Last Updated on October 22, 2023 by Kevin Chen
Image of transistor amplifier: source -Freepik
One of the critical functions of transistors is to serve as amplifiers. They level up signals in the circuit. If it is the power signal, they ensure that the output signal is higher than the input or incoming signal. And how do they execute the amplification role? Transistors have internal working mechanisms that ensure that process the input signals and ensure that the output signal is way higher than what came in.
How do I measure the amplification level of my transistor? Well, there are different parameters that we can use to gauge the amplification level of transistors. One of them is the gain.
So, what is the transistor amplifier gain? We can define a transistor amplifier gain as an increase in the input signal. To quantify this, we will simply have to compare the power of the output signal to that of the input signal. The difference between the two is what we will refer to as the transistor amplifier gain.
Simply put: Gain= Amplitude of the output signal/Amplitude of the input signal
The transistor amplifier gain is usually expressed in the form of ratio and it is a major component that is used in making critical decisions about the circuit. For example, it determines what level of amplification that you should apply to the input signal. Also, the gain ensures that the amplification does not alter the quality of the signal that is being amplified.
Before you buy a transistor amplifier, it will be prudent to calculate its gain. The purpose of this article is to help you do these calculations. We are going to highlight all the basics that you should know when calculating the transistor amplifier gain.
Calculating the gain of a transistor amplifier
How do calculate the voltage gain of a common emitter amplifier? As we have already defined, the gain is the ratio of the output power to the input signal power.
This gain is defined by this formula:
Gain = Output voltage /Input voltage
For an amplifier that has a voltage gain of 5, it means that its output voltage is five times higher than the input signal.
The gain tends to vary from one amplifier to another. It all depends on the input level and the required level of amplification.
Common emitter amplifier
Not all types and configurations of transistors can be used as amplifiers. So, before we even get deeper into the matters pertaining to the calculation of the gain, it will be a smart move to look at the type and configurations of the transistors that can serve as amplifiers.
The common emitter is the ideal configuration in which a transistor can serve as an amplifier. In fact, it is usually referred to as the common emitter amplifier. The two other transistor configurations are the common base and the common collector. The common emitter configuration is used mainly because of its numerous impressive features.
In the common emitter amplifier configuration, the signal gets into the amplifier through the base and out through the collector terminal. This configuration works on both the PNP and NPN transistors.
BJT transistor is the most common type of transistor amplifier and it perfectly suits the common emitter configuration.
How does a common emitter amplify the signals?
The amplification process in the common emitter amplifier is quite simple. It all starts with the application of signals at the emitter-base junction which is forward-biased. There will in a flow of electrons from the emitter to the collector. This will eventually cause an increase in the collector current.
The amplification of the signals follows a waveform pattern with a 180-degree shift between the input and output waves.
What is unity gain?
When calculating the gain of transistor amplifier, you are likely to encounter the term “unity gain”. Sometimes this type of gain is also referred to as a buffer or voltage-follower. Here, the signal gain is one. This means that the ratio of output to input is 1. Unity gain is obtained when the output signal is equal to the input signal.
Unity gain transistor amplifiers are mainly used for buffering the high-impedance signal sources. The transistor achieves this buffering by limiting the intake of input current into the circuit. By doing so, it separates the input second of the circuit from the output section. This mode of operation is quite common in the operational amplifiers (op-amps).
Electronic elements that affect the transistor amplifier gain
Other than just the input signal and the output signals, there are other key electronic elements that are likely to affect the gain of a transistor amplifier. These elements include the following:
- Circuit voltage divider: In most applications, resistors are used to work as voltage dividers and at the same time stabilize the circuit. They ensure that the transistor has a specific point of operation.
- Input capacitor: Capacitors are connected at the input junction to couple the incoming signal. The capacitor may control or regulate the flow of signals.
- Emitter bypass capacitor (CE): An emitter bypass capacitor is connected to the output terminal of the transistor amplifier. This capacitor provides low reactance to the amplified signal.
How does the signal frequency affect the transistor amplifier gain?
Frequency signal is another key factor that affects the voltage gain of a transistor amplifier. As we have stated, capacitors are among the key components installed in the circuit along with the transistor amplifiers. The reactance of these capacitors usually varies depending on the frequency of the input signals. This eventually ends up affecting the amplitude of the output signals
Low frequency will result in a low voltage gain while high frequency will result in a high voltage gain. An average voltage gain is obtained when the frequency stagnates at mid-range.
We have discussed how to calculate the signal gain of a transistor amplifier in a circuit. Next time you are working on such a circuit, you will be in a perfect position to know what to do and the figures that you should get. While at it, ensure that you buy high-quality BJT transistors from reputable sources.
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