Single stage Amplifier 1 Basic BJT Amplifiers Circuits

Single stage Amplifier 1

Basic BJT Amplifiers Circuits Single-Stage BJT Amplifiers To operate as an amplifier, the BJT must be biased to operate in active mode and then superimpose a small voltage signal vbe to the base. DC + small signal coupling capacitor (only passes ac signals)

Thermal Runaway 3

How to avoid thermal runaway n By using Heat sink n By providing proper circuit which reduces current when temperature increases 4

Transistor As a switch

Transistor As a switch n Two modes of transistor are used for the operation of transistor as switch. n Saturation mode – transistor act as on Switch n Cut-off mode – transistor act as off Switch n By controlling IB current we can use transistor as a switch. 6

Transistor biasing techniques n Base bias n Voltage divider bias n Emitter bias with feedback 7

Voltage-divider bias characteristics. Circuit recognition: The voltage divider in the base circuit. Advantages: The circuit Qpoint values are stable against changes in h. FE. Disadvantages: Requires more components than most other biasing circuits. Applications: Used primarily to bias linear amplifier. 8

MULTISTAGE AMPLIFIERS

Multistage Amplifiers Two or more amplifiers can be connected to increase the gain of an ac signal. The overall gain can be calculated by simply multiplying each gain together. A’v = Av 1 Av 2 Av 3 ……

Need of Multistage Amplifier n Many applications cannot be handle with single- transistor amplifiers in order to meet the specification of a given amplification factor, input resistance and output resistance n As a solution – transistor amplifier circuits can be connected in series or cascaded amplifiers n This can be done either to increase the overall small- signal voltage gain or provide an overall voltage gain greater than 1 with a very low output resistance

Block diagram of Multistage amplifier 12

Types of Multistage Amplifiers Depending up on type of Coupling used types of multistage amplifiers are: 1. Direct Coupled Amplifer 2. RC Coupled Amplifier 3. Transformer Coupled Amplifier

Direct Coupled Amplifier

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Transformer Coupled Amplifier 16

Frequency Response of TF coupled Amplifier

Application of Transformer Coupled Transistor Amplifier n In impedance matching n In last stage of multistage amplifier 18

RC coupled Transformer

Frequency Response of RC coupled Amplifier

Applications of RC coupled Amplifier n Used in PA system 21