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Build Practical Amplifiers by Cascading Single-Stage Transistor Amplifiers

Key Takeaways

  • To achieve the amplitude appropriate for many applications, several amplifiers must be connected together. In such cascaded circuits, a single transistor with biasing and auxiliary equipment forms a single-stage transistor amplifier.

  • A single-stage transistor amplifier only offers amplification with a biasing circuit and associated components. Without a biasing circuit, the transistor amplifier is non-operational. 

  • The various components of a single-stage amplifier are a biasing circuit, coupling capacitors, and bypass capacitors. 

Amplification graphic

The amplification of a weak signal is essential in electronic systems

Amplifiers are an inevitable part of electronic systems—there is a constant need to amplify weak signals. To do this, transistor amplifiers are commonly used. 

To achieve the amplitude appropriate for most applications, several amplifiers must be connected together. In such cascaded circuits, a single transistor with biasing and auxiliary equipment forms a single-stage transistor amplifier. When several single-stage transistor amplifiers are connected together to achieve voltage or signal amplification, it is considered a multi-stage transistor amplifier. 

Let’s take a closer look at single-stage transistor amplifiers. 

Single-Stage Transistor Amplifiers

When a transistor with a biasing circuit and associated components amplifies a signal, it forms a single-stage transistor amplifier. A weak signal given to a single-stage transistor amplifier is strengthened due to the amplification process. 

The signal applied to the transistor amplifier allows a small base current to flow in the circuit. Due to the transistor action, a large collector current starts flowing, and depending on the resistor value RL, a large voltage is dropped across it. This large voltage drop across the collector resistor is the output voltage of the common-base transistor amplifier. 

Circuit Components of a Single-Stage Transistor Amplifier

A single-stage transistor amplifier only offers amplification with a biasing circuit and associated components. Without a biasing circuit, the transistor amplifier is non-operational. The various components of a single-stage amplifier are:

Biasing circuit-This is the most important circuit in a transistor amplifier circuit. A biasing circuit provides biasing and stabilizes the operating point. The resistors R1, R2, and RE form the biasing circuit of a single-stage transistor amplifier. 

Input capacitor, Cin-Usually, electrolytic capacitors are used as input capacitors to couple the signal to the base of the amplifier. This allows only the AC signal at the base of the transistor, blocking the DC components in the signal. Due to this DC blocking function, these capacitors can also be called blocking capacitors. If Cin is not used, the source resistance comes across the resistor R2 and changes the bias point. 

Output capacitor, CC-The output capacitor is also called a coupling capacitor, as it couples the output of one stage to the next stage. If not used, the shunting effect of resistor RC changes the bias conditions. 

Emitter bypass capacitor, CE-The bypass capacitor bypasses AC currents from the emitter to the ground. If CE is not included in the circuit, the AC voltage drop across RE will reduce the output AC signal and affect the input to the next amplifier stage. 

Cascading Single-Stage Amplifiers

Generally, practical amplifiers are multi-stage transistor amplifiers. The single-stage transistor amplifier is the basic building block of such high gain amplifiers. The process of multi-stage amplifier analysis is equal to repeatedly analyzing the single-stage amplifier block present in it. The output of each single-stage amplifier is given as the input of the next stage, and this pattern continues until the final stage is reached. The amplified signal from the final stage amplifier is the output of the multi-stage amplifier circuit. 

Consider ‘n’ single-stage amplifiers connected together to amplify the input signal, Vin. The output of stage 1 is V1, stage 2 is V2, and so on. The input Vin is given as stage 1 input, and the output signal V1 of stage 1 is the input signal of stage 2. This pattern continues until the final amplifier stage. The output of the final stage, n, is the output of the multi-stage amplifier. 

The gain of the multi-stage voltage can be given as:

Voltage gain of the multi-stage amplifier, A =VnVin = Gain of stage1 x Gain of stage 2 x Gain of stage 3...x Gain of stage n-1 x Gain of stage n  = A1A2A3...An-1An= V1Vinx V2V1xV3V2...Vn-1Vn-2xVnVn-1

The foundation of a practical amplifier is the single-stage transistor amplifier, which can be of any three configurations, namely common emitter, common collector, and common base. The analysis of a practical amplifier becomes easier with a thorough understanding of single stages of amplification. You can accomplish voltage amplification using a practical amplifier by carefully designing and cascading single-stage amplifier blocks.

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