Understanding Input and Output Resistance

Input resistance is a performance indicator used to measure the impact of an amplifier on a signal source. The larger the input resistance, the smaller the current the amplifier draws from the signal source, and the larger the signal voltage obtained at the input end of the amplifier, that is, the signal source voltage is less attenuated. Theoretical basis: Us=(Rs+Ri)*I. Rs is the signal source internal resistance, Ri is the amplifier input resistance. Therefore, the amplifier circuit of an oscilloscope, voltmeter and other instruments used to measure signal voltage should have a large input resistance. If you want to draw a larger current from the signal source, you should make the amplifier have a smaller input resistance. Today qihe smt pick and place machine sharing the meaning of input resistance and output resistance

The output resistance is used to measure the load capacity of the amplifier. 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. This signal source provides the 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 end of the amplifier to the amplifier itself.

If the output resistance Ro is very small and meets the condition of R0<>RL, then when RL changes within a large range, the output signal current can be maintained constant. The ability of an amplifier to maintain a constant output signal voltage (or current) under different load conditions is called load capability. The output resistance Ro is a performance indicator that characterizes this ability.

The common collector amplifier, also known as the emitter follower, has a large input resistance and a small output resistance (generally 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. A smaller output resistance indicates a strong load capacity. When the load changes within a large range, the output signal voltage can basically be maintained constant. 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 multi-stage amplifier circuits as an input stage with high input impedance, an output stage with low output impedance, or as a buffer stage to achieve impedance transformation. For example, before performing correlated double sampling after the linear array CCD output, a first-level emitter follower must be connected to increase the current and improve the ability to drive the subsequent circuit. 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 when analyzing and designing the amplifier circuit.

The voltage amplification factor of the common emitter amplifier circuit is larger, and the output signal voltage is inverse phase with the input signal voltage. Its current amplification factor is also relatively large. His input resistance and output resistance are large and appropriate. This kind of circuit is often used in places where there are no special requirements for input resistance and output resistance. It is used as the input stage, intermediate stage or output stage of general low-frequency multi-stage amplifier circuits.

The voltage amplification factor of the common base amplifier circuit is also relatively large, and the output signal is in the same phase as the input signal voltage. Its current amplification factor is less than 1 and it 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 wide-band amplifiers and high-band amplifiers.

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