Time Base Circuit

ECE 322L: Microelectronics Lab Lab #3: Diode Recovery Time and Clipping Circuits Objectives: Understand the concept of reverse recovery time of a diode. Also learn how to design clipping circuits using diodes. Materials: Power supply Function generator Oscilloscope Solderless breadboard Hookup wire One 100Ω resistor Two 1N4004 diodes One 1N4148 diode Setup: Build the circuit shown below. Build the circuit initially using a 1N4004 diode.

id + vi(t) -

+ 100

vo(t) -

Background: Reverse Recovery Time: The reverse recovery time, trr, of a diode is a measure of how quickly a diode can switch from the forward biased to the reverse biased state. This value is important in the rectification of high frequency signals. It was previously stated that a reversed biased diode will allow little current to flow. However, if the diode is strongly forward biased, then reverse biased, it will take a short time for the accumulated charge carriers to be removed from the junction region allowing the diode to shut off the current. While these charge carriers are being removed, current will flow in the reverse bias direction. The circuit built in "Setup" above will be used for the measurement of the reverse recovery time of the diode(s). The typical diode current response for an input square wave voltage is shown in the figure below. The time ts (called the storage time) is the time required for the removal of the charge carriers from the junction region, while the time ta is the time for the diode to switch from on to off. Note that during the time ts a significant amount of reverse bias current does flow, and for the time ta after this, an exponentially decreasing amount flows until the diode is almost completely turned off. The total time ts + ta is defined as the reverse recovery time, trr. We will attempt to measure the reverse recovery time for two silicon diodes, the second of which is specially made for fast switching applications.

0

time

-IR ta

ts trr

Clipping Circuits: The ideal diode model predicts that the circuit below can not support an output voltage Vo(t) above 3.5V. If the input voltage Vi(t) exceeds 3.5V, then the diode becomes forward biased with zero voltage drop across the diode (VD = 0) and the output voltage saturates at 3.5V with the remaining voltage dropped across the resistor. If the input voltage is less than 3.5V, then the diode is reverse biased, diode and resistor currents are zero, and the resistor's voltage must therefore also be zero. In this case Vo(t) = Vi(t). The result is a circuit that "clips" the output voltage at 3.5V: ⎧ 3.5, Vi (t ) ≥ 3.5 Vo (t ) = ⎨ ⎩Vi (t ), Vi (t ) < 3.5

Switching the anode and cathode of the diode (so the diode points "up") results in a circuit that clips voltages below the 3.5V threshold (rather than above the threshold as the clipping circuit shown above does): ⎧ 3.5, Vi (t ) ≤ 3.5 Vo (t ) = ⎨ ⎩Vi (t ), Vi (t ) > 3.5

The threshold can be adjusted but changing the level of the voltage supply. It is also possible to put two diode/supplies in parallel. If this is done, then there is both an upper and a lower bound on output voltages (assuming that the voltage associated with the "down" diode is larger than the voltage associated with the "up" diode). Keep in mind that either threshold voltage could be negative. Lab assignment: 1) Using the circuit in "Setup" with a 1N4004 diode, make the input a 10 Vpp, 100 Hz square wave. Observe on the oscilloscope simultaneously the waveforms vi(t) and vo(t). Note that the reverse recovery time cannot be readily observed since it is significantly shorter than the 10 ms period of the square wave. Now begin increasing the frequency and adjusting the oscilloscope time base until the frequency is such that the reverse recovery time becomes readily measurable. Record the reverse recovery time for the diode and plot the waveforms of vi(t) and the diode current (vo(t)/100).

2) Replace the 1N4004 diode in the above circuit with a 1N4148 diode and attempt to repeat the reverse recovery time measurement. What do you observe? 3) Construct the circuit shown above in the "Clipping Circuits" section. Obtain simultaneous displays of vi(t) and vo(t) vs. time. Use the function generator to provide an input of 10 Vpp, 100 Hz sinusoid. Plot the oscilloscope displays, and from your plots determine the graphical transfer function (a plot of vo vs. vi). 4) Design and construct a circuit that will clip all voltages above +2V and below -3V. Test your circuit using a 10 Vpp, 100 Hz sinusoid. As always, make sure all plots have axes labeled (including units).