ELECTRONICS LABORATORY

SIMULATION OF BJT AMPLIFIER

Simulation of BJT Amplifier

Course – Section : ECE20L-E06

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Course Instructor : Engr. Julius Sese

 Simulation of BJT Amplifier

An amplifier is an electronic circuit that increases the amplitude of voltage, current, or power.
The main component of amplifier circuit is the transistor. The name transistor comes from
transfer and resistor. In order for the transistor to amplify voltage, this semiconductor device
must have a low input resistance and a high output resistance. So that when the electronic
current flows through the transistor device, the small input voltage becomes a bigger output
voltage.

To illustrate the amplifying action of the transistor, Figure 1 presents an NPN bipolar junction
transistor (BJT) with a forward-biased base-emitter PN junction and with a reverse-biased base-
collector PN junction. The PN junction has a voltage-controlled resistance. When the BE
junction is applied with forward bias, it results to having a lower resistance. On the other hand,
when the BC junction is applied with reverse bias, it produces a wider depletion region causing
a higher resistance. So when the transistor current flows in through a small-resistance FB input
BE junction, it results to a lower input voltage. As the transistor current flows out through a
high- resistance RB output BC junction, it yields a higher output voltage. This is the condition
that a BJT can amplify the voltage signal. This condition is called the active operation.

Figure 1. BJT as an Amplifier.

Table 1. Operating Conditions of BJT.

Bias of PN Junction
Operating Condition Transistor Application
BE Junction BC Junction
Active FB RB Amplifier
Saturation FB FB Close Switch
Cutoff RB RB Open Switch
Reverse Active RB FB Attenuator
Table 1 provides the different operating conditions of BJT, namely: active, saturation, cutoff,
and reverse active. The required condition of transistor in amplifier circuit is the active
condition. For an NPN transistor, the BE junction, with P-type base and N-type emitter, is
considered forward biased if its base voltage is higher than its emitter voltage, by at least the
amount of barrier potential which is around 0.7V. The BC junction, with P-type base and N-type
collector, is reverse biased if its collector voltage is higher than its base voltage. The transistor
terminal voltages, namely: the collector voltage VC, the base voltage VB, and the emitter
voltage VE; are measured from the transistor terminal to the ground of the amplifier circuit.
The voltage requirement of transistor device is provided by using a biasing circuit, like the
voltage divider bias.

Now, let us use the LTSPICE to run simulations and determine the characteristics of bipolar
junction transistor (BJT) amplifier, such as the Common-Emitter Amplifier. Here is a link about
‘Getting Started with LTSPICE’: https://learn.sparkfun.com/tutorials/getting-started-with-ltspice/all

1. Connect the circuit diagram of Common-Emitter Amplifier. Refer to Figure 2. The input
signal Vin is connected to the base terminal of transistor Q1, through the input coupling
capacitor C1. The output voltage is taken from the collector terminal of Q1, through the
output coupling capacitor C2, and is across the load resistor R5. The bias circuit of Q1 is the
voltage divider bias, comprised of the voltage dividers R1 and R2. The voltage across
resistor R2 provides the base voltage of Q1. The voltage across resistor R4 provides the
emitter voltage of Q1. The collector voltage of Q1 is the power supply VCC less the voltage
across the resistor R3. The base voltage must be greater than the emitter voltage by at
least 0.7V, to forward bias the base-emitter junction. The collector voltage must be lower
than the base voltage, to reverse bias the base-collector junction.

Note: The student or group may opt to change the part number or the values of
the components for improved performance of the amplifier.

2. Take a snapshot of your schematic diagram in LTSPICE and place it as Figure 3.

3. Fill-up the Table 2 with the components used in your Common Emitter Amplifier. You may
use the Bill of Materials feature in LTSPICE to provide you with the list of components.
 Figure 2. Example of BJT Amplifier.

Figure 3. BJT Amplifier connected as Common-Emitter Amplifier in LTSPICE.

Table 2. List of Components of BJT Amplifier.

Component Part No. Value Description

Q1 NPN

R1 25.5 k
R2 25.5k
R3 2.32k
R4 1.02k
R5 10.5k
C1 100𝜇
C2 100𝜇
C3 22𝜇
The amplifier circuit in Figure 2 has two voltage sources, the dc power supply VCC and the ac
input signal Vin. By applying the superposition theorem, the analysis of amplifier circuit can be
by three steps.

First is the large-signal analysis or the dc analysis of the amplifier circuit. The effect of the dc
power supply VCC to the amplifier circuit is established. In dc analysis, the ac input Vin is
shorted and the capacitors are treated as open circuits. The frequency is 0 Hz under the dc
condition. That is why the capacitive reactance of the capacitors become very high, hence the
capacitors are effectively open circuits. With open-circuit capacitors, the equivalent dc circuit of
amplifier in Figure 2 is shown in Figure 3. Using this equivalent circuit, the bias condition of
transistor is determined by computing the operating terminal voltages and the operating
transistor currents.

Figure 3. Equivalent DC Circuit of BJT Amplifier.

Second is the small-signal analysis or the ac analysis of the amplifier circuit. The effect of ac
input signal Vin to the amplifier circuit is established. In ac analysis, the dc power supply VCC is
shorted and the capacitors are treated as short circuits. When the VCC is shorted, the nodes of
resistors R1 and R3 become connected to circuit ground. In ac analysis, the transistor is
replaced with its current-controlled current source model. The equivalent ac circuit of amplifier
in Figure 2 is provided in Figure 4. Using this equivalent circuit, the ac parameters of amplifier is
determined. These amplifier parameters include the input resistance, output resistance, voltage
gain, current gain, and power gain. Third step is to combine the results of dc and ac analyses.

