Definition and measurement of passband

The passband is used to measure the amplification ability of the amplifier circuit for signals of different frequencies. Due to the presence of reactive elements such as capacitors, inductors and junction capacitance of semiconductor devices in the amplifier circuit, when the input signal frequency is low or high, the value of the amplification factor will decrease and a phase shift will occur. Usually, the amplifier circuit is only suitable for amplifying signals within a specific frequency range.

As shown in the figure, this is the amplitude-frequency characteristic curve of a certain amplifier circuit.

Lower cut-off frequency fL: When the signal frequency drops to a certain level, the value of the amplification factor drops significantly. The frequency at which the value of the amplification factor is equal to 0.707 times is called the lower cut-off frequency fL.

Upper cut-off frequency fH: When the signal frequency rises to a certain level, the value of the amplification factor will also decrease. The frequency at which the value of the amplification factor is equal to 0.707 times is called the upper cut-off frequency fH.

Passband fbw: The frequency band formed between fL and fH is called the mid-frequency band, or passband fbw.

fbw=fH-fL

Or it can be defined as: in the signal transmission system, the signal frequency at which the system output signal attenuates 3dB from the maximum value is the cutoff frequency, and the frequency band between the upper and lower cutoff frequencies is called the passband, which is represented by BW. The wider the passband, the stronger the adaptability of the amplifier circuit to signals of different frequencies.

The narrower the passband, the stronger the circuit's ability to select the center frequency of the passband.

What is the use of passband?

The passband is a measure of the circuit's ability to select frequencies within a certain range.

Bandwidth refers to the number of image points scanned by the electron gun per second, measured in MHz (megahertz), and indicates the frequency range that the display circuit can handle.

Let's take an example. For example, in standard VGA mode, if the refresh rate is 60Hz, the required bandwidth is 640×480×60=18.4MHz; in 1024×768 resolution, if the refresh rate is 70Hz, the required bandwidth is 55.1MHz. The above data is theoretical value, the actual required bandwidth is higher.

Early monitors had a fixed frequency, but today's multi-frequency monitors use automatic tracking technology to automatically synchronize the monitor's scanning frequency with the output of the graphics card, thus achieving a wider range of applications.

The larger the bandwidth value, the better the display performance.

Measurement of amplitude-frequency characteristics and passband

The passband BW is used to measure the amplification ability of the amplifier circuit for signals of different frequencies. Due to the presence of coupling capacitors, emitter bypass capacitors and junction capacitance of the PN junction inside the transistor in the amplifier circuit, the value of the amplification factor will decrease when the input signal frequency is low or high. In general, the amplifier circuit is only suitable for amplifying signals within a certain frequency range.
As shown in Figure 2-13, the relationship curve between the gain of the amplifier circuit and the input signal frequency is called the amplitude-frequency characteristic curve of the amplifier circuit. In the figure, Aum is the intermediate frequency amplification factor.

When the signal frequency drops to a certain level, the value of the amplification factor decreases significantly, and the frequency value that makes the amplification factor equal to 0.707Aum is called the lower cutoff frequency fL. When the signal frequency rises to a certain level, the value of the amplification factor will also decrease, and the frequency value that makes the amplification factor equal to 0.707Aum is called the upper cutoff frequency fH. The part where f is less than fL is called the low frequency band of the amplifier circuit, the part where f is greater than fH is called the high frequency band of the amplifier circuit, and the frequency band formed between fL and fH is called the mid-frequency band, also known as the passband BW of the amplifier circuit, BW=fH-fL.

The wider the passband, the stronger the adaptability of the amplifier circuit to signals of different frequencies. When the frequency approaches zero or infinity, the value of the amplification factor approaches zero. For a loudspeaker, its passband should be wider than the audio range (20Hz~20kHz) to amplify the sound signal without distortion. In practical circuits, sometimes we also hope that the frequency band is as narrow as possible, such as a frequency-selective amplifier circuit, which is expected to amplify only a single frequency signal to avoid interference and noise.

There are usually two methods for testing amplitude-frequency characteristics and passband.

(1) Point-by-point method

While keeping the input signal constant, change the input signal frequency and use an oscilloscope to measure the output voltage point by point. Record in a sequential list and plot the measured data point by point on the coordinate paper, i.e. the frequency characteristic curve, find out fL and fH, and calculate the passband BW.

(2) Sweep frequency method

The frequency sweeper is used to directly display the curve of the amplifier output signal amplitude changing with frequency on the screen, namely the Au-f curve. The passband BW is measured on the amplitude-frequency characteristic curve displayed on the screen.