обо UCD DUBLIN Experiment 4 UCD School of Electrical and Electronic Engineering Electronic Circuits Laboratory - MOSFET Amplifier Design Part B - Measurements and Resimulation Equipment: Power supply, signal generator, oscilloscope, breadboard and components, computer with LTspice circuit simulation software Introduction You have already examined the DC characteristics of a MOSFET (metal-oxide-semiconductor field effect transistor) and have seen how to use the transistor as a switch – operating either in the cutoff region (zero gate-source voltage and no drain current) or in the linear region (large gate- source voltage and very small drain-source voltage). You may wish to refer back to the instructions for that exercise, or your lecture notes, to refresh your memory. In this laboratory, you will design and simulate a circuit to amplify a small alternating voltage signal. The MOSFET in this case will be biased in its saturation region. You will design and simulate an amplifier using discrete components by (i) choosing the DC biasing circuit which sets the operating point of the transistor at your chosen bias current ID and voltage VDs values, (ii) selecting resistor values to set the required small-signal voltage gain for your design, and (iii) choosing values for the coupling capacitors. The laboratory is in two parts: (A) a preliminary design and simulation phase and (B) a measurement and resimulation phase. You should complete the amplifier design (choose the component values and validate the design by simulation) before considering the measurements. In the measurement phase, you will breadboard your design and measure it. You should check that the transistor is biased correctly and has the desired small-signal behaviour. Measure the actual component values, resimulate the circuit using LTspice, and explain any differences between the theoretical, simulated, and measured performance of the real amplifier. Specifications The small-signal voltage gain and input resistance should satisfy the following: Small-signal voltage gain |Ay| ≥10 [Use the value you chose in Part A] Input resistance Rin > 50 k In addition, the amplifier should have the following properties: • It must provide at least the specified voltage gain at all frequencies in the range 100 Hz to 10 kHz when connected to the specified load resistance RL > 20 kN. • It must operate from a single DC supply voltage of not more than 30 V. • There must be no DC current flowing in the source or in the load. Transistor The BS170 transistor you will use is an n-channel enhancement type MOSFET. It comes in a small plastic package, with three pins, arranged as shown in Table 1. Note that this is the view looking at the top of the package, with the pins underneath. 03/24 EEEN20040 Absolute Maximum ratings: Manufacturers specify these values as the maxima which the component can withstand, at room temperature. You should always keep these in mind when designing a practical circuit. Table 1 BS 170 maximum values for key parameters Parameter Symbol Max. Value DO Drain-source voltage VDS 60 V GO Gate-source voltage (continuous) VGS +20 V SO Drain current (continuous) ID 0.3 A BS170 Power dissipation (in free air) PD 0.6 W top view Operating parameters: When designing a small-signal amplifier, the parameters in Table 2 are useful. Note that there can be large variations in the values of the parameters from transistor to transistor, due to statistical variations in the manufacturing process. Manufacturers give upper and lower bounds, as well as typical values. These parameters also change with temperature. Table 2 BS 170 small-signal parameters Parameter Symbol Minimum Typical Maximum Units Gate threshold voltage Vt 0.8 2.1 3 V Transconductance gm 12 20 32 mS (at ID = 2 mA, VDS 10 V) = Output resistance (at ID = 2 mA, VDS 10 V) Το 20 30 40 ΚΩ = Both small-signal parameters (9m and ro) increase with VDs, but relatively slowly, as the channel length modulation parameter λ is small. Im increases with drain current, and 9m x √ D. % decreases with drain current, and ro × 1/ID. You can estimate the values of these parameters at whatever operating point you choose. 2 Part B: Measurement and Resimulation Phase When you designed your amplifier in Part A, you will have selected topology (a) or (b) in Fig. 1. (a) RG1 Vin C1 RG2 ли (b) VDD VDD RG1 RD C2 C1 Vout R$1 ли RS2 Cs RL RG2 ли VDD VDD R$1 RD C2 Vout Cs w R$2 ли RL Fig. 1 Amplifier topologies. In Part B, you will build and measure your circuit, resimulate it using the values that have been used in the laboratory and compare the simulated behaviour with the measured behaviour. Explain any differences you observe. B1 Measurements Component values Record the values of all components that were used in the experiment. Operating point With the input voltage set to zero, record the drain current and the gate, source and drain voltages at the operating point. Small-signal voltage gain (AC response) Set the input signal to a 1kHz sinewave with an amplitude of 100mV and measure the output signal. Record and plot the output voltage at 100Hz, 10kHz, 100kHz, 1MHz. Locate the frequencies at which the small-signal voltage gain falls by a factor of 1/√2 from its mid- frequency value. Large-signal behaviour Set the input signal to a 1kHz sinewave with an amplitude of 1V and plot the output signal. B2 Simulation using measured component values DC analysis The first step is to check the DC conditions, by running a DC operating point simulation. You will use a number of different simulation commands – place them neatly together underneath or to one side of your schematic. Then use the Run command from the Simulate menu to run the simulation. If all