An attenuator (https://www.easybom.com/c/rf-if-and-rfid/attenuators) is a circuit used to introduce a predetermined attenuation within a specified frequency range. It is generally indicated by the number of decibels of the attenuation introduced and the ohms of its characteristic impedance. Attenuators are widely used in cable television systems to meet the level requirements of multiple ports. Such as amplifier input, output level control, branch attenuation control. There are two types of attenuators: passive attenuators and active attenuators. The active attenuator cooperates with other thermal components to form a variable attenuator, which is used in the automatic gain or slope control circuit in the amplifier. Passive attenuators have fixed attenuators and adjustable attenuators.
Can the amplifier be used as an attenuator? This is a very interesting question. This may seem counterintuitive on the surface, but in fact people want to do this for some good reasons. For operational amplifiers, one of the most useful functions is impedance conversion. Before the operational amplifier, using a passive attenuator, or using an amplifier as an attenuator, will be able to give full play to this function of the operational amplifier. However, some issues must be paid attention to in advance.
When the amplifier is used as an attenuator, the gain of the amplifier is less than unity gain (G <1). Therefore, a prerequisite is that the amplifier must be configured as an inverter, because the inverting gain is G = -RF/RG, and the non-inverting gain is G=(RF/RG)+1. Through simple analysis, it seems that the only feasible configuration that can be used as an amplifier/attenuator is inverting. In fact, it is not necessary. As mentioned above, the passive attenuator added before the non-inverting amplifier provides in-phase output. But you can also use a differential amplifier or a differential amplifier, both of which have a gain of G = RF/RG. Therefore, in fact, either an inverting amplifier or a non-inverting amplifier can be used as an attenuator...or just as an amplifier.
Regardless of the mechanism and specific structure of the power attenuation, the attenuator can always be described by the two-port network shown in the following formula. The power at the input end of the signal is P1, the power at the output end is P2, and the power attenuation of the attenuator is A (dB). If P1 and P2 are expressed in decibel milliwatts (dBm), the relationship between the power at both ends is
P2 (dBm) = P1 (dBm)-A (dB)
It can be seen that the attenuation describes the degree of power reduction after the power passes through the attenuator. The amount of attenuation is determined by the material and structure of the attenuator. The decibel is used as the unit of attenuation, which is convenient for the calculation of the whole machine index.
As mentioned above, there are some issues to be aware of when using an amplifier as an attenuator. First of all, when the resistance of the feedback resistor used is very large, it will bring several hidden dangers: one is the increase of system noise, the other is the increase of the offset voltage, and the other is the stability problem. A larger feedback resistance, coupled with the amplifier's input and stray capacitance, will introduce a pole in the amplifier's feedback response, which will bring additional phase shift, thereby reducing the amplifier's phase margin and causing instability.
Another more important consideration is the noise gain and how it relates to amplifier stability. Remember, this refers to the noise gain, not the signal gain that determines the stability of the amplifier. Whether it is an inverting or non-inverting amplifier, this noise gain is the same, and it is equal to the non-inverting gain. For example, an inverting amplifier has a signal gain of -0.5 and a noise gain of 1.5 at the same time. Once the noise gain is determined, it can be converted into an open-loop gain and phase diagram to check the amplifier's phase margin and stability. If the selected noise gain has a phase margin of 45° or more, the amplifier will work stably. If it is less than 45°, problems may occur.
Summarized by Easybom (https://www.easybom.com).