Overview
ECT226 Course Project—Week 5
Transistor Amplifier and Switching CircuitsObjectives
- Learn how to calculate the voltage gain of an amplifier.
- Measure the input and output signals using an oscilloscope in Multisim.
- Observe and explain how a transistor acts as a switch.
Software
Procedures
Part 1: Using a BJT as an Amplifier
A BJT transistor can be built to amplify AC signals or used as an electronic switch. As shown in Figure 1.1, when the transistor is configured to work in the active region, it works as an amplifier. When the transistor is in the saturation region, VCE is very low (around 0.2 V) and the transistor acts like a closed switch. When the transistor is in the cut-off region, IC is near zero, VCE = VCC, and the transistor is like an open switch.
Figure 1.1: Collector curves of a BJT
- Open Multisim. Click (place transistor) and type 2N3904 to select the transistor, as shown in Figure 1.2.
- Build the common emitter transistor amplifier circuit, as shown in Figure 1.3.
- Set the function generator XFG1, as shown in Figure 1.4.
- Run the simulation. Configure the timebase and scales for channels A and B, as shown in Figure 1.5. Place the BLUE bar at a peak of the borrom waveform and the RED bar at at a peak of the top waveform. (The top waveform shows the input voltage and the bottom waveform shows the output voltage.)
- Read the peak voltages for Vin (under Channel_A) and Vout (under Channel_B).
- For all amplifiers, voltage gain (AV) is defined as the following.
- Now, repeat steps 3 to 6 with the following function generator settings:
- Copy and paste the following two screenshots based on the amplifier circuit with the function generator settings of step 7 to the ECT226 Project Deliverables Module 5.
- Read the peak voltages for Vin (under Channel_A) and Vout (under Channel_B) and calculate as shown in step 6. Record your values in Table 1.1.
- Copy/paste or take a screenshot of Table 1.1 and insert it in the ECT226 Project Deliverables Module 5.
Figure 1.2: Pick a NPN BJT transistor
Figure 1.3: The common emitter transistor circuit
Note the following in Figure 1.3.
- C1 and C2 are coupling capacitors used to block the DC components.
- Ce is a bypass capacitor used to bypass the AC voltage. It has no effect to DC.
- The oscilloscope XSC1 displays the input voltage Vin on channel A and the output voltage Vout on channel B.
Figure 1.4: The function generator provides a sinusoidal input: f = 1 kHz, VP = 40 mV
Figure 1.5: Adjust the timebase and vertical scales
Calculating AV based on the readings from Step 5, we see that:
Important: Keep both Vin and Vout either all mV or V. And, AV has no unit because the units will cancel out.
sinusoidal input: f = 1 kHz, VP = 50 mV
- The BJT amplifier circuit schematic from your Multisim design
- The simulation results displayed on the oscilloscope XSC1 along with proper settings and the placements of the BLUE and RED bars at the peaks of the waveforms.
Table 1.1
Vin (mV) | Vout (V) | Av |
Part 2: Using a BJT as a Switch
In this part of the project, you will observe how a transistor acts as a switch.
- Build the simulation circuit shown in Figure 2.1.
- Follow Figure 2.2 to get the clock input CLOCK_VOLTAGE.
- To configure the clock input to 2 Hz, double click on the symbol and type in the desired frequency.
- Run the simulation. Double click on the Oscilloscope to see the output voltage between the collector and ground. Paste the oscilloscope screenshot along with proper settings of the oscilloscope to see at least two cycles of the output signal to ECT226 Project Deliverables Module 5.
- Include the screenshot of the circuit created in figure 2.1 with LED ON.
Figure 2.1: The 2N3860 is used as a switch to turn the LED ON/OFF.
Figure 2.2: Get the CLOCK_VOLTAGE from sources
- Note that an oscilloscope is added to display the Vout between the collector and GND.
Part 3: Conclusion
Write a conclusion for your project answering the questions below and include it in your ECT226 Project Deliverables Module 5.