Multistage amplifier

transistor amplifier a part of multistage amplifier electronics class -12

1. MULTISTAGE AMPLIFIER INTRODUCTION: Generally ,voltage gain or power gain obtained from a single stage amplifier is less. Therefore in order to achieve greater voltage and power gain ,we have to use more than one stage of amplification. A circuit containing more than one stage of amplification is called multistage amplifier.

2. A V = Where, A V1 = V 1 V i V 1 A V2 = V 2 A V3 = V 3 V 2 X X X X Over all the voltage gain of a multistage amplifier is equal to the product of the gain of the individual stages.

3. Gain in decibel(dB) The decibel is a logarithmic measurement of the ratio of one power to another or one voltage to another. Usually ,the voltage gain or power gain of an amplifier is represented in decibels. Power gain in dB=10 log 10 A P 10 10

4. Note : 1dB= 1 10 th bel Power gain in dB Voltage gain in dB Current gain in dB 1dB = log P o We know that, P i Hence , Power gain in dB=10 log 10 A P G p = Current gain in dB=20 log 10 A i G v = G ì =

5. Why do we measure gain in dB? It allows us to denote, both very small as well as very large quantities of linear scale. The human ear responds to the intensity of sound which is used by logarithmic scale. Note : The overall voltage gain of a multistage amplifier in decibels is given by

6. 2 . 3.

7. Types of coupling amplifiers: RC coupling amplifier. Direct coupling amplifier. Transformer coupling amplifier.

8. Advantages of RC coupling: It is inexpensive. It has good frequency response. It has low distortion. Overall gain is high. Disadvantages of RC coupling: The gain is reduced because of loading effect. It cannot amplify very low frequency signal. Power loss due to large number of resistors. Applications: Used as a voltage amplifiers such as radio and TV receivers.

9. This amplifier is also called as DC amplifier and is used to amplify very low frequencies including Direct current or zero frequency. Two stage direct coupled amplifier:

10. The output of first stage is directly connected to the base of the next transistor. Moreover ,there is no input and output coupling capacitors. Required DC conditions of the first stage are obtained from R1 and R2 [along with Vcc]. The DC voltage of first stage provides the required DC bias for the second stage. The signal to be amplified is applied directly to the input of the first stage. Due to transistor action, it appears in the amplified form across the collector resistor of transistor T1. This voltage then drives the base of the second transistor T Advantages: 1. The circuit arrangement is very simple. 2. The circuit cost is less. 3. It can amplify very low frequencies [even zero frequency]. Disadvantages: 1. It cannot amplify high frequency signals. 2. It has poor temperature stability. Working: Applications: Used in electrocardiogram, computers and instrumentation amplifiers. Regulator circuits.

11. Capacitive reactance is defined as a opposition offered by the capacitor to the flow of alternating current. It is denoted by Coupling capacitance C C : couples the Ac signal to next stage.

12. Frequency response of RC coupled amplifier The curve drawn between voltage gain and the signal  frequency  of an  amplifier  is known as  frequency response . 

13. At low frequency : C C offers large reactance. Hence, gain decreases with decrease in frequencies.

14. At high frequency: C C offers very low reactance. loading effect increases voltage gain decreases

15. At mid frequency: As frequency increases, X C decreases, A V increases and also at the same time, decrease in X C increases the loading effect which decreases the gain A V .

16. LOW FREQUENCY RANGE MID FREQUENCY RANGE HIGH FREQUENCY RANGE At low frequency, C C offers large reactance. Hence, gain decreases with decrease in frequencies. At mid frequency, As frequency increases, X C decreases, A V increases and also at the same time, decrease in X C increases the loading effect which reduces the gain A V . Therefore, gain remains almost constant. At high frequency, C C offers very low reactance. therefore loading effect increases which reduces the voltage gain.

17. Frequency response of direct coupled amplifier