How do you measure input and output impedance?

Input resistance is a measure of the amplifier’s effect on the signal source. The larger the input resistance, the smaller the current taken by the amplifier from the signal source, and the larger the signal voltage obtained at the input terminal of the amplifier, that is, the less attenuation of the signal source voltage. Theoretical basis: Us=(Rs+Ri)*I. Rs is the internal resistance of the signal source, and Ri is the input resistance of the amplifier. Therefore, the amplifier circuits of instruments such as oscilloscopes and voltmeters for measuring signal voltages should have relatively large input resistances. If you want to get a larger current from the signal source, you should make the amplifier have a smaller input resistance.Today qihe smt pick and place machine will share some tips for measuring input and output impedance .

The output resistance is used to measure the strength of the amplifier’s load capacity. When the amplifier outputs the amplified signal to the load resistor RL, for the load RL, the amplifier can be equivalent to a signal source with internal resistance Ro, and this signal source provides output signal voltage and output signal current to RL. Ro is called the output resistance of the amplifier, which is the AC equivalent resistance seen from the output terminal of the amplifier to the amplifier itself. If the output resistance Ro is very small and satisfies the condition of R0<>RL, then when RL changes within a wide range, the output signal current can be maintained constant. The ability of the amplifier to maintain a constant output signal voltage (or current) under different load conditions is called the load capacity. The output resistance Ro is a performance index that characterizes this capability.

The common-collector amplifier, also known as an emitter follower, has a large input resistance and a small output resistance (usually a few ohms or hundreds of ohms). In order to reduce the output resistance value, you can choose a tube with a large B value and a small output resistance, which means that it has a strong load capacity. When the load changes in a large range, it can basically maintain a constant output signal voltage. The common collector circuit cannot amplify the voltage signal (always less than 1), but it can amplify the current signal and amplify the power. This circuit is often used in the input stage of high input impedance and the output stage of low output impedance in multi-stage amplifier circuits, or as a buffer stage for realizing impedance transformation. For example, after the output of the line array CCD, it is necessary to connect a first-stage emitter follower to increase the current and improve the ability to drive the subsequent circuit before performing correlated double sampling. Because the signal source resistance will affect the output resistance of the circuit, the influence of the signal source internal resistance Rs should be considered. In addition, the load resistance RL will affect the input resistance Ri, which should be paid attention to in the analysis and design calculation of the amplifying circuit.

The voltage amplification factor of the common-emitter amplifying circuit is relatively large, and the output signal voltage is inverse to the input signal voltage. His current magnification is also relatively large. His input resistance and output resistance are large and suitable. This kind of circuit is often used in places where there is no special requirement for input resistance and output resistance, as the input stage, intermediate stage or output stage of a general low-frequency multi-stage amplifier circuit.

The voltage magnification of the common-base amplifier circuit is relatively large, and the output signal is in phase with the input signal voltage. His current magnification factor is less than 1 and cannot amplify the current. The input resistance of this circuit is small and the output resistance is moderate. Because of its good frequency characteristics, it is often used in broadband amplifiers and high-frequency amplifiers

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