Figure 4. Equivalent AC Circuit of BJT Amplifier.

4. Run a DC simulation in LTSPICE. Measure the following operating voltages:
Collector DC voltage source (VCC) = 12V
Collector voltage (VC) = 3.81574V
Base voltage (VB) = 4.52428V
Emitter voltage (VE) = 3.71631V _

5. Based on the measured DC voltages, answer the following questions. Briefly explain your
every answer.

5.1. How much is the voltage across the base-emitter junction (VBE) of the transistor?
How is the base-emitter junction of the transistor biased?

The voltage across the base-emitter junction is 0.808V and the base-
emitter junction of the transistor is forward biased.

5.2. How much is the voltage across the base-collector junction (VBC) of the transistor?
How is the base-collector junction of the transistor biased?

The voltage across the base-collector junction is 0.709V and the base-collector
junction of the transistors is in reversed biased.

5.3. Based on the conditions of BE and BC junctions, what is the operating condition of
the transistor? What is the impact of this condition to the amplifying action of the
transistor?

The working state of the semiconductors is dynamic wherein the BE and BC

intersections are in forward one-sided and invert one-sided, individually.
The effect of this condition to the enhancing activity of the semiconductor is that
the semiconductor application turns into an intensifier.
6. Run the transient simulations in LTSPICE and display the waveforms of input voltage at node
IN and of output voltage at node OUT. Adjust the magnitude of sine-wave input voltage Vin
until the waveform of output voltage shows no distortion or clipping. Take a snapshot of the
simulation waveforms and place it in Figure 5.

Figure 5. LTSPICE Simulation Waveforms of Common-Emitter Amplifier.

7. Measure the parameters of input voltage at node IN and of output voltage at node OUT.
Input voltage (Vin) = 1.0μ V
Input frequency (fin) = 1000kHz _ _
Output voltage (Vout) = 20.0μV _ _
Output frequency (fout) = 1000kHz _
8. Remove the bypass capacitor C3 in the circuit in Figure 2. Run the transient simulations in
LTSPICE and display the waveforms of input voltage at node IN and of output voltage at
node OUT. Adjust the magnitude of sine-wave input voltage Vin until the waveform of
output voltage shows no distortion or clipping. Take a snapshot of the simulation
waveforms and place it in Figure 6.

Figure 6. LTSPICE Waveforms of Common-Emitter Amplifier without Bypass Capacitor.

9. Measure the parameters of input voltage at node IN and of output voltage at node OUT.
Input voltage (Vin, unbypassed) = 1.0mv
Input frequency (fin, unbypassed) = 1000kHz

Output voltage (Vout, unbypassed) = 600μV

Output frequency (fout, unbypassed) = 1000kHz _
10. Reconnect the bypass capacitor C3 in the circuit but remove the load resistor R5 in Figure 2.
Run the transient simulations in LTSPICE and display the waveforms of input voltage at node
IN and of output voltage at node OUT. Adjust the magnitude of sine-wave input voltage Vin
until the waveform of output voltage shows no distortion or clipping. Take a snapshot of the
simulation waveforms and place it in Figure 7.

Figure 7. LTSPICE Waveforms of Common-Emitter Amplifier without Load Resistor.

11. Measure the parameters of input voltage at node IN and of output voltage at node OUT.
Input voltage (Vin, unloaded) = 1.0 𝜇V
Input frequency (fin, unloaded) = 1000 kHz

Output voltage (Vout, unloaded) = 0

Output frequency (fout, unloaded) = 0
 12. Calculate the voltage gain of BJT Common-Emitter Amplifier with load resistor and with
bypass capacitor. Use the measurements in number 7.
Voltage Gain with Load Resistor and Bypass Capacitor (AV, loaded, bypassed) = 20.0μV

13. Find the voltage gain of BJT Common-Emitter Amplifier with load resistor but without
bypass capacitor. Use the measurements in number 9.
Voltage Gain with Load Resistor, Without Bypass Capacitor (AV, loaded, unbypassed) = 0.6 μV

14. What is the voltage gain of BJT Common-Emitter Amplifier without load resistor but with
bypass capacitor? Use the measurements in number 11.
Voltage Gain without Load Resistor, With Bypass Capacitor (AV, unloaded, passed) = 0

15. Briefly discuss how to determine the voltage gain of a BJT amplifier.

I determined the voltage gain of a BJT amplifier using the voltage

gain formula. The formula of voltage gain that was used was 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝐺𝑎𝑖𝑛 = − 𝑅𝐿/𝑅𝐸

16. Explain the effect of bypass capacitance to the voltage gain of a BJT amplifier.
The impact of the detour capacitance to the voltage gain of a BJT is that it shorts the sign of AC
through the producer resistor wherein it keeps the producer at AC ground. The addition of the
intensifier is at most extreme and is equivalent to Rc/r'e. In this way, huge estimation of the detour
capacitor is normal for the reactance over the enhancer's recurrence range being little in worth being
contrasted with the RE.

17. What is the effect of load resistance to the voltage amplification of a BJT amplifier?
At the point where the resistor is connected through the yield utilizing the coupling capacitor, it
can put load on the given circuit. The opposition gatherer through the recurrence signal is
essentially Rc equidistant with RL.

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