# Single Stage Amplifier

By Mohit Uniyal|Updated : September 30th, 2022

A Single Stage Amplifier can amplify effectively if the transistor is biased correctly. So, to improve the strength of the weak signal, single-stage amplifiers play a vital role. In the applications like communication and medical and scientific investigations for showing the results in the indicating instruments, we will use a single-stage amplifier.

In Multistage amplifiers, we will connect multiple single-stage amplifiers in a cascade. So, to understand the operation of multistage amplifiers, one should know the operation of a single-stage amplifier. So, in this article, we will focus on the operation of Single stage amplifier and its essential parameters.

## What is a Single Stage Amplifier?

An electronic circuit is said to be a single-stage amplifier if it consists of a single transistor with proper bias and additional components based on the requirement that will provide an output, which is then an amplified version of any input quantity like the voltage, current, and power. That means Transistor is the main component in single-stage Amplifiers.

Among all the transistors, BJT is the basic one. Hence, it is easy to understand single-stage amplifiers using JFET or MOSFET after studying and understanding the working of amplifiers using BJT in a single stage. We can analyze the multistage amplifiers by analyzing every single circuit stage.

### Simple Circuit Diagram of Single Stage Amplifier

The simple circuit diagram of a single-stage amplifier is shown below.

## Practical Circuit Diagram of Single Stage Amplifier

In the simple circuit diagram of a single-stage amplifier, we have considered one Bipolar Junction Transistor (BJT) and four resistors. The resistors R1 and R2 are used to provide a bias to the Transistor in the circuit. The resistors RC and RE are connected to BJT's collector and emitter terminals, respectively.

Basically, in this circuit diagram, the BJT and one resistor are connected to each terminal of the BJT. For practical purposes, we will connect one capacitor to each terminal of the BJT to pass the AC and block DC at the input and output terminals of the BJT.

The practical circuit diagram of a single-stage amplifier is shown below. Here, we considered the load resistor, RL.

## Equivalent Circuits of Single-Stage Amplifier

We will provide the DC supply to the single-stage amplifier circuit for biasing the Transistor. To get the amplified signal at the output, we will apply the AC signal at the input terminals of the Transistor. For easy analysis of the circuit, we will use the superposition theorem. Since we have two types of signals accordingly, we will get the following two equivalent circuits.

• DC Equivalent Circuit: We will get the DC equivalent circuit of a single-stage amplifier by open circuit the capacitors and making the AC voltage value zero by short-circuiting the AC voltage terminals. This DC equivalent circuit is shown below. Since only the DC supply is present in the circuit, we can calculate the DC voltages and DC currents in the circuit.
• AC Equivalent Circuit: We will get the AC equivalent circuit of a single-stage amplifier by short-circuiting the capacitors and making the DC voltage value zero by short-circuiting the DC voltage terminals. This AC equivalent circuit is shown below. Since only AC supply is present in the circuit, we can calculate the AC voltages and AC currents.
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## FAQs on Single Stage Amplifier

• In multistage amplifiers, we will have multiple single-stage transistors. It will be very complex if we analyse the multistage amplifier directly. Hence, we will split the multistage amplifier into multiple single stages. So that by analysing each single stage transistor of that circuit first, we can easily analyse the overall multistage amplifier.

• The number of transistors present in a single-stage amplifier is one. That means we have only one transistor stage between the input and output of the amplifier. In this amplifier, along with the transistor, there will be additional components for providing bias, etc.

• There will be less distortion in the amplitude and frequency. It provides large bandwidth. These are the advantages of single-stage RC coupled amplifiers. There will be a reduction in the voltage gain at both low and high frequencies. We cannot use this amplifier at the output or last stage since its impedance matching is poor. These are the disadvantages of single-stage RC coupled amplifiers.

• We have the basic electrical quantities like voltage and current. The multiplication of these two quantities is known as power. Since we have three types of electrical quantities, we will have three types of amplifiers: voltage amplifiers, current amplifiers, and power amplifiers

• We can classify the amplifiers into two types based on the number of stages. These are single-stage amplifiers and Multistage amplifiers. There will be a single transistor (stage) in a single-stage amplifier. Whereas in multi-stage amplifiers, multiple transistors will be there.

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