Why does gain decrease with frequency

These results will finally be synthesized in the conclusion to plot the global frequency response of a Common Emitter Amplifier. Before defining in details the frequency response, we need to present the unit of decibel dB and the logarithmic scale related to it.

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.

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 :. 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. This sets a 0 dB reference when the gain is maximum. Each time that the power is halved, a reduction of 3 dB of the normalized gain is observed.

Halving the voltage signal corresponds to a reduction of 6 dB and follows the same pattern as presented for the power gain. The most common tool used to represent the frequency response of any system is the Bode plot. It consists of the normalized gain A V dB as a function of the frequency in log scale.

A simplified Bode graph of an amplifier is shown in the Figure 1 below :. The light blue curve is called the asymptotic representation while the dark blue curve is the real frequency response of the circuit.

The quantity f hc -f lc is called the bandwidth and represents the frequency range where the gain is above the -3 dB plateau. One last observation can be given about the slope of the frequency response out of the bandwidth.

First of all, they are not necessary identical for low and high frequencies. 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. What is important to keep in mind is that capacitors have a property called reactance that is an equivalent of the resistance. The reactance X C of capacitors depends on the frequency and the value of the capacitor, it satisfies the following formula :.

Independently of the value of the capacitor, when the frequency is low, X C tends to be high. Near DC signals, capacitors behave therefore as open circuits.

On the other hand, when the frequency increases X C tends to zero and capacitors act as short circuits. At low input frequencies, the coupling capacitors will more likely block the signal, since X C 1 and X C3 are higher, more voltage drop will be observed across C 1 and C 3.

This results in a lower voltage gain. There is another type of capacitors that affect the frequency response of the amplifier and is not represented in Figure 2. They are known as internal transistor capacitors and represented in Figure 3 below :.

Whereas the coupling and bypass capacitors act as high-pass filter they block low frequencies , these internal capacitors behave differently. However, if the frequency increases, more signal passes through them instead of going in the base branch of the transistor, therefore decreasing the voltage gain. A very important formula is given in Equation 3 and links the cutoff frequency of a RC filter :. First of all we consider the input high-pass filter R in C 1.

As explained in previous tutorials, R in is the total input impedance of the amplifier. The low cutoff frequency of the bypass structure is thus :. One last thing we need to understand before plotting the Bode graph is about the slope out of the midrange values.

This value means for high-pass filters resp. When multiple filters are blocking the same range of frequencies, the roll-off is enhanced. This information can be synthesized in a Bode plot showing the low frequency response of the CEA in asymptotic representation :.

High frequency response As stated previously, it is the internal transistor capacitors that will limit the gain at high frequencies acting as low-pass filters. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Variation of voltage gain with frequency Ask Question.

Asked 5 years, 7 months ago. Active 3 years, 9 months ago. Viewed 7k times. I know the answer but not reason Any explanation at the level of high school 12th class would be helpful. JM97 JM97 1 1 silver badge 8 8 bronze badges. Add a comment. Active Oldest Votes. Darko Darko 6 6 silver badges 12 12 bronze badges. Could you explain for reason being c the correct answer.

Show 1 more comment. Again, this is either a bad question or it assumes context within your particular class. Olin Lathrop Olin Lathrop k 36 36 gold badges silver badges bronze badges. So, I would like to answer it. So, overall, we will get a graph like this Voltage gain versus frequency :.

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