WIDEBAND AMPLIFIERS

When analyzing amplifiers mathematically, it is convenient to assume that the gain calculations are not affected by the reactive elements that might be present in the circuit. How­ever, in reality, capacitances and inductances play a major role in determining how the amplifier performs over a range of frequencies. The effect of inductances can be minimized but it is impossible to ignore the presence of capacitances. This effect is more pronounced particularly while analyzing multistage amplifiers. Coupling capacitors and bypass capaci­tors can reduce the gain of an amplifier at lower or higher frequencies, because the capacitive reactance is inversely pro­portional to the frequency. In other words, as the frequency increases, the capacitive reactance decreases because

1

x =

j(2n fC)

Therefore, if there is a grounded bypass capacitor, signal cur­rents may be inadvertently diverted to ground instead of be­ing transmitted to the output. This is because bypass capaci­tors offer low reactances to signal currents at higher frequencies. However, the bypass capacitors offer high re­actances to signals at lower frequencies, and therefore diver­sion of such currents to ground does not pose a major problem.

Figure 1 is a representation of a frequency plot of an am­plifier. Here, the output voltage or power gain is plotted against a range of frequencies (for a given constant input volt­age). The frequency axis is normally plotted on a logarithmic scale. The unit for the y axis is decibels (dB); the number of decibels of gain is given by

201ogi° ^

or

Table 1. Bandwidth Values for Selected Electronic Signals

Signal Type

Frequency Range

Electrocardiograms

0.05 to 100 Hz

Audio signals (human ear)

20 Hz to 15,000 Hz

AM radio waves

550 kHz to 1600 kHz

FM radio waves

88 MHz to 100 MHz

Microwave and satellite signals

1 GHz to 50 GHz

amplifier characteristic be assigned a voltage level of Vflat. Then the frequencies at which voltage levels have dropped to

0. 707Vflat are denoted by fL and fH. The range of frequencies that lies between f L and f H is known as the bandwidth. In other words, the bandwidth can be defined as the frequency range over which the amplifier gain remains within 29.3% of its maximum value (3 dB level, or 1 — 0.707 = 0.293).

The bandwidth of an amplifier depends upon the applica­tions and signal type involved. Bandwidth values for some selected electronic signals are given in Table 1.

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