Single Stage Transistor Amplifiers

1. SingleStageTransistor Amplifiers

2. Introduction

3. When only one transistor with associated circuitry is used for amplifying a weak signal, it is called “single stage transistor amplifier”. Although a weak amplifier consists of a number stages, yet such a complex circuit can be conveniently split up into separate single stage. By analyzing carefully only a single stage and using this single stage repeatedly, we can effectively analyze the complex circuit. Therefore, single stage amplifier analysis is of great value in understanding the practical amplifiers circuits.

4. Circuits Elements of Transistor Amplifiers

5. A transistor can accomplish faithful amplification only if proper associated circuitry is used with it. The various circuit elements of a single stage transistor amplifiers are. . .

6. (i) A biasing circuit which establish the proper operating point. This ensures faithful amplification of the signal. Generally potential divider method of bias is employed for this purpose.

7. (ii) An inputcapacitorCin which couples the signal to the base of the transistor. If its not used, the signal source resistance will change the bias conditions of the circuit. This capacitor only allows a.c. signal to flow but isolates the signal source from the biasing circuit.

8. (iii) A coupling capacitor Cc which couples one stage of amplification to the next. If it not used, the bias conditions of the next stage will be drastically changed. This capacitor isolates the D.C. of one stage from the next but allows the passage of A.C. signal.

9. (iv) An emitter by pass capacitor CE in parallel with emitter resistance RE . This capacitor provides a low reactnce path to the amplified A.C. Signal. If it not used, then amplified A.C. signal flowing through RE will cause a voltage drop across it, thereby reducing the output volage.

10. D.C & A.C. Equivalent Circuits. In a transistor amplifier, both D.C and Ac. Conditions prevail. The D.C. source set up D.C currents and voltages whereas sources (i.e signal) produces fluctuations in transistor currents and voltages.

11. Therefore. A simple way to analyze the action of a transistor amplifier is to split the analysis into two parts vizd.c. equivalent circuits and A.C. Equivalent circuits

12. (i) In drawing the D.C. equivalent circuit, only D.C. conditions are considered i.e. it is presumed that no signal is applied. For this purpose, all A.C. sources are reduced to zero and all the capacitors are considered open. If we carefully apply these steps to a transistor amplifier, we shall get the D.C. equivalent circuit.

13. (ii) In drawing A.C. equivalent circuit, only A.C. conditions are considered. For this purpose, all D.C. sources are reduced to zero and capacitors are considered short.

14. LOAD LINE ANALYSIS. The output characteristics are determined experimentally and indicate the relation between VCE and IC graphically . It is well know that relation between VCE and IC linear so that it can be represented by a straight line on the output characteristics.

15. This is known as “load-line”. As in a transistor both D.C. and A.C. Conditions exist, therefore, there are two types of load lines namely; D.C Load Line and A.C. Load Line. The former determines the locus of IC and VCE in the zero signal conditions while the later shows these values when signal is applied.

16. Voltage Gain. The voltage gain of an amplifier is the ratio of output voltage to the input voltage i.e.

17. Voltage = Output Voltage SignalFor CE amplifier, it can be easily proved that:Voltage gain= b x RACRIN

18. where: RAC = effective collector load RIN = input resistance of the transistor

19. Power Gain. The ratio of power output to input power of an amplifier is called power gain i.e.

20. Power Gain = Output Power Input PowerFor CE amplifier, it can be proved that:Voltage gain= b2x RACRin