ECEN 325 Lab : MOFET Amplifier Configurations Objective The purpose of this lab is to examine the properties of the MO amplifier configurations. C operating point, voltage gain, and input and output impedances of common-source and common-drain topologies will be studied. 2 Introduction 2. MOFET C Biasing Figures (a) and (b) show typical resistive biasing circuits for NMO and PMO transistors, respectively. V R2 V R2 V (a) V (b) Figure : Resistive C biasing circuit for (a) NMO (b) PMO Assuming that the transistors are active, C solutions for Figs. (a) and (b) can be found as = k n W NMO: 2 L (V V tn ) 2 V R2 V = k n R V R2 = ( + V ) = V + 2 + R 2 = k p W PMO: 2 L (V V tp ) 2 V R2 V = k p R 2 R V R2 = ( + V ) = V + 2 + W L (V V tn ) 2 () W L (V V tp ) 2 (2) Both quadratic equations above have two solutions, where the solution satisfying V > V tn and V > V tp should be chosen for the NMO and PMO circuits, respectively. After determining V or V, can be calculated from the linear equation. To verify that the transistors are active, the following should be satisfied NMO: V V ov + V ( + ) V V tn (3) PMO: V V ov + V ( + ) V V tp (4) 2.2 MOFET mall-ignal AC models Figure 2 shows the AC small-signal models for NMO and PMO transistors in the active region. mall-signal parameters in Fig. 2 can be calculated as = k W L V ov = 2k W L = λ (5) where λ is the channel length modulation parameter. For typical discrete MOFET circuit implementations, will not have a significant impact, therefore will be ignored. For small-signal AC analysis, π-model and T-model provide identical results, however the T-model allows more intuitive analysis with simpler calculations. Table shows node impedances and node-to-node gains for generic MOFET configurations, which are derived by substituting the transistor with its T-model, where =. c epartment of Electrical and Computer Engineering, Texas A&M University
NMO PMO π model T model π model T model s s g g Figure 2: mall-signal AC models for NMO and PMO transistors Table : MOFET Node Impedances and Node-to-Node ains when = NMO PMO Impedance NMO PMO ain Z gate Z gate Z gate = = + Z Z source Z Z source Z source = Z Z = Z source Z drain Z Z Z drain Z drain = vg = + 2.3 MOFET Amplifier Configurations 2.3. Common-ource Configuration Figures 3 and 4 show the common-source configurations for NMO and PMO transistors, respectively. AC analysis of this configuration yields A v = V o,ac = ( ) = = R = = (6) 2
V R V o V o,dc V o,ac R V R Figure 3: (a) NMO Common-ource Configuration (b) C equivalent (c) AC equivalent V R V o V R V o,dc R V o,ac Figure 4: (a) PMO Common-ource Configuration (b) C equivalent (c) AC equivalent Typical design specifications for the common-source configuration includes: 0-to-peak unclipped output voltage swing: ˆV o Voltage gain: A v = V o,ac Input and output resistances: and TH at the maximum output level Based on the typical specifications, design procedure for the common-source amplifiers in Figs. 3 and 4 can be given as follows: Choose V R V to reduce the variation of as a function of V t. To have an unclipped output swing of ˆV o, V R should be chosen such that ( ˆV o V R V ov ) V R ˆV o. Choice of V R does not only affect the available signal swing at the output, but also determines the gain as well as the linearity of the amplifier as follows: A v = = V R = k W L V R V ov k W 2 L V 2 ov = 2V R V ov mall-signal condition: ˆs 2V ov ˆV o 2V R V ov 2V ov ˆV o 4V R To maximize the available gain and linearity, choose V R = ˆV o V R V ov. ince V ov = 2V R A v, V R = ˆV o V R + 2 A v 3
To avoid clipping or distortion due to possible variation of V t, V R may be chosen slightly less than the value given in the equation above. Calculate V ov = 2V R A v, then = k W 2 L V ov 2. Calculate = V R and = V R Find and such that V R2 = V R + V t +V ov and d =, which yields = where d is the desired input resistance. 2.3.2 Common-rain (ource Follower) Configuration d V R + V t + V ov = d d Figures 5 and 6 show the common-drain configurations (also known as source follower) for NMO and PMO transistors, respectively. C analysis of this configuration can be performed using the same equations given in () and (2), whereas AC analysis yields A v = V o,ac = R i = = + (7) V o V o,dc R V o,ac Figure 5: (a) NMO Common-rain (ource Follower) Configuration (b) C equivalent (c) AC equivalent V o,ac V o V o,dc R Figure 6: (a) PMO Common-rain (ource Follower) Configuration (b) C equivalent (c) AC equivalent In typical multi-stage amplifiers, source follower is directly connected to a gain stage, such as a common-source amplifier, without the extra biasing resistors and. Therefore, C voltage levels in a source follower is typically dependent on the previous amplifier stage. 4
