UNIT-1: TRANSISTORS, UJTs, AND THYRISTORS

1. With neat figure, explain the construction and operational principle of an Unijunction

Transistor (UJT). (10 marks) (July 2013)

2. Find the values of resistors Rb, Rc, Re and the transistor gain β, for the circuit. Ib=40µA,

Ic=4mA, Ve=2V, Vce=12V, Vcc=15V. Assume that the transistor used in the circuit is a silicon

transistor. (10 marks) (July 2013)

3. Sketch and explain with the circuit, the combination clippers which limit the output between ±5V.Assume diode voltage is 0.7V. (10 marks) (Dec 2013)

There are a variety of diode networks called clippers that have 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

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. The series

configuration is defined as one where the diode is in series with the load, while the parallel variety

has the diode in a branch parallel to the load.

4) Find the values of resistance RB, RC, RE and transistor gain β, for the circuit shown in

Fig. Q1.c Given that IB = 40 μA, IC = 4mA, VE = 2V, VCE = 12V and supply voltage VCC

= 15V. Assume the transistor used in circuit is a silicon transistor. (6 marks) (Dec 2013)

5) Explain the criteria for selecting a suitable operating point and factors affecting the stability.

(10 marks) (June 2014)

We know that the load line is constructed on the output characteristics joining (0, VCC/RC) & (VCC, 0)

the effect of variation in IB on the Q-point is shown in fig (1).

If the value of IB is increased by decreasing the value of base substance RB. The Q-point moves up the

load line gradually towards saturation. Now, if VCC is kept constant & if collector resistance RC is

increased.

Keeping base current IB fixed, the Q - point moves to the left into saturation as shown in fig (2).

Fig (2) Effect of variation in collecter resistance on Q-point.

If we now keep RC fixed while decreasing VCC, the Q-point once again moves towards the left into

saturation as shown in fig (3).

6) What is Clamping? With neat diagram and waveform, explain the working of negative clamper

and also write the condition for stiff clamper. (7 marks) (June 2014)

The 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. The magnitude of Rand C must be chosen such 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 nonconducting. Throughout the analysis we will assume that for all practical purposes the

capacitor will fully charge or discharge in five time constants.

There are two basic types of clampers:

1. A positive clamper shifts its input waveform in a positive direction, so that it lies above a dc

reference voltage. For example, the positive clamper shifts the input waveform so that it lies above 0 V

(the dc reference voltage).

2. A negative clamper shifts its input waveform in a negative direction, so that it lies below a dc

reference voltage. The direction of the diode determines whether the circuit is a positive or negative

clamper. Clamper operation is based on the concept of switching time constants.

Circuit Diagram Input Output

7. Explain Varactor diode with its characteristic curves. (3 marks) (June 2014) Varactor [also called varicap, VVC (voltage-variable capacitance), or tuning] diodes are semiconductor,

voltage-dependent, variable capacitors. Their mode of operation depends on the capacitance that exists at

the p-n junction when the element is reverse biased. Under reverse-bias conditions, it was established that

there is a region of uncovered charge on either side of the junction that together the regions make up the

depletion region and define the depletion width Wd.

As the reverse-bias potential increases, the width of the depletion region increases, which in turn reduces

the transition capacitance. The characteristics of an typical commercially available varicap diode in figure

above. Note the initial sharp decline in CT with increase in reverse bias. The range of VR for VVC diodes

is limited to about 20 V. When reverse voltage

across a varactor diode is increased, the width Wd of the depletion layer increases. Therefore, the total

junction capacitance CT of the junction decreases. On the other hand, if the reverse voltage across the

diode is lowered, the width Wd of the depletion layer decreases. Consequently, the total junction

capacitance CT increases.

8. Explain how transistor can be used as switch (5 marks) (Dec 2013/ Dec 2014 2015)

Saturation region, - switch closed, Cut-off region – switch open, with Explanation of Propagation delay, Timing state diagram.

Solid state switches are one of the main applications for the use of Transistors. In the application of transistor as a switching element, the device is operated alternately between the cut-off condition and the saturation condition the transistor is allowed to make a transition from cut-off to saturation state and vice-versa by using a pulse wave form. Consider Cut-off characteristics

• The input and base are grounded (0V) , Base-emitter voltage VBB< 0.7V , Base-emitter

junction is reverse biased.

