1. Chapter 3
Diode Circuits

2. INTRODUCTION
In the field of electronics, the simplest and the most fundamental non-linear circuit element is the diode.
The p–n junction diode is considered to be a circuit element. For easy and lucid evaluation of the diode element, the concept of load line is extremely important.
Among the many applications of diodes, their use in the design of rectifiers, which convert ac to dc, is the most common.
The piecewise linear model is used in certain applications of diodes, namely clippers, rectifiers and comparators.
Many more such circuits are possible with one or more diodes being implemented in them.

3. ANALYSIS OF DIODE CIRCUITS
The basic diode circuit consists of a voltage source in series with a resistor and a diode.
The circuit might be analysed properly to obtain the instantaneous current and diode voltage.
For such an analysis to be done, the concept of load line and its effective use in various circuits has to be thoroughly understood.
The concept of load line is absolutely essential.

4. LOAD LINE AND Q-POINT
The applied load will normally have an impact on the region (or point) of operation of a device. If the analysis is performed in a graphical manner, a line can be drawn on the characteristics of the device to represent the applied load.
The intersection of the load line with the characteristics will determine the point of operation of the system. Physically, this point of operation mainly determines the conditions under which the device is to be operated in a circuit.
This case takes care of the various intriguing attributes of the circuit. This kind of an analysis is known as the load-line analysis.
Analysis of a basic diode circuit

5. LOAD LINE AND Q-POINT
Under normal conditions, this resistance is approximately 10 Ω. Applying Kirchoff’s voltage law (KVL) of circuit theory, to the series, we obtain:
If VD = 0 V, we can calculate ID and plot the magnitude of ID on the vertical axis.
If ID = 0 A, we can calculate VD and plot the magnitude of VD on the horizontal axis.
I-V characteristics of the diode

6. The load line on the characteristics of the diode
LOAD LINE AND Q-POINT
A straight line drawn between two points will define the load line.
If the value of R is changed, the intersection on the vertical axis will change. This affects the slope of the load line, and gives a different point of intersection between the load line and the device characteristics.
The point of intersection between the device characteristics and the load line (VD0 , ID0 ) is called the point of operation or the quiescent point (Q-point) as defined by a dc network.
Illustration of a load line when: (1) voltage is constant and resistance varies (2) voltage varies and the resistance is constant
The load line on the characteristics of the diode

7. Change of Q-point with changes in supply voltage and load
LOAD LINE AND Q-POINT
Change of Q-point with changes in supply voltage and load
Load line

8. RECTIFIERS Full-wave Rectifier
The full-wave rectifier can be classified into two distinct types.
(i) Centre-tapped transformer full-wave rectifier:- It comprises of two half-wave circuits, connected in such a manner that conduction takes place through one diode during one half of the power cycle and through the other diode during the second half of the cycle.
Full-wave rectifier

9. CLIPPER AND CLAMPER CIRCUITS
A clipper is a type of diode network that has the ability to “clip off” a portion of the input signal without distorting the remaining part of the alternating waveform.
The half-wave rectifier is an example of the simplest form of diode clipper—one resistor and a diode.
Depending on the orientation of the diode, the positive or negative region of the input signal is “clipped” off.
There are two general categories of clippers: series and parallel.
Series clipper:- A series clipper and its response for two types of alternating waveforms are provided.
Series clipper circuit
Response of clipper circuit

10. CLIPPER AND CLAMPER CIRCUITS
Key points
The first step is to find out in which interval of the input signal the diode is in forward-bias.
The direction of the diode suggests that the signal vi must be positive to turn it on. The dc supply further requires the voltage vi to be greater than v volts to turn the diode on.The negative region of the input signal turns the diode into the OFF state. Therefore, in the negative region the diode is an open circuit.
Series clipper with a dc supply

11. Determining the transition level of the input signal
CLIPPER AND CLAMPER CIRCUITS
Determine the applied voltage (transition voltage) that will cause a change in state for the diode. For the ideal diode the transition between states will occur at that point on the characteristics where vd = 0 V and id = 0 A. Applying this condition, it is recognized that the level of vi that will cause a transition in state is:
vi = V
For an input voltage greater than V volts, the diode is in the short-circuit state, while for input voltage less than V volts it is in the open-circuit or OFF state (as it is reverse-biased).
Determining the transition level of the input signal
Determining vo in the clipper circuit

12. CLIPPER AND CLAMPER CIRCUITS
Be continually aware of the defined terminals and polarity of vo. When the diode is in the short-circuit state, the output voltage vo can be determined by applying KVL in the clock-wise direction:
It can be helpful to sketch the input signal above the output and determine the output at instantaneous values of the input. It is then possible to sketch the output voltage from the resulting data points of vo.
Determining levels of vo

