**Q1. If in Fig. 1; R1 = 10 kΩ ; R2 = 2.2 kΩ ; RC = 3.6 kΩ ; RE = 1.1. kΩ and VCC = + 10 V, find the d.c. power drawn from the supply by the amplifier.**

**Solution:**

The current I1 flowing through R1 also flows through R2 ( assumption because IB is small).

D.C. voltage across R2, V2 = I1 R2 = 0.82 mA × 2.2 kΩ = 1.8V

D.C. voltage across RE, VE = V2 – VBE = 1.8V – 0.7V = 1.1V

D.C. emitter current, IE = VE/RE = 1.1V/1.1 kΩ = 1 mA

∴ IC ≅ IE = 1 mA

Total d.c current I_{T }drawn from the supply is;

I_{T} = IC + I1 = 1 mA + 0. 82 mA = 1.82 mA

∴ D.C. power drawn from the supply is

Pdc = V_{CC} I_{T} = 10V × 1.82 mA = 18.2 mW

**Q2. Determine the a.c. load power for the circuit shown in Fig. 2.**

**Solution : **

The reading of a.c. voltmeter is 10.6V. Since a.c. voltmeters read r.m.s. voltage, we have,

**Q3. In an RC coupled power amplifier, the a.c. voltage across load RL (= 100 Ω ) has a peak- to-peak value of 18V. Find the maximum possible a.c. load power.**

**Solution : **

The peak-to-peak voltage, VPP = 18V.

Therefore, peak voltage (or maximum voltage) = VPP/2 and the r.m.s value, VL = VPP/2√2 .

**Q4. A power amplifier operated from 12V battery gives an output of 2W. Find the maximum collector current in the circuit.**

**Solution : **

Let IC be the maximum collector current.

Power = battery voltage × collector current

or 2 = 12 × IC

∴ IC = 2/12 = 1/6 A = 166.7 mA

**Q5. A voltage amplifier operated from a 12 V battery has a collector load of 4 kΩ. Find the maximum collector current in the circuit.**

**Solution : **

**Q6. A power amplifier supplies 50 W to an 8-ohm speaker. Find (i) a.c. output voltage (ii) a.c. output current.**

**Solution : **

**Q7. Calculate the (i) output power (ii) input power and (iii) collector efficiency of the amplifier circuit shown in Fig. 3. It is given that input voltage results in a base current of 10 mA peak.**

**Solution:**

**Fig.3 **

First draw the *d.c. *load line by locating the two end points *viz*.,

*I _{C }*

_{(sat) }=

*V*/

_{CC}*R*= 20 V/20 Ω = 1 A = 1000 mA

_{C}

and *VCE *= *VCC *= 20 V as shown in Fig. 4.

The operating point *Q*of the circuit can be located as under :

**Q8. A power transistor working in class A operation has zero signal power dissipation ****of 10 watts. If the a.c. output power is 4 watts, find : (i) collector efficiency (ii) power rating of transistor.**** **

**Solution :**** **

Zero signal power dissipation, *Pdc *= 10 W

a.c. power output, *Po *= 4 W

** ( ii) **The zero signal power represents the worst case

*i*.

*e*. maximum power dissipation in a transistor occurs under zero signal conditions.

∴ Power rating of transistor = **10 W**

It means to avoid damage, the transistor must have a power rating of atleast 10 W.* *

**Q9. A class A power amplifier has a transformer as the load. If the transformer has ****a turn ratio of 10 and the secondary load is 100 ****Ω, find the maximum a.c. power output. Given that zero signal collector current is 100 mA.**** **

**Solution : **

** **Secondary load, *R _{L} *= 100 Ω

**Q10. A class A transformer coupled power amplifier has zero signal collector current ****of 50 mA. If the collector supply voltage is 5 V, find (i) the maximum a.c. power output (ii) the power rating of transistor (iii) the maximum collector efficiency.**

** ****Solution : **

It means that power rating of the transistor is twice as great as the maximum a.c. output power. For example, if a transistor dissipates 3 W under no signal conditions, then maximum a.c. output power it can deliver is 1.5 W.

**Q11. A power transistor working in class A operation is supplied from a 12-volt ****battery. If the maximum collector current change is 100 mA, find the power transferred to a 5****Ω loudspeaker if it is :**** **

**(i) directly connected in the collector**

**(ii) transformer-coupled for maximum power transference**** **

**Find the turn ratio of the transformer in the second case.**** **

**Solution :**** **

**( i) Loudspeaker directly connected. **Fig.5 shows the circuit of class

*A*power amplifier with loudspeaker directly connected in the collector.

Therefore, when loudspeaker is directly connected in the collector, only 50 mW of power is transferred to the loudspeaker.

**( ii) Loudspeaker transformer coupled. **Fig. 6 shows the class

*A*power amplifier with speaker transformer coupled. As stated before, for impedance matching, step-down transformer is used.

**Q12. A common emitter class A transistor power amplifier uses a transistor with ****β =100.****The load has a resistance of 81.6 ****Ω, which is transformer coupled to the collector circuit. If the peak values of collector voltage and current are 30 V and 35 mA respectively and the corresponding minimum values are 5 V and 1 mA respectively, determine :**

**(i) the approximate value of zero signal collector current**

**(ii) the zero signal base current**

**(iii) Pdc and Pac (iv) collector efficiency (v) turn ratio of the transformer.**

** ****Solution:**** **

In an ideal case, the minimum values of *vCE *(*min*) and *iC *(*min*) are zero. However, in actual practice, such ideal conditions cannot be realised. In the given problem, these minimum values are 5 V and 1mA respectively as shown in Fig. 7.** **

**Q13. For a class B amplifier using a supply of VCC = 12V and driving a load of 8Ω, determine (i) maximum load power (ii) d.c. input power (iii) collector efficiency***.*

Q14. **A class B push-pull amplifier with transformer coupled load uses two transistors rated 10 W each. What is the maximum power output one can obtain at the load from the circuit?**

**Solution :**** **

The power dissipation by each transistor is *PT *= 10W. Therefore, power dissipated by two transistors is

* P*_{2T} = 2 × 10 = 20W.

** ****Q15. A class B amplifier has an efficiency of 60% and each transistor has a rating ****of 2.5W. Find the a.c. output power and d.c. input power.**

**Solution :**

The power dissipated by each transistor is *PT *= 2.5W.

Therefore, power dissipated by the two transistors is *P*2*T *= 2×2.5 = 5W.