R=2k
VR_E is the voltage across R=470
And VB is the voltage across the base, VE is the voltage across the emitter, VC is the voltage
across the collector. We can use figure_1 from the top for further clarification.
The calculated value of all Currents andβDC ofthe resistor in the circuit from our
measurementis stated in the table bellow
IR_1 (in mA)
IR_2 (in mA)
IB (in mA)
IR_C (in mA)
IR_E (in mA)
βDC (IC/IB )
The measured Amplitude of the AC signal from the oscilloscope at the output of:
The signal Generator: 95.3mVpp
The base of the transistor: 96.8mVpp
The collector of the transistor: 388.9mVpp
The Load resistor RL: 389.8mVpp
The compute gain (in both absolute and dB) from the measured values of peak AC voltage:
Abs Gain (Vin/Vout)
Gain in dB
1) Amplitude of the AC signal from the oscilloscope for a signal generator
2) Amplitude of the AC signal from the oscilloscope for the base of transistor:
3) Amplitude of the AC signal from the oscilloscope across the base of transistor:
4) Amplitude of the AC signal from the oscilloscope across the base of transistor:
Response to the question that are stated in lab manual
1) How do your measured values for voltages and currents in the circuit compare to
predicted values using the simple 3-step method above? How do your voltage and current
values compare to PSPICE predictions using the “.op” (DC operating point) analysis?
They are almost the same. For instance, the predicted value of the emitter (IE) and
capacitor current (IC) is about 3.115 where as our measured values are 3.155 and 3. 165
respectively. It is almost the same with our SPICE results as well.
1) What is the gain if the signal generator voltage is increased to 1Vpp? …5Vpp? Why does
Because the voltage and current do not vary greatly from their operating point value.
2) From your Q-point values measured in Part 1, compute the small-signal value of rπ
(equation 4.37 in the text), and draw the small-signal equivalent circuit for this amplifier,
shown in Figure 4.34(b) in the text. (For our circuit, note that RE2 is zero (i.e., a short
circuit) and CE is not present, in Hambley’s Figure 4.34. You may assume Rs = 50 Ω.
You may also assume that the transistor’s βDC and βAC are the same.)
Rπ = BVT/ICQ = (450.719* 0.026V) / 3.155mA = 3714.32
3) In your write-up, calculate the predicted small-signal gain from your equivalent circuit and
compare it to your measured gain (for the 100mVpp input signal only). As Hambley derives
on page 254 for this circuit, the voltage gain is approximately |Av| = R′L / RE ≅ RC / RE. How
good an approximation is this?
|Av| = R′L / RE ≅ RC / RE = (2k / 470)= 4.255
The calculated small signal gain is 4.255 where as our measured value is about 3.9.
Hence, it is not accurate but it is still close enough.
4) How does your measured gain compare to a PSPICE prediction using the “.ac” (AC small
Our measured gain is almost the same as our PSPICE prediction using AC small signal
Discussion and Analysis:
The regions of interest in a transistor are the active region, cutoff region and the saturation region.
These regions are extensively used when the transistor is used in digital circuits. In the active
region, the transistor behaves as an ideal current source because the collector current is
independent of the value of the collector voltage. The collector current is dependent on the base
current. The dc common emitter current gain parameter of the transistor is the ratio of the current
across the collector to the current across the base. When the transistor is biased in the active
region it operates as an amplifier. The analysis suggests that the base current into the transistor is
negligibleand a small sinusoidal signals, Vsignal that superimposed on the DC voltage will give a
sinusoidal collector current and a superimposed on the DC current IC at the Q-point. Depending
upon the configuration of the resistors in the collector, the emitter, and the load, there will be an
ideal Q-point for maximum distortion-free output signal amplitude.
Vbase= (2.2k/12.2k)*12v = 2.164v ,Ic= IE= VE / RE = 3.115A ,Vemitter= Vbase– 0.7v = 1.464v
The use and characteristics of the Bipolar Junction Transistor as an amplifier was
explored in this experiment using the DC operating point (Q-Point) for transistor circuit which
function as a single stage transistor amplifier and the distinct disadvantages of the DC Operating
point (Q-Point) for a transistor amplifier. We also verify the correspondence of a small-signal
gain of a single stage to the predicted gain from an analysis of the small-signal equivalent circuit.
There were some percent error exist between our measured value and the predicted value. These
errors occurred due to the inaccuracy of the equipment we were used in order to build the signal
circuit. There were three fundamental configurations layer covered that are known as the Bipolar
Junction Transistor. We call the layer as the emitter, the base and the collector. Each layer has
unique beneficial characteristics as well as limitations. It is very crucial that the canonic cells are
well understood, as they will give a circuit designer the ability to evaluate complex circuit
topologies virtually by inspection.