13. CLIPPER AND CLAMPER CIRCUITS
For Vm > V, the diode is in the short-circuit state and vo = Vm – V. At vi = V, the diode changes state and vi = – Vm, vo = 0 V. The complete curve for vo can be sketched.
Determining vo when vi Vm
Sketch for vo

14. CLIPPER AND CLAMPER CIRCUITS
Parallel clipper:- Input vi is applied for the output vo. The analysis of parallel configuration is very similar to the series configuration.
Parallel clipper
Response of parallel clipper

15. CLIPPER AND CLAMPER CIRCUITS
Break region
There is a discontinuity at the voltage Vγ. Actually the transition of a diode state is not exactly abrupt but gradual.
Thus, a waveform, which is transmitted through the clipper circuit, will not show an abrupt clipping. Instead, it will show a gradual broken region, exhibiting the regions of un-attenuated and attenuated transmission. Now, we will estimate the range of this break region. The output current of a diode is given by:
The incremental diode resistance r = dv/dI is given by:
Again for meaningful clipping to be done, the applied signal must vary from one side of the break point to a point well on the other side.
at diode break point

16. CLIPPER AND CLAMPER CIRCUITS
A clamping network is one that will “clamp” a signal to a different dc level. The network must have a capacitor, a diode, and a resistive element, but it can also employ an independent dc supply to introduce an additional shift.
Before further probing into the clamper circuit one must have a basic understanding of a transient RC circuit.
From the basic understanding of a series RC transient circuit applied across a dc voltage EO, the instantaneous charge across the capacitor at any time is given by
Q0=E0C where, C is the capacitance of the capacitor.
We know that the time constant τ = RC. The rise time becomes smaller if we decrease the time constant.
Charging of a RC circuit

17. CLIPPER AND CLAMPER CIRCUITS
The discharge will occur quickly if the time constant of the circuit is decreased.
The magnitude of R and C must be so chosen that the time constant, τ = RC, is large enough to ensure that the voltage across the capacitor does not discharge significantly during the interval the diode is non-conducting.
Discharging of an RC circuit
The clamping circuit will clamp the input signal to the zero level.
Simple clamper circuit

18. State of the circuit in the negative half-cycle
CLIPPER AND CLAMPER CIRCUITS
During the interval 0 –T/2 the network will appear, with the diode in the ON state effectively “shorting out” the effect of the resistor R.
The resulting RC time constant is so small that the capacitor will charge to V volts very quickly. During this interval the output voltage is directly across the short circuit and vo = 0 V.
The diode will now be in the open-state condition. Applying KVL around the input loop of figure will result in:
The negative sign results from the fact that the polarity of 2 V is opposite to the polarity defined for vo.
For a clamping network the total swing of the output is equal to the total swing of the input.
State of the circuit in the negative half-cycle

19. COMPARATORS
The diode circuit which has been used in the design of the clipping circuit can also be used for the purpose of comparison, hence the name comparator.
The basic principle on which the comparator works is the switching of the diodes.
This action corresponds to the phase when the diode conducts; and when it does, the comparator circuit is used to compare the input arbitrary voltage with the reference voltage.
A diode comparator
Input signal with the threshold voltage
Corresponding output waveform

20. ADDITIONAL DIODE CIRCUITS
Voltage Multiplier
The voltage multiplier is a passive circuit, similar to the rectifier circuit and gives an output which is approximately equal to a certain multiple of the peak value of the peak input voltage.
Here it is possible to obtain a dc voltage equal to the peak value of the applied ac voltage.
Peak Detector
The half-wave rectifier circuit can be suitably manipulated to obtain the peak detector circuit.
The working principle depends on the charging and discharging of the capacitor, and also on the conducting and non-conducting regions of the diode. Its simple circuit consists of a diode kept in series with a resistor and a load at the end.
Peak detector
Voltage doublers

21. ADDITIONAL DIODE CIRCUITS
Digital Circuits
In various kinds of analog to digital converters, and in many digital circuits, diodes are extensively brought to use.
Their primary domain of operation is switching, i.e., to keep a portion of a large circuit in the ON state, selectively for a given interval of time.
They, in coherent action with resistors, form many important logic families, which are used in digital electronic circuits.
Switching Regulators
Power supplies with switching regulators offer great versatility, as the design of power supplies employing this type of regulator can be lighter and more compact.
Another advantage of these power supplies is that the circuit can be designed to give an output voltage that is higher than the unregulated voltage or, has a different polarity.
But there is also a disadvantage. The circuit becomes more complex because of the control circuitry.
The structure of the converter can be of three types: buck, boost and buck–boost.