OCR Electronics for A2 MOSFETs Variable resistors

Transcription

1 Resistance characteristic You are going to find out how the drain-source resistance R d of a MOSFET depends on its gate-source voltage V gs when the drain-source voltage V ds is very small. 1 Assemble the circuit chunk shown opposite. The drain should not be connected to anything at this stage. 2 Use a voltmeter to verify that the value of V gs can be varied from 0 V to 3.5 V by rotating the potentiometer shaft. 3 Assemble this voltage divider and op-amp with negative feedback. If all is well, the voltage at V out will be 50 mv. 4 Now connect the drain and source of the MOSFET as a pull-down resistor R d for the inverting input of the op-amp, as shown below. 5 Record the values of V gs required to set the values of V out shown in the table. V out V gs R d 3.20 V 1.60 V 0.80 V 0.40 V 0.20 V 0.10 V 6 The circuit above is a non-inverting amplifier, with a fixed input signal of 50 mv and a variable pull-down resistor R d made by the MOSFET. Calculate the value of R d with this formula. V V out in R f = 1 + where V in = 0.05 V and R f = 4700 Ω R d Michael Brimicombe 2009 page 1

2 7 Use your results to plot a graph of R d against V gs on the axes opposite. 8 The circuit on your breadboard is essentially an amplifier whose gain G is controlled by the voltage at the gate. Transfer the results from the table on the previous page and calculate the gain for each value of V gs. Then plot the results on the axes below. V out V in V gs G 3.20 V V V V V V 0.05 Michael Brimicombe 2009 page 2

3 Musical MOSFETs You are going to use your MOSFET to make the voltage-controlled volume control in the musical instrument whose block diagram is shown below. 1 Start off by assembling the relaxation oscillator shown opposite. Run the i.c. off supply rails at +5 V and - 5 V (instead of the more usual 0 V.) Note the use of an LDR as feedback resistor, so that the frequency of the signal at S is determined by the illumination of the circuit. 2 Use an oscilloscope to verify that the signal at S is a triangle wave oscillating between about +10 mv and -10 mv. The frequency should decrease as you shade the LDR with your hand. 3 Add the volume control circuit shown opposite. The voltage at G determines how much of the signal at S gets through to T. Use the potentiometer to set G such that shading the LDR with your hand dramatically reduces the gain of the system from 1 to 0. 4 Add the inverting amplifier shown below. Verify that its output signal has an amplitude of up to 2 V, depending on the illumination of the volume control. 5 Finally, use the op-amps in a L272M i.c. to construct a power amplifier so that you can listen to the output of your system. You should be able to alter the pitch with one hand and the volume with the other. Michael Brimicombe 2009 page 3

4 Analogue switches You are going to compare the transfer characteristic of an analogue switch made from a single MOSFET with that of the switches in a 4066 i.c. 1 Assemble the circuit shown opposite. 2 Use a voltmeter to set D to 0 V by adjusting the potentiometer. 3 Connect the voltmeter to S. If all is well, S should be at -5 V when the switch C is open and 0 V when it is closed. 4 Complete the tables below. G D S G D S +5 V +5.0 V -5 V +5.0 V +5 V +4.0 V -5 V +4.0 V +5 V +3.0 V -5 V +3.0 V +5 V +2.0 V -5 V +2.0 V +5 V +1.0 V -5 V +1.0 V +5 V 0.0 V -5 V 0.0 V +5 V -1.0 V -5 V -1.0 V +5 V -2.0 V -5 V -2.0 V +5 V -3.0 V -5 V -3.0 V +5 V -4.0 V -5 V -4.0 V +5 V -5.0 V -5 V -5.0 V 5 On the axes opposite, sketch a graph to show the transfer characteristic of the analogue switch when the gate is held at +5 V. Label the graph open. 6 Sketch another graph to show the transfer characteristic when the gate is held at -5 V. Label the graph closed. Michael Brimicombe 2009 page 4

5 7 Assemble the circuit shown opposite. Use one of the analogue switches in a 4066 i.c. Run the i.c. off supply rails at +5 V and -5 V. 8 Complete the tables below. G X Y G X Y +5 V +5.0 V -5 V +5.0 V +5 V +4.0 V -5 V +4.0 V +5 V +3.0 V -5 V +3.0 V +5 V +2.0 V -5 V +2.0 V +5 V +1.0 V -5 V +1.0 V +5 V 0.0 V -5 V 0.0 V +5 V -1.0 V -5 V -1.0 V +5 V -2.0 V -5 V -2.0 V +5 V -3.0 V -5 V -3.0 V +5 V -4.0 V -5 V -4.0 V +5 V -5.0 V -5 V -5.0 V 9 On the axes opposite, sketch a graph to show the transfer characteristic of the analogue switch when the gate is held at +5 V. Label the graph open. 10 Sketch another graph to show the transfer characteristic when the gate is held at -5 V. Label the graph closed. Michael Brimicombe 2009 page 5

6 Digital to analogue conversion You are going to use the analogue switches in a 4051 multiplexer i.c. to design a two-bit digital-toanalogue converter. Each different two-bit word EF at the inputs results in a different voltage at the output H, as shown in the table below. input word EF voltage at H V 00 0 V 11-1 V 10-2 V 1 Determine the values of resistors X and Y required in the resistor ladder. You should be able to make them from combinations of 10 kω and 22 kω resistors. 2 Assemble the resistor ladder. Use a voltmeter to test it. 3 Add the multiplexer shown below. Use a 4051 i.c. with C held low and pin 7 at -5 V. Connect the Q n inputs to appropriate points on the resistor ladder. 5 Add a couple of push switches and pull-down resistors (as shown above) to control the multiplexer. Verify that the whole system behaves as required. 6 If you have time, expand the system so that it converts three-bit words as shown below. input output +3 V +2 V +1 V 0 V -1 V -2 V -3 V -4 V Michael Brimicombe 2009 page 6