• Base-collector junction in reverse biased. , Transistor is fully - off (Cut-off region)

• No collector current flows (IC = 0) , Vout= VCE = VCC = “1”

• Transistor operator as an “opens switch” .

Saturation characteristics

• The input and base are connected to VCC , Base-emitter voltage VBE< 0.7V

• Base-emitter junction is forward biased, Transistor is fully - on (saturation region)

• Max collector current flows (IC = VCC/RL) VCE = 0 (ideal saturation)

• Vout= VCCE = “0”, Transistor operates as a closed switch

• Base-collector junction is forward biased.

9. Determine the value of the resistors Re and Rc for the circuit shown in figure given that

R1=5K Ω, R2=1K Ω, β=200, Vceq=5v and Iceq=2mA for the silicon made transistor.

(8 marks) (Dec 2014 2015)

Applying KVL to Collector Emitter loop, Vcc-IcRc-Vce-IeRe = 0 as Ie~=Ic,

12 - 2e-3*(Rc+Re) – 5 = 0 Rc + Re = 3.5Kohm

Consider Approx. method as I1> Ib Vb= VccR2 / R1+R2 = 2V. Ve = Vb – Vbe

Re = 0.65Kohm and Rc = 2.85Kohm

10. Briefly discuss the working operation of silicon controlled rectifier.

(7 marks) (Dec 2014 2015)

The four-layer construction in Silicon Controlled Rectifier or SCR is shown below. We ground both

the cathode and the gate, and apply a positive voltage to the anode; no current will flow through this

device. This device acts as a switch that is cheaper than a relay and is able to handle the large power

dissipation that we are expecting. The rectifier circuit (anode-cathode) a low forward resistance and a

high reverse resistance. Controlled from an off state (high resistance) to the on state (low resistance)

by a signal applied to the third terminal, the gate. Once it is turned ON it remains even after removal

of the gate signal, as long as a minimum current, the holding current, is maintained in the main or

rectifier circuit.

Figure: SCR Schematic Symbol 11. With the help of circuit diagram, explain the accurate method of voltage divider bias

(8 marks) (June 2015)

This method is always valid must be used when R2 > .1 β RE

Perform Thevenin’s Theorem

Open the base lead of the transistor and the Voltage Divider bias circuit is:

Calculate RTH We may apply KVL to the

input, which gives us:

12. For the emitter bias network determine: (4marks) (June 2015)

i) Ib ii) Ic iii) Vce iv) Vc v) Ve vi) Vb vii) Vbc viii) i/p resistance

This circuit differs from base-bias circuit in two important aspects. First, it uses two separate d.c.

voltage sources; one positive (+ VCC) and the other negative (– VEE). Normally, the two supply

voltages will be equal. For example, if VCC = + 20V (d.c.), then VEE = – 20V (d.c.). Secondly, there

is a resistor RE in the emitter circuit.

13. Explain the gate characteristics of SCR (8marks) (June 2015)

Figure: SCR Characteristics

It is the curve between anode-cathode voltage (V) and anode current (I) of an SCR at constant gate

current. Fig. 20.7 shows the V-I characteristics of a typical SCR.

(i) Forward characteristics. When anode is positive w.r.t. cathode, the curve between V and I is

called the forward characteristic. In Fig. 20.7, OABC is the forward characteristic of SCR at IG = 0. If

the supply voltage is increased from zero, a point is reached (point A) when the SCR starts

conducting. Under this condition, the voltage across SCR suddenly drops as shown by dotted curve

AB and most of supply voltage appears across the load resistance RL. If proper gate current is made to

flow, SCR can close at much smaller supply voltage.

(ii) Reverse characteristics. When anode is negative w.r.t. cathode, the curve between V and I is

known as reverse characteristic. The reverse voltage does come across SCR when it is operated with

a.c. supply. If the reverse voltage is gradually increased, at first the anode current remains small ( i.e.

leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts

conducting heavily in the reverse direction as shown by the curve DE. This maximum reverse voltage

at which SCR starts conducting heavily is known as reverse breakdown voltage.