Bipolar junction transistors amplifiers

Bipolar junction transistors amplifiers

I. Introduction

HE Bipolar-Junction-Transistors-Amplifiers are active devices used in many applications such as switching and amplifying and etc. The DC biasing determines the operating point of the device and its performance characteristics. The BJT transistor structure contains three regions, the emitter, base and collector. The objective of this lab is to design and gain a understanding of how an amplifier can be built by using a BJT.


The objective of this lab is to design and gain an understanding of the physical structure, operation, and characteristics of the bipolar junction transistors (BJT). In particular how to determine the gain, input resistance, output resistance, and the frequency bandwidth of the amplifier by simulation, hand calculations, actual measurement. The last objective is to recognize the discrepancies between the three and why they may differ from each-other.


A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. This mode of operation is contrasted with unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due to the flow of charges injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices.

The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is represented by β; it is approximately the ratio of the DC collector current to the DC base current in forward-active region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications. Another important parameter is the common-base current gain, α. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and 0.998. Alpha and beta are more precisely related by the following identities (NPN transistor):

IV. Design Approaches/Trade-offs

The performance of this lab will depend on how well the circuit is developed. If the circuit is developed correctly the results should be similar to the simulation results that were obtained by PSpice and the hand calculations (analysis). The performance of the lab also depends on how well the equipment is calibrated and accurate the components tolerance is.

This is not a very cost effective lab except for the development and time it took to construct the lab components. But to save money for a lab project, whether it's the testing or developing phase of a new design, depending on what the schematic is, a circuit can be reduced, if done correctly.

V. Hand Calculations

The hand calculations used for this lab (See figure 1) can be found below.


-Rc||Rl/Re = Av

Ri = R1||R2||Rib

Rib = Rpi + (1+Beta)Re

Rpi = Beta(Vt)/Icq

AV = 7, choose Re = 1kohms

7 = Rc||RL/Re

7 = Rc||10k/Rc + 10k

7Rc + 70k = 10RcK

Rc = 3Rc = 70 Rc = 23.3Kohms

Choose R2 = 15kohms

10 - Icq(34) -1 = (Icq -0.1)(7.6744)

9 - 34Icq = 7.67(Icq - 0.1)

9 - 34Icq = 7.67Icq - 0.767

9 = 41.674Icq - 0.767

9.767 = 41.674Icq

Icq = 0.234mA

Icq + Icq/Beta = Ie

Ie =0.234+( 0.234/200) Ie = 0.23517

Ie (Vbe) = Vb Vb = (0.23517)(0.65)

Vb = 0.885

15k(10)/R1 + 15K = 0.885

150k = 0.885R1 + 13.275

136.725 = 0.885R1

R1 = 154.49kohms

Ri = R1||R2||Rib

Rib = Rpi + (1 + Beta)Re

Rib = 22.1 + (201)(1k) Rib = 223.1

Ri = 154.49||15||223.1

Ri = 12.88kohms

VI. Circuit Schematics

The circuit schematics below were built in PSpice and allowed our team to analyze the circuit digitally before performing the physical build.

VII. Component List

The following is a list of components that were used in constructing the BJT amplifier from the giving specs in lab

3. Component values were selected by the professor.

  • A digital multimeter for measuring circuit voltages, resistor resistances, and capacitor capacitance.
  • A oscilloscope for viewing the input and output waveforms of the circuit.
  • A power supply capable of producing Vcc = 10V
  • A Pulse Generator capable of delivering input voltage of (100mV) and a signal at about 2Khz.
  • A 2N3904 Transistor, V(BE) = .65V, Vt = 0.026V Beta (B) = 200
  • 5 resistors RL = 10kohms, Rc = 23.2kohms, (raised to 33kohms), Re = 1kohms, R1 = 154.49kohms (raised to 160kohms) and R2 = 15kohms.
  • Two capacitors C1 and C2 = 0.1uF
  • Bread board with wires.
  • NOTE: Resistors can normally provide around +/-5%-25% difference between actual and designed values while Capacitors generally provide around 20%-50% difference between actual and designed values. You can add resisters in series as (R1+R2) to closer approximate required resistance values and you can add Capacitors in parallel as (C1+C2) to closely approximate required capacitance.

VIII. PSpice simulation Results

The P-Spice simulation results below confirmed our circuit schematics and allowed our team to confirm the circuit digitally before performing the physical build.

Key: Green line = Vout, Purple line = Vin

Scale: Vout ( Y - Axis) Range: -800mV to 800mV (400mV increments)

Vin (X - Axis) Range: 1.000s to 1.0045s (0.005s increments)

PSpice results show that when Vin ranges from -100mV to 100mV. It is showing that Vin is 200mV peak-peak.

Vout = Pspice results show that Vout ranges from 667.466V to -694.957V. Vout = 1.362mV. Which produces a gain of 6.81.

DC Bias Results

The graph above is the actual experimental results. It shows

that the Vpp voltage/200mv is appx the gain (Av). Rc and R1 were raised to increased to achieve a close gain of 7.

IX. Experimental Data

The above diagram is the experimental data.The change in the Vpp voltage alters between 1.35V and 1.38 depending on what resistor values were used. The gain is approximately 7, according to this result Vpp/.2mV = 1.35/0.2.


The analysis/PSpice/Experimental data results were all accurate, but the results differed between the three. The reasons that the results were different is because the experimental results have equipment calibrations, component tolerances, and actual measures values from the components. The PSpice results is a close estimate of what the results showed actually be. For example figure 1 shows a graph of what the output showed look like. And the actual results verifies that to be true. The analysis is a good estimate of what the PSpice results should be. If the PSpice results match the analysis results, then it's time to work on the actual lab. All three were not in total, agreement however, the results were close to each other, and can be proved by applying the PSpice Results from Figure 1 to the experimental results.

XI. Conclusions

The DC Bias values that were calculated by hand are similar to that of the P-Spice results. The results from the different methods proves that the P-Spice and hand calculation are correct. The measured characteristics closely matched those in the specifications. A 2N3004 Transistor was used , a requirement of a gain of 7 was closely achieved because we obtained a gain of 6.89 from the actual experiment results. The input resistance that had to be at least 12K was achieved because the input resistance that we resulted in was 12.9Kohms. The signal was undistorted which is a requirement for the lab. There was no clipping, or saturation. Look at figure 5. This BJT Amplifier lab may be improved to achieve a closer gain of 7 by altering Rc. Depending on the other resistor values, whether or not to increase or decrease the resistance value.

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