3 Calculations. Using the 2N7000 NMO transistor, design the common-source amplifier in Fig. 3(a) with the following specifications: upply Voltage, 5V 0-to-Peak Output wing, ˆV o V Voltage ain, A v 25 Input Resistance, 0kΩ TH for 5kHz V (0-to-peak) ine Wave Output Voltage, V o 5% how your design procedure and all your calculations. 2. Using 2N7000 and the same, and values from your common-source amplifier, calculate A v, and for the source follower in Fig. 5. 4 imulations For all simulations, provide screenshots showing the schematics and the plots with the simulated values properly labeled.. raw the common-source amplifier schematic in Fig. 3(a) using the calculated component values and 2N7000 transistor. (a) Perform C operating point or interactive simulation to obtain the C solution for V R2, V R, V R, V o,dc and. (b) Perform AC simulation to obtain A v and. (c) Apply a 5kHz 40mV sine wave signal to the input and obtain the time-domain waveforms for the input and output voltages using transient simulation. Perform Fourier simulation to measure the total harmonic distortion (TH) on the output waveform. (d) Increase the input amplitude to measure the clipping levels at the output voltage V o. 2. raw the source follower schematic in Fig. 5(a) using the calculated component values and 2N7000 transistor. (a) Perform C operating point or interactive simulation to obtain the C solution for V R2, V R and. (b) Perform AC simulation to obtain A v, and. (c) Apply a 5kHz 0.8V sine wave signal to the input and obtain the time-domain waveforms for the input and output voltages using transient simulation. Perform Fourier simulation to measure the total harmonic distortion (TH) on the output waveform. 5 Measurements For all measurements, provide screenshots showing the plots with the measured values properly labeled.. Build the common-source amplifier in Fig. 3(a) using the simulated component values and 2N7000 transistor. (a) Measure the C values for V R2, V R, V R, V o,dc and I C using the voltmeter scope. (b) Measure A v and using the network analyzer. (c) Apply a 5kHz 40mV sine wave signal to the input and obtain the time-domain waveforms for the input and output voltages using the scope. Measure the total harmonic distortion (TH) on the output waveform using the spectrum analyzer. (d) Increase the input amplitude to measure the clipping levels at the output voltage V o using the scope. 2. Build the source follower circuit in Fig. 5(a) using the simulated component values and 2N7000 transistor. (a) Measure the C values for V R2, V R and using the voltmeter scope. (b) Measure A v, and using the network analyzer. (c) Apply a 5kHz 0.8V sine wave signal to the input and obtain the time-domain waveforms for the input and output voltages using the scope. Measure the total harmonic distortion (TH) on the output waveform using the spectrum analyzer. 5
6 Report. Include calculations, schematics, simulation plots, and measurement plots. 2. Prepare a table showing simulated and measured results. 3. Compare the results and comment on the differences. 7 emonstration. Build the common-source amplifier in Fig. 3(a) and the source follower in Fig. 5(a) on your breadboard and bring it to your lab session. 2. Your name and UIN must be written on the side of your breadboard. 3. ubmit your report to your TA at the beginning of your lab session. 4. For the common-source amplifier in Fig. 3(a): Measure A v and using the network analyzer. Apply a 5kHz 40mV sine wave input and show the time-domain output voltage using the scope. With the 5kHz 40mV sine wave input, measure the TH at the output using the spectrum analyzer. 5. For the source follower in Fig. 5(a): Apply a 5kHz 0.8V sine wave input, and show the time-domain waveforms at the input and the output using the scope. 6