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1、精選優(yōu)質(zhì)文檔-傾情為你奉上Transimpedance amplifierFrom Wikipedia, the free encyclopediaFig. 1. Simplified transimpedance amplifierIn, aamplifier(TIA) is a current-to-voltage converter, most often implemented using an. The TIA can be used to amplifythe current output of, photo multiplier tubes, photo detectors an

2、d other types of sensors to a usable voltage. Current-to-voltage converters are used with sensors that have a current response that is more linear than the voltage response. This is the case withwhere it is not uncommon for the current response to have better than 1% linearity over a wide range of l

3、ight input. The transimpedance amplifier presents a low impedance to the photodiode and isolates it from the output voltage of the operational amplifier. In its simplest form a transimpedance amplifier has just a large valued feedback resistor, Rf. The gain of the amplifer is set by this resistor an

4、d because the amplifier is in an inverting configuration, has a value of Rf. There are several different configurations of transimpedance amplifiers, each suited to a particular application. The one factor they all have in common is the requirement to convert the low-level current of a sensor to a v

5、oltage. The gain, bandwidth, as well as current and voltage offsets change with different types of sensors, requiring different configurations of transimpedance amplifiers.In the circuit shown in Fig. 1 the photodiode is connected between ground and the inverting input of the op-amp. The other input

6、 of the op-amp is also connected to ground. This provides a low impedance load for the photodiode, which keeps the photodiode voltage low. The photodiode is operating in photovoltaic mode with no external bias. The high gain of the opamp keeps the photodiode current equal to the feedback current thr

7、ough Rf. The input offset voltage due to the photodiode is very low in this self-biased photovoltaic mode. This permits a large gain without any large output offset voltage. This configuration is used with photodiodes that are illuminated with low light levels and require a lot of gain.Fig. 2. Trans

8、impedance amplifier with a reverse biased photodiode The above equation is the DC and low frequency gain of a transimpedance amplifier. If the gain is large anyat the non-inverting input of the opamp will result in an output DC offset. An input bias current on the inverting terminal of the opamp wil

9、l similarly result in an output offset. To minimize these effects transimpedance amplifiers are usually designed with FET input op-amps that have very low input offset voltages.Fig. 2 shows a inverting TIA with the photodiode driven by a laser diode and operating in themode. A positive voltage at th

10、e cathode of the photodiode applies a reverse bias. This reverse bias increases the width of the depletion region and lowers the junction capacitance, improving the high frequency performance. The photoconductive configuration of a transimpedance photodiode amplifier is used where fast switching spe

11、ed is required but high gain is not. The feedback capacitor, Cfis usually required to improve stability.Bandwidth and stabilityFig. 3. Incremental model showing sensor capacitanceThe frequency response of a transimpedance amplifier is inversely proportional to the gain set by the feedback resistor.

12、The product of the gain, Vin/Vo, is very close to being a constant for any given op-amp. The sensors that transimpedance amplifiers are used with usually have more capacitance than an op-amp can handle. Fig. 3 models the sensor as a current source and a capacitor, Ci.This capacitance across the inpu

13、t terminals of the op-amp, which includes the internal capacitance of the op-amp, introduces a low-pass filter in the feedback path. The low pass response of this filter can be characterized as the feedback factor , which attenuates the feedback signal. This places a greater demand on the amplifier

14、gain.where is the reactance of the capacitance Ci.When the effect of this low-pass filter response is considered, the circuits response equation becomes the following:Where is the open loop gain of the op-amp.Fig. 4. Bode plot of uncompensated transimpedance amplifierAt low frequencies the feedback

15、factor, , has little effect on the amplifier response. The amplifier response will be close to the ideal,as long as the loop gain,is much greater than unity. Theof a transimpedance amplifier with no compensation is shown in Fig. 4. The vertical axis represents logarithmic gain and the horizontal axi

16、s the log of frequency. The flat curve with the peak, labeled I-TO-V GAIN, is the frequency response of the transimpedance amplifier. The peaking of the gain curve is typical of uncompensated or poorly compensated transimpedance amplifiers. The curve labeled AOLis the open-loop response of the amplifier. The feedback factor, plotted as a reciprocal, is labeled 1/. In Fig. 5 the 1/ curv