Let’s consider a Common Emitter Amplifier (CEA) which configuration is shown in Figure 2 : fig 2 : Common Emitter Amplifier Moreover, as we will see later, the slope has a value that depends on the reactance of the components that induce a dependency with the frequency. First of all, they are not necessary identical for low and high frequencies. One last observation can be given about the slope of the frequency response out of the bandwidth. The quantity f hc-f lc is called the bandwidth and represents the frequency range where the gain is above the -3 dB plateau. In Figure 1, two different cutoff frequencies can be distinguished : f lc for “low cutoff” and f hc for “high cutoff”. The light blue curve is called the asymptotic representation while the dark blue curve is the real frequency response of the circuit. A simplified Bode graph of an amplifier is shown in the Figure 1 below : fig 1 : Typical Bode graph of an amplifier It consists of the normalized gain A V(dB) as a function of the frequency in log scale. The most common tool used to represent the frequency response of any system is the Bode plot. Halving the voltage signal corresponds to a reduction of 6 dB and follows the same pattern as presented for the power gain. Therefore A P=-3 dB corresponds to A V,mid/2, A P=-6 dB corresponds to A V ,mid/4 and so on …įor this same frequency, the voltage (or current) is multiplied by a factor √2=0.7. Each time that the power is halved, a reduction of 3 dB of the normalized gain is observed. The frequency at which the power drops to 50 % of its midrange value is known as the cutoff frequency and noted f c. It is important to note that when the power is divided by two, we observe that A P(dB)=10log(0.5)=-3 dB. This sets a 0 dB reference when the gain is maximum. Therefore, when A V=A V,mid, the normalized gain (written indifferently A V) is A V(dB)=0. Where A V,mid is called the midrange gain and represents the maximum gain of the amplifier in its frequency working range, for example 20 Hz – 20 kHz for an audio amplifier. Often, it is not the gain A V(dB) that is investigated but rather a normalized ratio A V/A V ,mid(dB)=20log(A V/A V,mid). While the gains in linear scale are always positive (A P,A V≥0), their equivalent in dB can either be positive if an amplification is being realized (A P,A V>1) or negative if the input signal is attenuated (A P,A V<1). If we consider an amplifier with power gain A P and voltage gain A V, the power and voltage gain in dB are defined by : eq 1 : Power and voltage gain in dB When studying the frequency response, it is indeed more suitable to convert either the power or voltage gain into dB and to represent the frequency scale in a logarithmic (log) scale. ![]() Definitionsīefore defining in details the frequency response, we need to present the unit of decibel (dB) and the logarithmic scale related to it. These results will finally be synthesized in the conclusion to plot the global frequency response of a Common Emitter Amplifier. ![]() In the rest of the article a method to establish the low and high frequency responses is presented. In the second section, we will understand which component affects the frequency response and how. First of all, the notion of frequency response is detailed along with some basic related concepts and we will present how to quantify it. ![]() In this tutorial, we will focus on this important feature of amplifiers. The frequency response depends directly on the components and the architecture chosen for the design of the amplifier. In the frequency range that amplifiers have been designed for, they must deliver a constant and acceptable level of gain. This characteristic is known as the frequency response.įrequency response is one of the most important property of amplifiers. As such for any electronic circuit, the behavior of amplifiers is affected by the frequency of the signal on their input terminal.
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