6 002 Demo 11 11A 11B Load Set up demo 11 set Load Set up demo 11A set Load Set up demo 11B set Gate Delay Agarwal Fall 00 Lectures 12 and 13 Purpose This demo examines gate delay using two cascaded MOSFET inverters and also an RC circuit model A square wave input is applied to the first inverter and the three relevant waveforms input middle output are shown on the scope By increasing the frequency of the input the gate delay becomes more pronounced relative to the square wave period The middle waveform exhibits the usual decaying exponential response resulting in a delayed reaction by the second inverter The second portion of this demo shows the response of an RC LPF circuit to a square wave input This is used to motivate and justify the gate to source capacitance model of the MOSFET Steps 1 With the square wave input to the cascaded inverters set to a low frequency 100 Hz observe the input the middle waveform between the two inverters and the output on the scope Note that the signals are all consistent with the S or SR models of the MOSFET 2 With the square wave input set to a higher frequency 3 kHz observe again the three waveforms on the scope Note the slow rise of the middle waveform and the corresponding delay in the output transition and the subsequent change in duty cycle 3 Switch to the RC circuit driven by a square wave input Note the decaying exponential response This motivates the gate to source capacitance model for the MOSFET Note it might be desirable to show the capacitor current using the voltage across the resistor 4 Input Slow rise middle waveform delayed ouput Square wave and RC circuit output Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY Description Gate Delay 1 First set FG1 100 HZ square wave Amplitude 5 v p p offset 2 5 v p p observe the 3 waveforms CH1 CH2 and CH3 and see no delays 2 Change frequency FG1 from 100 HZ to 3 KHZ amplitude and offset remain the same Go to load setup and load Demo 11A observe the 3 waveforms again CH1 CH2 and CH3 and see the delays 3 Change the switch on the board to RC CKT set FG2 frequency 1 KHZ square wave Amplitude 5 v p p offset 2 5 v p p Go to load setup and load Demo 11B observe CH1 and CH4 waveforms Note FG1 and FG2 should be set HiZ See schematic diagrams and waveforms on the next for more info and pins out Also see Fg1 and Fg2 on the next page This demo we use power supply to charge the capacitor and discharge it thru a bulb Oscilloscope Setup CH V DIV OFFSET MODE FUNC 1 on 5 10 57 DC off 2 on 5 4 78 DC off 3 on 5 DC off 15 4 4 off MATH VERTICAL HORIZONTAL off Horizontal 1 m Acquisition Trigger CH4 Waveform Generator Setup Power Supply Setup UNIT WAVE AMP FG1 Square 5 2 5 500 HZ 5 2 5 1 KHZ FG2 Square OFFSET FREQ 6 25 0 5 25 OUTPUT 0 on Trigger INT Note For Prof Agarwal load Demo 11AA set also he changed FG1 freq to 500 HZ Note Demo 11C to show only input and output gate waveform is hidden but can be Ckicked to show Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY
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