analog circuit presentation of multistage transistor

analog circuit presentation of multistage transistor

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  ANALYZE THE HIGH FREQUENCYMODEL OF MULTISTAGE TRANSISTOR ASHMITA BHATTACHARJEE ECE X(1) UNIVERSITY ROLL:- 12000323132 ANALOG CIRCUIT (EC402)
• 2.
  INTRODUCTION A Multistage Amplifieris obtained by connecting several single-stage amplifiers in series or cascaded form. Whenever we are unable to get the required amplification factor, input, and output resistance values by using a single-stage amplifier, that time we will use Multistage amplifiers. Based on the requirement, we will connect the number of transistors to the output of a single-stage amplifier. For easy analysis of a multistage amplifier, first, we must split it into several single-stage amplifiers and then analyze each of them. So, in this article, we will focus on the operation of Multistage amplifiers and their types. We will use the respective multi-stage amplifier based on the requirement and application.
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  In practical applications,the output of a single state amplifier is usually insufficient, though it is a voltage or power amplifier. Hence they are replaced by Multi-stage transistor amplifiers. In Multi-stage amplifiers, the output of first stage is coupled to the input of next stage using a coupling device. These coupling devices can usually be a capacitor or a transformer. This process of joining two amplifier stages using a coupling device can be called as Cascading. The following figure shows a two-stage amplifier connected in cascade. The overall gain is the product of voltage gain of individual stages. Where AV = Overall gain, AV1 = Voltage gain of 1st stage, and AV2 = Voltage gain of 2nd stage. If there are n number of stages, the product of voltage gains of those n stages will be the overall gain of that multistage amplifier circuit.
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  Purpose of couplingdevice The basic purposes of a coupling device are To transfer the AC from the output of one stage to the input of next stage. To block the DC to pass from the output of one stage to the input of next stage, which means to isolate the DC conditions. Types of Coupling Joining one amplifier stage with the other in cascade, using coupling devices form a Multi-stage amplifier circuit. There are four basic methods of coupling, using these coupling devices such as resistors, capacitors, transformers etc. Let us have an idea about them. Resistance-Capacitance Coupling This is the mostly used method of coupling, formed using simple resistor- capacitor combination. The capacitor which allows AC and blocks DC is the main coupling element used here. The coupling capacitor passes the AC from the output of one stage to the input of its next stage. While blocking the DC components from DC bias voltages to effect the next stage. Let us get into the details of this method of coupling in the coming chapters.
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  Impedance Coupling The couplingnetwork that uses inductance and capacitance as coupling elements can be called as Impedance coupling network. In this impedance coupling method, the impedance of coupling coil depends on its inductance and signal frequency which is jwL. This method is not so popular and is seldom employed.
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  Transformer Coupling The couplingmethod that uses a transformer as the coupling device can be called as Transformer coupling. There is no capacitor used in this method of coupling because the transformer itself conveys the AC component directly to the base of second stage. The secondary winding of the transformer provides a base return path and hence there is no need of base resistance. This coupling is popular for its efficiency and its impedance matching and hence it is mostly used.
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  Direct Coupling If theprevious amplifier stage is connected to the next amplifier stage directly, it is called as direct coupling. The individual amplifier stage bias conditions are so designed that the stages can be directly connected without DC isolation. The direct coupling method is mostly used when the load is connected in series, with the output terminal of the active circuit element. For example, head-phones, loud speakers etc.
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  Role of Capacitorsin Amplifiers Other than the coupling purpose, there are other purposes for which few capacitors are especially employed in amplifiers. To understand this, let us know about the role of capacitors in Amplifiers. The Input Capacitor Cin The input capacitor Cin present at the initial stage of the amplifier, couples AC signal to the base of the transistor. This capacitor Cin if not present, the signal source will be in parallel to resistor R2 and the bias voltage of the transistor base will be changed. Hence Cin allows, the AC signal from source to flow into input circuit, without affecting the bias conditions
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  Coupling Capacitor CC Thecapacitor CC is the coupling capacitor that connects two stages and prevents DC interference between the stages and controls the operating point from shifting. This is also called as blocking capacitor because it does not allow the DC voltage to pass through it. In the absence of this capacitor, RC will come in parallel with the resistance R1 of the biasing network of the next stage and thereby changing the biasing conditions of the next stage. Amplifier Consideration For an amplifier circuit, the overall gain of the amplifier is an important consideration. To achieve maximum voltage gain, let us find the most suitable transistor configuration for cascading.
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  CC Amplifier •Its voltagegain is less than unity. •It is not suitable for intermediate stages. CB Amplifier •Its voltage gain is less than unity. •Hence not suitable for cascading. CE Amplifier •Its voltage gain is greater than unity. •Voltage gain is further increased by cascading.
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  CONCLUSION The power gainotherwise voltage gain can be achieved by the single-stage amplifier but it is not enough in practical application. For that, we have to use multiple stages of amplification for achieving the required voltage gain or power. This kind of amplifier is termed as a multistage amplifier analysis. In this amplifier, the first stage output is fed to the next stage input. Such type of connection is commonly known as cascading. This article discusses an overview of the multi-stage amplifier and its frequency response.
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