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| SEC. 16-1 FREQUENCY
RESPONSE OF
AN AMPLIFIER The frequency response is the graph of voltage gain versus input frequency. An ac amplifier has a lower and an upper cutoff frequency. A dc amplifier has only an upper cutoff frequency. Coupling and bypass capacitors produce the lower cutoff frequency. Internal transistor capacitances and stray-wiring capacitances produce the upper cutoff frequency. SEC. 16-2 DECIBEL POWER GAIN Decibel power gain is defined as 10 times the common logarithm of the power gain. When the power gain increases by a factor of 2, the decibel power gain increases by 3 dB. When the power gain increases by a factor of 10, the decibel power gain increases by 10 dB. SEC. 16-3 DECIBEL VOLTAGE GAIN Decibel voltage gain is defined as 20 times the common logarithm of the voltage gain. When the voltage gain increases by a factor of 2, the decibel voltage gain increases by 6 dB. When the voltage gain increases by a factor of 10, the decibel voltage gain increases by 20 dB. The total decibel voltage gain of cascaded stages equals the sum of the individual decibel voltage gains. SEC. 16-4 IMPEDANCE MATCHING In many systems, all impedances are matched because this produces maximum power transfer. In an impedance-matched system, the decibel power gain and the decibel voltage gain are equal. SEC. 16-5 DECIBELS ABOVE A REFERENCE Besides using decibels with power and voltage gains, we can use decibels above a reference. Two popular references are the milliwatt and the volt. Decibels with the 1 milliwatt reference are labeled dBm, and decibels with the 1 volt reference are labeled dBV. SEC. 16-6 BODE PLOTS An octave refers to a factor of 2 change of frequency. A decade refers to a factor of 10 change in frequency. A graph of decibel voltage gain versus frequency is called a Bode plot. Ideal Bode plots are approximations that allow us to draw the frequency response quickly and easily. SEC. 16-7 MORE BODE PLOTS In a lag circuit, the voltage gain breaks at the upper cutoff frequency and then rolls off at a rate of 20 dB per decade, equivalent to 6 dB per octave. We can also draw a Bode plot of phase angle versus frequency. With a lag circuit, the phase angle is between 0 and .90°. SEC. 16-8 THE MILLER EFFECT A feedback capacitor from the output to the input of an inverting amplifier is equivalent to two capacitors. One capacitor is across the input terminals, and the other is across the output terminals. The Miller effect refers to the input capacitance being Av . 1 times the feedback capacitance. 16-9 RISETIME-BANDWIDTH RELATIONSHIP When a voltage step is used as the input to a dc amplifier, the risetime of the output is the time between the 10 and 90 percent points. The upper cutoff frequency equals 0.35 divided by the risetime. This gives us a quick and easy way to measure the bandwidth of a dc amplifier. 16-10 FREQUENCY ANALYSIS OF BJT STAGES The input coupling capacitor, output coupling capacitor, and emitter bypass capacitor produce the low cutoff frequencies. The collector bypass capacitor and the input Miller capacitance produce the high cutoff frequencies. Frequency analysis of bipolar and FET stages is typically done with MultiSim or an equivalent circuit simulator. 16-11 FREQUENCY ANALYSIS OF FET STAGES The input and output coupling capacitors of a FET stage produce the low cutoff frequencies (like a BJT stage). The drain bypass capacitances, along with the gate capacitance and input Miller capacitance, produce the high cutoff frequencies. Frequency analysis of BJT and FET stages are typically done with MultiSim or an equivalent circuit simulator. |