Nov 05, 2015

Semiconductor photodiodes are used for a wide variety of purposes from high-speed telecommunications systems to lower speed industrial control applications. Typically, larger area photodiodes are used in low-speed applications and are often used at zero bias. It is well known that the speed of a photodiode is determined by intrinsic processes involving the light wavelength and semiconductor material comprising the junction photodiode as well as the reverse-bias voltage applied to the junction. Furthermore, a reverse-bias voltage applied to a junction photodiode lowers the capacitance which will interact with the amplifier circuit to determine the overall circuit speed as well as total noise of the combined photodiode-amplifier circuit.

This article demonstrates how the speed of a typical mid-frequency Si photodiode depends on the reverse-bias voltage applied to the photodiode.

The AD8065 has a GBW product of 65MHz and a total equivalent input capacitance in this circuit of 6.6pF with a slew-rate of ~ 180V/µs, sufficiently fast for the signal levels in the measurement. For the circuit shown with Rf=1kΩ and Cf = 7.5pF (including stray) chosen to provide approximately the maximally flat transimpedance response, the circuit Tz bandwidth (for an ideal current source) is about 21MHz. The Digilent Analog Discovery scope used for the measurements has a -3db bandwidth comparable to this. Therefore the measurement BW limitation for the Tz circuit and scope combined is somewhat less than 20MHz.

To verify the Tz circuit bandwidth WITHOUT using a photodiode and LED optical source as a current source, the circuit below, using a voltage source Vs can be used with the corresponding Thevenized equivalent current source shown below (the output section with Ro, Rt is not shown for simplicity) . A simulation shows that although this test circuit is somewhat different than the original Tz circuit with an equivalent shunt resistance Rs of 5kΩ from inverting input to ground, the resultant Tz circuit response is almost identical with 20.5MHz BW and with almost identical Q value. A shunt capacitance of 10pF was added to represent the worst-case photodiode capacitance (at zero bias). The total capacitance from the INV input to ground is therefore about 18pF (including op-amp and stray). The Tz bandwidth doesn't change significantly as the shunt capacitance changes from 10 to 3 pF (corresponding to the range of PD capacitance change with the reverse bias range studied here):

The frequency response of this test-circuit is shown below, with Vs set at 1Vpeak, corresponding to an equivalent midband photocurrent of 200µA or an output voltage of 200mVpeak. The equivalent circuit, with the original voltage source, is essentially an inverting amplifier with a midband gain of 0.1 or -20dB (note that Ro and Rt at the output comprise a -6dB divider). The response is nearly flat to 10MHz with a very slight rise corresponding to a value of Q ~ 0.75 and consistent with f-3db ~ 20MHz: