The HP 5082-4204 PIN Photodiode will be used as an example for our discussion. Its characteristics are given in Figure 5.26. It is typical of many commercially available PIN photodiodes. As in most high-speed photodiode applications, the diode is operated in the reverse-biased or photoconductive mode. This greatly lowers the diode junction capacitance, but causes a small amount of dark current to flow even when the diode is not illuminated (we will show a circuit which compensates for the dark current error later in the section).

This photodiode is linear with illumination up to approximately 50 to 100uA of output current. The dynamic range is limited by the total circuit noise and the diode dark current (assuming no dark current compensation).

f1 = 274kHz

C2 = 0.76pF

f2 = 2.1MHz.

As in most noise analyses, only the key contributors need be identified. Because the noise sources combine in an RSS manner, any single noise source that is at least three or four times as large as any of the others will dominate.

The factor of 1.57 converts the approximate single-pole bandwidth of 2.1MHz into the equivalent noise bandwidth.

The output noise due to the input voltage noise is obtained by multiplying the noise gain by the voltage noise and integrating the entire function over frequency. This would be tedious if done rigorously, but a few reasonable approximations can be made which greatly simplify the math. Obviously, the low frequency 1/f noise can be neglected in the case of the wideband circuit. The primary source of output noise is due to the high-frequency noise-gain peaking which occurs between f i and fu. If we simply assume that the output noise is constant over the entire range of frequencies and use the maximum value for AC noise gain [1+(C1/C2)], then

The total rms noise referred to the output is then the RSS value of the two components:

TOTAL RTO NOISE =

C1 = 5.8pF

C2 = 0.76pF

274kHz 2.1MHz 16MHz

DYNAMIC RANGE = 92dB

Large value feedback resistor circuits require these types of amplifiers. For biased circuits a bipolar operational amplifier can also be considered especially for Avalanche photodiodes. Bipolar amplifiers are usually only used with feedback resistors of 50k ohms and lower, because the output offsets caused by their high input bias current. Bipolar amplifiers have lower input voltage noise, but higher input current noise FET amplifiers. They are typically much faster than FET and CMOS type amplifiers and maybe considered for high-speed circuitry.

Feedback capacitance values for an overshoot of 5% and 25% are calculated. The 5% overshoot capacitance value is the smallest capacitance recommended in a production circuit. This is because of the tolerance of the components. A capacitance value for a 25% can be used, but the devices will need to be tested for overshoot because the component tolerances may cause the circuit to have excessive ringing and instability. The faster the circuit is the more noise it will have and less stable it will be; therefore don't design the circuit for more bandwidth than is required.

That's basically all you need to know to design transimpedance amplifier circuits. Power supply bypass capacitors recommended by the operational amplifier manufacturer should always be used and are not shown in any of the model or circuitry schematics