Small signal amplifier

The following figure shows the characteristics curve of a transistor. The operating point of a transistor is determined by the collector-base current and VBE (voltage between base and emitter). This point is also called as Collector Current Point (CIP).

Small signal amplifier
Small signal amplifier

The following figure shows the biasing configuration of an amplifier. Common Emitter Amplifier (CeA), Common Base Amplifier (CeB), and Common Collector Amplifier (CeC) are three different types of amplifiers that are used in designing small signal amplifiers. These three types of amplifiers have different circuit configurations, like input stage, output stage, and power supply circuit. In this chapter, we will discuss about these three types of amplifiers with their respective circuit configurations and applications.

FIGURES

As shown in the figure, we have three curves for a bipolar junction transistor. The base current curve is used for determining the operating point of a transistor and it shows how much current flows through the base when an external voltage is applied to it.

FIGURES
FIGURES

The collector current curve is used for determining the operating point of a transistor and it shows how much current flows through the collector when an external voltage is applied to it.

The emitter current curve is used for determining the operating point of a transistor and it shows how much current flows through the emitter when an external voltage is applied to it.

CHARACTERISTICS

If you have a look at the characteristics curve of a transistor, you will see that it has an operating point where it is on its linear region. The operating point of a transistor is determined by its current gain, voltage gain and voltage operation point. For example, if we want to find out the current gain of a transistor then we can calculate it from its current gain curve as shown below:

CHARACTERISTICS
CHARACTERISTICS

I = VBE/2πR

So, if we want to find out the voltage gain of a transistor then we can do it by using the input/output impedance (Z) or by using beta relationship as shown below:

V = I (β) / Z

Since in case of bipolar junction transistors there are two different regions called forward bias region and reverse bias region so to know what is going on inside these regions it is important to know what direction they are biased. In case of NPN transistors they are biased with positive voltage while in case of PNP transistors they are biased with negative voltage.

Small signal amplifiers are used in many applications such as data acquisition, control systems, and analog-to-digital converters. Here you will learn about how small signal amplifiers are designed and what are the main components of a small signal amplifier.

Designing Small Signal Amplifiers

There are three types of small signal amplifiers, namely Class B, Class AB and Class A. A class B amplifier is designed to amplify an input signal with a frequency between 100Hz to 200kHz. It has two complementary outputs: one with low output impedance and the other one with high output impedance. In this case, it also has high gain which helps in increasing the gain at higher frequencies. The main drawback of this type of amplifier is that it has very low power efficiency due to which it consumes more power than other types of amplifiers.

Design Small Signal Amplifiers
Design Small Signal Amplifiers

A class AB amplifier is designed to amplify an input signal with a frequency between 100Hz to 200kHz. It has two complementary outputs: one with low output impedance and the other one with high output impedance. This type of amplifier also provides high gain at high frequencies, but it consumes less power than class B type of amplifiers as well as it has good power efficiency too because it uses less power than class B type of amplifiers.

Magnitude

Small signal name is given as they amplify the input signals that are very small in magnitude usually in mV. While designing the amplifiers, you need to make sure of two things:

Magnitude
Magnitude

Biasing point (operating point)

Biasing configuration (common emitter, common base, and common collector)

The biasing point of an amplifier actually decides to determine the shape of output signal and the biasing configuration helps to determine the phase of output signal, type of gain, and input/output impedance.  Transistors are used for designing amplifiers and the most widely used transistors for that are bipolar junction transistors. If you generally discuss the biasing point or operating point of a transistor you can say that it’s the point that is set with dc biasing. For understanding the operating point of a transistor, have a look of transistor characteristics curves. The following figure shows the characteristics curve of a transistor.

In this figure we can see that Q2 has high gain due to its high transconductances value while Q1 has low gain due to its low transconductances value, but it can be changed by adjusting collector current or base current through control grid bias voltage Vg (DC).

The very first step in designing an amplifier is to understand the biasing point and operating point of a transistor. The following figure shows the biasing point of a transistor which is referred to as bias voltage or collector current. A small signal amplifier is designed by using transistors with given characteristics curves, like dual base, high gain, low cutoff frequency etc.

About this item

The following figure shows the operating point of a transistor which is referred to as saturation current or trans-conductance ratio. The output impedance of an amplifier determines how much power can be dissipated by it. A small signal amplifier needs very low output impedance because it amplifies signals with very small magnitude. When input signal is amplified then output signal must be amplified with the same magnitude, but if you try to apply too much power then it will damage your amplifier due to its high output impedance.

The following figure shows about variable gain and fixed gain amplifiers with their input/output impedances and also showing about their phase shift between input and output signals.

The following figure shows about common emitter transistors with their collector currents and base currents at different voltages Vc=0V and Vb=5V respectively.

A small-signal amplifier is an electronic circuit that amplifies the signal input to it. A small-signal amplifier is used to amplify signals in the mV range.

Small signal amplifiers are used for amplifying the signals which are very small in magnitude and are used for driving low impedance loads such as LEDs or sensors. The biasing point of a transistor determines to determine the shape of output signal and it also determines to determine the phase of output signal, type of gain, and input/output impedance.

The small signal amplifiers are used in a variety of applications such as low noise amplifiers, oscillators, and general-purpose amplifiers. Small signal amplifiers are constructed by using transistors and diodes. They amplify signals that are very small in magnitude(mV). The input signal is applied to the base of the transistor through amplifier stages which amplify the signal and provide it to the collector of the transistor. The collector of the transistor is connected to output terminals.

Product Description

The small signal amplifier can be built up with two types of configurations: common emitter (CE), common base (CB), and common collector (CC). In each configuration there are three basic elements namely source stage, sink stage, and an active element called emitter follower. In CB configuration, both source and sink are biased together. In CE configuration, only source is biased while in CC configuration both source and sink are biased together. So, it can be seen that CE configuration has more gain than CB configuration but less gain than CC configuration. This means that CE configuration gives larger amplification than CB or CC configurations but less amplification than CE or CC configurations respectively.

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