University Of Nottingham

School of Electrical and Electronic Engineering

Analogue Electronics

Example Sheet 1 - Diodes.



1. Find the current flowing through a diode at room temperature (T=300K) when a forward bias of 0.2 volts is applied across the diode.  Repeat for a reverse bias of 0.2 volts.  Assume n=1 and that the reverse saturation current is 5?A.
[Ans: 11.4mA, 5?A]


2. Calculate the values of the static and the dynamic forward resistance of a junction diode conducting a DC current of 2mA if the reverse saturation current is 2?A.
[Ans: 89.4?, 13?]


3. A diode and a resistor are connected in series across a DC source of 0.2 volts.  If the reverse saturation current is 3?A, find the value of the resistor such that 0.1 volts are dropped across it.
[Ans: 714? or 0.33?]


4. For the circuit below determine the supply voltage if the saturation current of the diode is 2?A.  Determine the supply voltage if the current I is increased to 4mA and a resistor of 30? is connected in series with the anode.




         


[Ans: 219mV, 397mV]


5. Estimate Vo in each of the circuits below, stating any assumptions made.











[Ans: 4.4V, 0.6V]




6. Open file es1q06.sch and set Vs1 to 10V and Vs2 to 5V. Next add a voltage marker at point A and point D and simulate the circuit.  Next, add markers at points B and C and re-simulate the circuit.  Explain why B and C are at the levels they are.  Finally without using PSpice you should easily be able to determine the voltage at A, B, C and D if Vs1=8V and Vs2=3V.











7. A 1V r.m.s. sine wave is applied to the diode bridge as shown.  Sketch the output vo when:
(a) Vs =10V
(b) Vs=-10V


















[Ans: 1V r.m.s. , 0V]


8. Estimate the dynamic resistance of the diode in the circuit shown.  Hence find vo when the small signal input is applied.










[Ans: 2.5?, 25?A]







9. a) The circuit below shows a diode clamping circuit which adds a positive DC level to an AC signal.  Outline its mode of operation, assuming a silicon diode and that RL is a high resistance.
  








b) Open file es1q09.sch and simulate it.  It is currently set so that CRL = 1/f.  Change the values of C and RL and discuss the relationship between these and 1/f to ensure a good clamping action.

     c) Show that if the diode is turned around a negative DC voltage is added to the input signal.


[Tutorial Note: This type of circuit may also be used to restore a DC component to a signal that has been AC (capacitively) coupled.  Capacitive coupling will be introduced in future lectures on transistor amplifiers.]



10.  a) Open file es1q10a.sch.  This shows a positive biased clipper. Change VBB to 3 and 8 volts simulating the circuit each time, and note the effect changing this has on the output.  You should now be able to sketch the output (without using PSpice) for VBB=5.3V

       b) Open file es1q10b.sch.  This shows a negative biased clipper. Change VBB to 3 and 8 volts simulating the circuit each time, and note the effect changing this has on the output.  You should now be able to sketch the output (without using PSpice) for VBB=4.5V



11.  a) The circuit below shows a circuit combining a positive based clipper with a negative based clipper.  Using your knowledge gained in example 9, determine the output waveform for a sinusoidal input voltage of 10 volts, assuming the diodes are silicon ones.











       b) Explain how the circuit produces this waveform.



12.  a) Open file es1q12.sch and simulate it. Next go to the get new part menu and add a smoothing capacitor across the output (starting value=1?C) Increase the capacitance to 10?C and then to 100?C.  You should be able to see, and explain the effect this has.

       b) Measure the ripple on the output waveform for 100?C (using the cursor) and compare this to the value obtained from the formula given in the lecture notes.
1 of 2

Posted in Posted by waytofeed at 9:44 AM