is well, a window will appear with the Operating Point (aka Bias Point) information. The bias point information will show you the voltages at each node, and the current through all of the devices in your circuit. In particular, the currents at the gate, drain and source of the transistor are shown as IG, ID, and Is. No units are given – voltages are always in volts, currents are always in amperes and conductances are always shown in siemens. (Some might be negative ― remember that the current flowing into the MOSFET at the drain flows out through the source!) From the View menu, choose SPICE Error Log. In LTspice, this menu option will allow you to view the operating points of every semiconductor device in your simulation. Using the information in this window, find the operating point, ID and VDs. Also find the key small-signal parameters, 9m and ro (which is given in the reciprocal form of drain-source conductance, gas). Record these four values in your report, as you will need them later. Transient analysis This analysis will show how the amplifier responds to its input signal waveform. It uses the full nonlinear model for the transistor, so if your input signal is too large, you will see clipping effects. If you followed the instructions correctly when drawing your circuit, the signal source is already set up for the transient analysis. Return to the Edit Simulation Cmd box, and click on the Transient tab. Choose an input signal of 200mV peak-peak at 1 kHz, which has a period of 1 ms. To see even a rough sinusoidal signal on the output graphs, you will need at least 10 points per cycle; 20 or 50 points would be better. On the other hand, a smaller Maximum Time Step means that the simulation will take longer to run. Choose a suitable Maximum Time Step value. The Stop Time defines for how long the simulation will run (simulation time, not real time!). You probably want to see about 5 cycles of the output signal, so you need to let the simulation run for about 5 ms. Place the transient command neatly on your schematic, near to the DC operating point command. Then run the simulation. You should see a new plot window, which may show one of the signals in your circuit plotted against time. Delete any unwanted trace(s) and plot the signals at the input and output of your amplifier. Is the output as expected? Using the cursors, measure the amplitude of the output signal. Label the cursor position. (To do this, go to Plot Settings, Notes and Annotations, Label Cursor Position.) You already know the amplitude of the input signal, so you can calculate the voltage gain of the amplifier at 1 kHz. Record this result. Add the current in the load resistance to your graph. (To do this, hover the mouse over the load resistance, which will cause the pointer to change to a current-clamp symbol. Then left-click to add the trace to your graph. Note that a new vertical scale appears on the right-hand side.) Measure the amplitude of this current using the cursor, and label this on your plot. Is this consistent with the amplitude of the output voltage? Before you print your graph, make sure to turn on the Print Monochrome option in the file menu. Otherwise, your labels and annotations may not print out. Repeat the transient simulation with an input signal amplitude of 1V. AC analysis This analysis will show the small-signal behaviour of the amplifier over a range of frequencies. If you followed the instructions correctly when drawing your circuit, the signal source is already for the AC analysis. Return to the Edit Simulation Cmd box and choose the AC Analysis set tab. up For the Type of Sweep, select Decade to get a logarithmic frequency axis on the plots. Type the number of points you want in each decade (a factor of 10 in frequency)-20 points per decade will give an acceptable plot, but about 50 would be preferable. Then set the range of frequency you want to examine, up to at least 10 MHz. Note that M or MHz will be interpreted as mHz (milli-hertz); if you want megahertz you must type MEG! Run the simulation. You should see a graph of the magnitude (left y-axis scale) and phase (right y-axis scale) of the input and output signals, plotted against frequency. As the input signal amplitude is set to 1 for AC Analysis, the output signal is simply the small-signal gain. You can delete the input signal trace. The gain is shown in decibels, or dB. Left click the left axis and change the representation from Decibel to Linear. Use the cursor to measure the magnitude of the output signal at 1 kHz—this will give the gain at 1 kHz. Label the cursor position. Is this consistent with the results of the transient analysis? Return the magnitude scale to decibels. What is the mid-frequency gain in decibels? You may notice that the gain of the amplifier falls at low frequencies. Use the cursor to find at what frequency the gain has fallen by 3 dB (a factor of 1/√2) from its mid-frequency value. Note this value in your report. This reduction in gain is due to one (or more) of the capacitors in the circuit―remember that the reactance of a capacitor increases as the frequency decreases, and note that two of the capacitors are in series with the signal path through the amplifier. To see what is happening, add the gate voltage and the source voltage traces to the plot. Which capacitor do you think is responsible for the fall in gain at low frequencies? Do you see a reduction in gain at high frequencies as well? If not, increase the maximum simulation frequency to 100 MHz. You should now see the gain drop off at high frequencies. What causes this reduction in gain? B3 Comparison Compare the measured and resimulated data. Explain any differences you observe. 5/n