AB 4 : NMOS Common-Source Ampliϐier and NPN Common-Emitter Ampliϐier Lab
Objective:
To study NMOS-based common- source (CS) and NPN-based common-emitter (CE) ampliϐiers by:
Completing the DC and small-signal analysis based on their theoretical behavior.
Simulating the ampliϐiers to compare the results with the paper analysis.
Implementing the circuits in an experimental setting, taking measurements, and comparing its performance with theoretical and simulated results.
Measuring their output resistance.
Qualitatively seeing the impact of transistor-to-transistor variations.
Materials: 1. Laboratory setup, including breadboard 2. 1 enhancement-type NMOS transistor (e.g., 2N7000) and 1 NPN-type bipolar transistor (2N2222) 3. 3 large (e.g., 47µF) capacitors 4. Several wires and resistors of varying sizes
Part 1:Design and Simulation of NMOS Common-Source Ampliϐier
Consider the circuit above.
Design the ampliϐier to achieve a small-signal gain of at least AV = −5V /V . Use supplies of V+ = −V− = 15V , Rsig = 50Ω, RL = 10kΩ, RG = 10kΩ, and design the circuit to have ID = 1mA. Obtain the data sheet for the NMOS transistor that will be used. 1 DC Operating Point Analysis
Using a DC model of the circuit, (the three coupling capacitors are replace with ”large-valued” capacitors that create open circuits and you may also omit vsig, Rsig, and RL from the circuit), you have enough information to calculate VOV = VGS−Vtn. Calculate and write down the value of VOV and gm.
What is the value of VGS?
Remember: Your actual transistor will have a value of Vtn that will vary from his nominal value, which will alter your measurement results slightly.
Calculate r0.
You now have enough information to calculate RS. Calculate and show your calculations for RS. Is this value available in your kit or can it be created by combining resistors?
Note: At this stage we do not know neither VDS nor RD. AC Analysis
Replace the circuit with its small-signal model. In the small-signal model, the capacitors will be at short circuits (what happens to RS?) and replace V+ with an AC ground. What happens to V−? Label the gate of the transistor as vi , i.e., the small-signal voltage at the input.
What is the ratio of vi/vsig? How would you approximate it in further calculations?
Derive the expression for Av = v0/vi . What is the value of RD that produces a small-signal focus gain of at least AV = −5V /V ? Is the value for RD available in your kit or can it be created by combining resistors?
What is the DC voltage at the drain? Does this satisfy the assumption that the transistor should be operating in the saturation region? Explain.
What is the output resistance, Ro?
Simulation
Simulate your circuit. Use capacitive values CC1 = CC2 = CS = 47µF, and the values of RS and RD based on your preceding calculations. Use a 10mVpk−pk, 1kHz sinusoid with not DC component applied at vsig.
From your simulation, report the DC values of VGS, VDS, and ID. How closely do they match your calculations? (Remember: The simulator has his own more complex model of the real transistor, so there should be some small variations.)
From your simulation, report AV . How closely does it match your calculations?
Part 2:Prototyping
Assembled the circuit onto your breadboard using the speciϐied component values and those just calculated. Note that Rsig represents the output resistance of the function generator, and therefore you should not include in your circuit.
Part 3:Measurement
DC bias point measurements: Using a digital multimeter, measure the DC voltages of your circuit at the gate (VG) , source (VS), and drain (VD) of your transistor.
AC measurements: Using a function generator, apply to your circuit a 10mVpk−pk, 1kHz sinusoid with no DC component. (Note: Some function generators allowed only implicit small as 50mVpk−pk. If this is the case, use that value instead. Some distortion may occur in the output waveform.) 2
Using an oscilloscope, generate plots of vo and vi vs. t.
Output resistance Ro: Replace RL with a 1MΩ resistor and repeat the AC measurement. What is the amplitude of the output waveform? Adjust RL until you ϐind a value such that the amplitude of the output waveform is approximately 50% of what is was for the 1MΩ load. This new value of RL is the output resistance Ro.
How does it compare to the value calculated earlier? Hint: It cannot be greater than the value of RD.
Using a digital multimeter, measure all resistors to three signiϐicant digits. Part 4:Post-Measurement Exercise
Calculate the values of VGS and VDS that you obtained in the lab. How do they compare to your pre-lab calculations? Explain your discrepancies.
