高速跨阻因素放大器

　　Introduction The purpose of a transimpedance circuit is to convert an input current from a current source （typically a photodiode） into an output voltage. The simplest method to achieve this conversion is to use a resistor connected to ground. However， the achievable gain using this method is limited by the following factors： 1. The current source input impedance; 2. The load impedance; and 3. Desired bandwidth. A closed-loop approach， using an operational amplifier， is normally beneficial to most applications as it has the potential of eliminating those issues. A typical circuit with all necessary components for this analysis is shown in Figure 1. In this circuit， the generator is a photodiode， whose role is to convert the photons into a current. This current is then amplified by the feedback resistor RF 。 Working with an ideal amplifier for now， we can see that because no bias current is present， all the signal generated by the photodiode is going to be converted to the output. Note that the source impedance is not of interest here; it is isolated from the output. Note also that the output is low impedance， allowing a wide variety of load to be connected. The bandwidth is going to be a function of the source capacitance （CS）， the feedback capacitance （CF ）， and the gain bandwidth product of the actual amplifier used.