Features that a high-frequency signal amplifier should have

Amplifier circuits can be said to be the basic unit of analog signal processing circuits, especially for high-frequency receivers and transmitters. In the receiver, the amplifier circuit amplifies the small signal in µV input from the antenna, and in the transmitter, the power amplifier circuit also amplifies the signal to a signal level in W. In this chapter, three different types of amplifier circuits will be analyzed in turn: small signal high-frequency amplifiers, wide-band amplifiers, and power amplifiers. 2-1 The characteristics required for high-frequency amplifier circuits are that the power gain for the frequency band used should be high. In DC amplifiers and low-frequency amplifier circuits, gain generally refers to voltage gain; while in high-frequency circuits, gain is generally expressed as power gain. For example, in the circuit shown in Figure 2-1, the signal input from the antenna is -30dBm (0.00lmW). When the power gain of the high-frequency amplifier is 25dB, the output signal becomes -5dBm.

Figure 2-1 The gain of a high-frequency circuit is the power gain (in a high-frequency amplifier, the power gain of each stage is generally limited to 20~30dB. As shown in the figure, if the input signal is -30dBm and the gain is 25dB, the amplified signal becomes -5dBm.)

The noise generated must be very small. S/N is called the signal-to-noise ratio, which is often used to indicate the quality of the signal and reflects the ratio of the useful signal to the noise in the specific signal.
As shown in Figure 2-2, since the amplifier generates noise while amplifying the signal, the S/N value of the signal at the output end is smaller than that at the input end, and the quality will deteriorate. The change in signal S/N caused by amplification can be expressed by the noise index NF. The NF of an ideal amplifier is 0dB.
Figure 2-3 shows an example of improving NF. In a receiver with a noise index of 8dB, a high-frequency amplifier of the pre-amplifier is connected. At this time, the NF of the pre-amplifier is 2dB, and the power gain is 25dB. Therefore, the NF after connecting the pre-amplifier can be expressed by the following formula.

Substituting the numerical value into this formula, we can get the NF after connecting the preamplifier as NF=10log[1.58+(6.3-1)/316]=2.03dB. It can be seen that adding a preamplifier can improve the overall NF and obtain a high-gain, low-noise amplifier.

Figure 2-2 Amplifier Noise Index NF (S/N is a value that indicates signal quality. In high-frequency amplifiers, noise is generated inside the amplifier, causing the S/N to deteriorate. The noise index NF can be used to analyze the situation in which the S/N is reduced due to internal noise. An amplifier with no internal noise is called an ideal amplifier, and its NF is 0dB.)

Figure 2-3 Improvement of Noise Index NF

The intercept point (IPterceptpoint) should be high. The input signal of the high-frequency amplifier circuit has many complex components, especially when the selectivity of the high-frequency amplifier circuit is not very high, it is more likely to input multiple components of the input signal. Therefore, in the high-frequency amplifier circuit, the signals of multiple components will interfere with each other and generate redundant signals. In addition, since the non-linear part of the amplifier circuit will also generate high harmonics, these signals will intermodulate and generate redundant signals. Due to the existence of the above situation, the generated signal becomes a pseudo image (SpuriOtIS) component, high harmonic component and other noise that interferes with the receiver. Here, the third intermodulation distortion component with the most serious impact is compared with the signal, and this comparison can be expressed by the intercept point (Intercept Point). Figure 2-4 shows the intercept point method. The point on the figure where the signal level is equal to the level of the third intermodulation distortion is called the intercept point. Since the actual amplifier level has reached saturation, the imaginary extension line shown by the dotted line is used to find the intercept point.

Figure 2-4 Intercept point (In high-frequency amplifier circuits, interference signal components other than the target signal are called false image components. Among the false image components, the third-order intermodulation distortion is the most problematic. The point where the basic signal and the third-order intermodulation distortion are the same is called the intercept point.)

Summary: In the actual high-frequency amplifier circuit design, the most important thing is to design according to the above three points, which is very important.