Based on the measured values of VD and VS and your measured resistor values, what is the real value of ID based on your lab measurements?
What is the measured value of Av? How does it compare to your pre-lab calculations? Explain any discrepancies.
Hint: The single biggest source of variations from your pre-lab simulation results will be due to variations in the transistor threshold voltage Vtn. Remember: Its value would be somewhere within the range indicated on the transistor data sheet.
Part 5: Design and Simulation of Common-Emitter Ampliϐier
Consider the circuit shown below. Design the ampliϐier to achieve a small-signal gain of at least AV = −200V /V . Use supplies of V+ = −V− = 15V , Rsig = 50Ω, RL = 10kΩ, RB = 10kΩ, and design the circuit to have IC = 1mA. Although there will be variations from transistor to transistor, you may assume a value of β of 100 in your calculations. Obtain the data sheet for the NPN transistor that will be used. 3 DC Operating Point Analysis
Using a DC model of the circuit, (the three coupling capacitors are replace with ”large-valued” capacitors that create open circuits and you may also omit vsig, Rsig, and RL from the circuit), calculate and write down the value of IB and IE. What is the value of VB?
Calculate a value for RE that produces a base-emitter voltage drop value of 0.7V . What is VE?
You now have enough information to calculate RS. Calculate and show your calculations for RS. Is this value available in your kit or can it be created by combining resistors?
Note: At this stage we do not know neither VCE nor RC . AC Analysis
Replace the circuit with its small-signal model (VA is large, so you may ignoore ro). In the small-signal model, replace the capacitors with short circuits (what happens to RE?) and replace V+ with an AC ground. What happens to V−? Label the base of the transistor as vi , i.e., the small-signal voltage at the input. What are the values of gm and rπ?
What is the ratio of vi/vsig? Can you approximate it?
Derive the expression for Av = v0/vi . What is the value of RC that produces a small-signal focus gain of at least AV = −200V /V ? Is this resistor value available in your kit or can it be created by combining resistors?
What is the DC voltage at the collector? Does this satisfy the assumption that the transistor should be operating in the active region? Explain.
What is the output resistance, Ro?
Simulation
Simulate your circuit. Use capacitive values CC1 = CC2 = CE = 47µF, and the values of RE and RC based on your preceding calculations. Use a 10mVpk−pk, 1kHz sinusoid with not DC component applied at vsig.
From your simulation, report the DC values of VBE, VCE, IB, IC , and IE. How closely do they match your calculations? (Remember: The simulator has his own more complex model of the real transistor, so there should be some small variations.)
From your simulation, report AV . How closely does it match your calculations?
Part 6:Prototyping
Assembled the circuit onto your breadboard using the speciϐied component values and those just calculated. Note that Rsig represents the output resistance of the function generator, and therefore you should not include in your circuit. Part 7:Measurement
DC bias point measurements: Using a digital multimeter, measure the DC voltages of your circuit at the base (VB) , emitter (VE), and collector (VC ) of your transistor.
AC measurements: Using a function generator, apply to your circuit a 10mVpk−pk, 1kHz sinusoid with no DC component. (Note: Some function generators allowed only implicit small as 50mVpk−pk. If this is the case, use that value instead. Some distortion may occur in the output waveform.) 4
Using an oscilloscope, generate plots of vo and vi vs. t.
Output resistance Ro: Replace RL with a 1MΩ resistor and repeat the AC measurement. What is the amplitude of the output waveform? Adjust RL until you ϐind a value such that the amplitude of the output waveform is approximately 50% of what is was for the 1MΩ resistor. This new value of RL is the output resistance Ro. How does it compare to the value calculated earlier? Hint: It cannot be greater than the value of RC .
Using a digital multimeter, measure all resistors to three signiϐicant digits.
Part 8:Post-Measurement Exercise
Calculate the values of VBE and VCE that you obtained in the lab. How do they compare to your pre-lab calculations? Explain your discrepancies.
Based on the measured values of VB, VC , and VE and your measured resistor values, what is the real value of all currents based on your lab measurements? How does it compare to your pre-lab calculations?
Based on the actual value of the measured currents, what is the actual value or β for your transistor?
What is the measured value of Av? How does it compare to your pre-lab calculations? Explain any discrepancies.
Hint: The single biggest source of variations from your pre-lab simulation results will be due to variations in β.
